title: "Yellowstone National Park" content: Fish and Wildlife Service removed the Northern Rocky Mountain wolf population from the endangered species list. Black bears are common in the park and were a park symbol due to visitor interaction with the bears starting in 1910. Feeding and close contact with bears has not been permitted since the 1960s to reduce their desire for human foods. Yellowstone is one of the few places in the United States where black bears can be seen coexisting with grizzly bears. Black bear observations occur most often in the park's northern ranges and in the Bechler area which is in the park's


title: "Yellowstone National Park" content: southwestern corner. , an estimated 700 grizzly bears were living in the Greater Yellowstone Ecosystem, with about 150 grizzlies living wholly or partially within Yellowstone National Park. The grizzly was initially listed as a threatened species in the contiguous United States on July 28, 1975 by the Fish and Wildlife Service. The grizzly bear was taken off the endangered species list in 2007. Opponents of delisting the grizzly expressed concerns that states might once again allow hunting and that better conservation measures were needed to ensure a sustainable population. A federal district judge overturned the delisting ruling in 2009, reinstating


title: "Yellowstone National Park" content: the grizzly; however, the grizzly was once again removed from the list in 2017. In September 2018, a US district judge ruled that the grizzly's protections must be restored in full, arguing the Fish and Wildlife Service was mistaken in removing the bear from the threatened status list. Regardless of the rulings, hunting is prohibited within Yellowstone National Park. Hunters who legally hunt animals outside park boundaries may transport the carcass through the park with a permit. Population figures for elk are in excess of 30,000—the largest population of any large mammal species in Yellowstone. The northern herd has decreased


title: "Yellowstone National Park" content: enormously since the mid‑1990s; this has been attributed to wolf predation and causal effects such as elk using more forested regions to evade predation, consequently making it harder for researchers to accurately count them. The northern herd migrates west into southwestern Montana in the winter. The southern herd migrates southward, and the majority of these elk winter on the National Elk Refuge, immediately southeast of Grand Teton National Park. The southern herd migration is the largest mammalian migration remaining in the U.S. outside of Alaska. In 2003 the tracks of one female lynx and her cub were spotted and followed


title: "Yellowstone National Park" content: for over . Fecal material and other evidence obtained were tested and confirmed to be those of a lynx. No visual confirmation was made, however. Lynx have not been seen in Yellowstone since 1998, though DNA taken from hair samples obtained in 2001 confirmed that lynx were at least transient to the park. Other less commonly seen mammals include the mountain lion and wolverine. The mountain lion has an estimated population of only 25 individuals parkwide. The wolverine is another rare park mammal, and accurate population figures for this species are not known. These uncommon and rare mammals provide insight


title: "Yellowstone National Park" content: into the health of protected lands such as Yellowstone and help managers make determinations as to how best to preserve habitats. Eighteen species of fish live in Yellowstone, including the core range of the Yellowstone cutthroat trout—a fish highly sought by anglers. The Yellowstone cutthroat trout has faced several threats since the 1980s, including the suspected illegal introduction into Yellowstone Lake of lake trout, an invasive species which consume the smaller cutthroat trout. Although lake trout were established in Shoshone and Lewis lakes in the Snake River drainage from U.S. Government stocking operations in 1890, it was never officially introduced


title: "Yellowstone National Park" content: into the Yellowstone River drainage. The cutthroat trout has also faced an ongoing drought, as well as the accidental introduction of a parasite—whirling disease—which causes a terminal nervous system disease in younger fish. Since 2001, all native sport fish species caught in Yellowstone waterways are subject to a catch and release law. Yellowstone is also home to six species of reptiles, such as the painted turtle and Prairie rattlesnake, and four species of amphibians, including the Boreal Chorus Frog. 311 species of birds have been reported, almost half of which nest in Yellowstone. As of 1999, twenty-six pairs of nesting


title: "Yellowstone National Park" content: bald eagles have been documented. Extremely rare sightings of whooping cranes have been recorded, however only three examples of this species are known to live in the Rocky Mountains, out of 385 known worldwide. Other birds, considered to be species of special concern because of their rarity in Yellowstone, include the common loon, harlequin duck, osprey, peregrine falcon and the trumpeter swan. As wildfire is a natural part of most ecosystems, plants that are indigenous to Yellowstone have adapted in a variety of ways. Douglas-fir have a thick bark which protects the inner section of the tree from most fires.


title: "Yellowstone National Park" content: Lodgepole Pines —the most common tree species in the park— generally have cones that are only opened by the heat of fire. Their seeds are held in place by a tough resin, and fire assists in melting the resin, allowing the seeds to disperse. Fire clears out dead and downed wood, providing fewer obstacles for lodgepole pines to flourish. Subalpine Fir, Engelmann Spruce, Whitebark Pine, and other species tend to grow in colder and moister areas, where fire is less likely to occur. Aspen trees sprout new growth from their roots, and even if a severe fire kills the tree


title: "Yellowstone National Park" content: above ground, the roots often survive unharmed because they are insulated from the heat by soil. The National Park Service estimates that in natural conditions, grasslands in Yellowstone burned an average of every 20 to 25 years, while forests in the park would experience fire about every 300 years. About thirty-five natural forest fires are ignited each year by lightning, while another six to ten are started by people— in most cases by accident. Yellowstone National Park has three fire lookout towers, each staffed by trained fire fighters. The easiest one to reach is atop Mount Washburn, which has interpretive


title: "Yellowstone National Park" content: exhibits and an observation deck open to the public. The park also monitors fire from the air and relies on visitor reports of smoke and/or flames. Fire towers are staffed almost continuously from late June to mid-September— the primary fire season. Fires burn with the greatest intensity in the late afternoon and evening. Few fires burn more than , and the vast majority of fires reach only a little over an acre (0.5 ha) before they burn themselves out. Fire management focuses on monitoring dead and down wood quantities, soil and tree moisture, and the weather, to determine those areas


title: "Yellowstone National Park" content: most vulnerable to fire should one ignite. Current policy is to suppress all human caused fires and to evaluate natural fires, examining the benefit or detriment they may pose on the ecosystem. If a fire is considered to be an immediate threat to people and structures, or will burn out of control, then fire suppression is performed. In an effort to minimize the chances of out of control fires and threats to people and structures, park employees do more than just monitor the potential for fire. Controlled burns are prescribed fires which are deliberately started to remove dead timber under


title: "Yellowstone National Park" content: conditions which allow fire fighters an opportunity to carefully control where and how much wood is consumed. Natural fires are sometimes considered prescribed fires if they are left to burn. In Yellowstone, unlike some other parks, there have been very few fires deliberately started by employees as prescribed burns. However, over the last 30 years, over 300 natural fires have been allowed to burn naturally. In addition, fire fighters remove dead and down wood and other hazards from areas where they will be a potential fire threat to lives and property, reducing the chances of fire danger in these areas.


title: "Yellowstone National Park" content: Fire monitors also regulate fire through educational services to the public and have been known to temporarily ban campfires from campgrounds during periods of high fire danger. The common notion in early United States land management policies was that all forest fires were bad. Fire was seen as a purely destructive force and there was little understanding that it was an integral part of the ecosystem. Consequently, until the 1970s, when a better understanding of wildfire was developed, all fires were suppressed. This led to an increase in dead and dying forests, which would later provide the fuel load for


title: "Yellowstone National Park" content: fires that would be much harder, and in some cases, impossible to control. Fire Management Plans were implemented, detailing that natural fires should be allowed to burn if they posed no immediate threat to lives and property. 1988 started with a wet spring season although by summer, drought began moving in throughout the northern Rockies, creating the driest year on record to that point. Grasses and plants which grew well in the early summer from the abundant spring moisture produced plenty of grass, which soon turned to dry tinder. The National Park Service began firefighting efforts to keep the fires


title: "Yellowstone National Park" content: under control, but the extreme drought made suppression difficult. Between July 15 and 21, 1988, fires quickly spread from throughout the entire Yellowstone region, which included areas outside the park, to on the park land alone. By the end of the month, the fires were out of control. Large fires burned together, and on August 20, 1988, the single worst day of the fires, more than were consumed. Seven large fires were responsible for 95% of the that were burned over the next couple of months. A total of 25,000 firefighters and U.S. military forces participated in the suppression efforts,


title: "Yellowstone National Park" content: at a cost of 120 million dollars. By the time winter brought snow that helped extinguish the last flames, the fires had destroyed 67 structures and caused several million dollars in damage. Though no civilian lives were lost, two personnel associated with the firefighting efforts were killed. Contrary to media reports and speculation at the time, the fires killed very few park animals— surveys indicated that only about 345 elk (of an estimated 40,000–50,000), 36 deer, 12 moose, 6 black bears, and 9 bison had perished. Changes in fire management policies were implemented by land management agencies throughout the United


title: "Yellowstone National Park" content: States, based on knowledge gained from the 1988 fires and the evaluation of scientists and experts from various fields. By 1992, Yellowstone had adopted a new fire management plan which observed stricter guidelines for the management of natural fires. Yellowstone climate is greatly influenced by altitude, with lower elevations generally found to be warmer year-round. The record high temperature was in 2002, while the coldest temperature recorded is in 1933. During the summer months of June to early September, daytime highs are normally in the range, while night time lows can go to below freezing (0 °C) especially at higher


title: "Yellowstone National Park" content: altitudes. Summer afternoons are frequently accompanied by thunderstorms. Spring and fall temperatures range between with nights in the teens to single digits (−5 to −20 °C). Winter in Yellowstone is accompanied by high temperatures usually between zero and 20 °F (−20 to −5 °C) and nighttime temperatures below 0 °F (−18 °C) for most of the winter. Precipitation in Yellowstone is highly variable and ranges from annually near Mammoth Hot Springs, to in the southwestern sections of the park. The precipitation of Yellowstone is greatly influenced by the moisture channel formed by the Snake River Plain to the west that


title: "Yellowstone National Park" content: was, in turn, formed by Yellowstone itself. Snow is possible in any month of the year, but most common between November and April, with averages of annually around Yellowstone Lake, to twice that amount at higher elevations. Tornadoes in Yellowstone are rare; however, on July 21, 1987, the most powerful tornado recorded in Wyoming touched down in the Teton Wilderness of Bridger-Teton National Forest and hit Yellowstone National Park. Called the Teton–Yellowstone tornado, it was classified as an F4, with wind speeds estimated at between . The tornado left a path of destruction wide, and long, and leveled of mature


title: "Yellowstone National Park" content: pine forest. The climate at Yellowstone Lake is classified as subarctic (Dfc), according to Köppen-Geiger climate classification, while at the park headquarters the classification is humid continental (Dfb). Yellowstone ranks among the most popular national parks in the United States. Since the mid-1960s, at least 2 million tourists have visited the park almost every year. Average annual visitation increased to 3.5 million during the ten-year period from 2007 to 2016, with a record of 4,257,177 recreational visitors in 2016. July is the busiest month for Yellowstone National Park. At peak summer levels, 3,700 employees work for Yellowstone National Park concessionaires.


title: "Yellowstone National Park" content: Concessionaires manage nine hotels and lodges, with a total of 2,238 hotel rooms and cabins available. They also oversee gas stations, stores and most of the campgrounds. Another 800 employees work either permanently or seasonally for the National Park Service. Park service roads lead to major features; however, road reconstruction has produced temporary road closures. Yellowstone is in the midst of a long term road reconstruction effort, which is hampered by a short repair season. In the winter, all roads aside from the one which enters from Gardiner, Montana, and extends to Cooke City, Montana, are closed to wheeled vehicles.


title: "Yellowstone National Park" content: Park roads are closed to wheeled vehicles from early November to mid April, but some park roads remain closed until mid-May. The park has of paved roads which can be accessed from five different entrances. There is no public transportation available inside the park, but several tour companies can be contacted for guided motorized transport. In the winter, concessionaires operate guided snowmobile and snow coach tours, though their numbers and access are based on quotas established by the National Park Service. Facilities in the Old Faithful, Canyon and Mammoth Hot Springs areas of the park are very busy during the


title: "Yellowstone National Park" content: summer months. Traffic jams created by road construction or by people observing wildlife can result in long delays. The National Park Service maintains 9 visitor centers and museums and is responsible for maintenance of historical structures and many of the other 2,000 buildings. These structures include National Historical Landmarks such as the Old Faithful Inn built from 1903 to 1904 and the entire Fort Yellowstone – Mammoth Hot Springs Historic District. An historical and educational tour is available at Fort Yellowstone which details the history of the National Park Service and the development of the park. Campfire programs, guided walks


title: "Yellowstone National Park" content: and other interpretive presentations are available at numerous locations in the summer, and on a limited basis during other seasons. Camping is available at a dozen campgrounds with more than 2,000 campsites. Camping is also available in surrounding National Forests, as well as in Grand Teton National Park to the south. Backcountry campsites are accessible only by foot or by horseback and require a permit. There are of hiking trails available. The park is not considered to be a good destination for mountaineering because of the instability of volcanic rock which predominates. Visitors with pets are required to keep them


title: "Yellowstone National Park" content: on a leash at all times and are limited to areas near roadways and in "frontcountry" zones such as drive in campgrounds. Around thermal features, wooden and paved trails have been constructed to ensure visitor safety, and most of these areas are handicapped accessible. The National Park Service maintains a year-round clinic at Mammoth Hot Springs and provides emergency services throughout the year. Hunting is not permitted, though it is allowed in the surrounding national forests during open season. Fishing is a popular activity, and a Yellowstone Park fishing license is required to fish in park waters. Many park waters


title: "Yellowstone National Park" content: are fly fishing only and all native fish species are catch and release only. Boating is prohibited on rivers and creeks except for a stretch of the Lewis River between Lewis and Shoshone Lake, and it is open to non-motorized use only. Yellowstone Lake has a marina, and the lake is the most popular boating destination. In the early history of the park, visitors were allowed, and sometimes even encouraged, to feed the bears. Visitors welcomed the chance to get their pictures taken with the bears, who had learned to beg for food. This led to numerous injuries to humans


title: "Yellowstone National Park" content: each year. In 1970, park officials changed their policy and started a vigorous program to educate the public on the dangers of close contact with bears, and to try to eliminate opportunities for bears to find food in campgrounds and trash collection areas. Although it has become more difficult to observe bears in recent years, the number of human injuries and deaths has taken a significant drop and visitors are in less danger. The eighth recorded bear-related death in the park's history occurred in August 2015. Other protected lands in the region include Caribou-Targhee, Gallatin, Custer, Shoshone and Bridger-Teton National


title: "Yellowstone National Park" content: Forests. The National Park Service's John D. Rockefeller, Jr. Memorial Parkway is to the south and leads to Grand Teton National Park. The famed Beartooth Highway provides access from the northeast and has spectacular high altitude scenery. Nearby communities include West Yellowstone, Montana; Cody, Wyoming; Red Lodge, Montana; Ashton, Idaho; and Gardiner, Montana. The closest air transport is available by way of Bozeman, Montana; Billings, Montana; Jackson; Cody, Wyoming, or Idaho Falls, Idaho. Salt Lake City, to the south, is the closest large metropolitan area. The entire park is within the jurisdiction of the United States District Court for the


title: "Yellowstone National Park" content: District of Wyoming, making it the only federal court district that includes portions of more than one state (Idaho, Montana, and Wyoming). Law professor Brian C. Kalt has argued that it may be impossible to impanel a jury in compliance with the Vicinage Clause of the Sixth Amendment for a crime committed solely in the unpopulated Idaho portion of the park (and that it would be difficult to do so for a crime committed solely in the lightly populated Montana portion). One defendant, who was accused of a wildlife-related crime in the Montana portion of the park, attempted to raise


title: "Yellowstone National Park" content: this argument but eventually pleaded guilty, with the plea deal including his specific agreement not to raise the issue in his appeal. Yellowstone National Park Yellowstone National Park is an American national park located in Wyoming, Montana, and Idaho. It was established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872. Yellowstone was the first national park in the U.S. and is also widely held to be the first national park in the world. The park is known for its wildlife and its many geothermal features, especially Old Faithful geyser, one of


title: Yerevan content: began in 1885 (with a reading of on 12 July 2018), as another sign of increasing hot extremes in a warming world. Among European capital cities Yerevan has highest difference between average summer (June–August) and winter (December–February) temperatures. The Yerevan TV Tower is the tallest structure in the city, and one of the tallest structures in the Transcaucasian region. The Republic Square, the Yerevan Opera Theatre, and the Yerevan Cascade are among the main landmarks at the centre of Yerevan, mainly developed based on the original design of the academician Alexander Tamanian, and the revised plan of architect Jim Torosyan.


title: Z content: Z Z (named "zed" or "zee" ) is the 26th and final letter of the modern English alphabet and the ISO basic Latin alphabet. In most English-speaking countries, including the United Kingdom, Canada, India, Ireland, New Zealand, Zambia and Australia, the letter's name is "zed" , reflecting its derivation from the Greek "zeta" (this dates to Latin, which borrowed X, Y, and Z from Greek, along with their names), but in American English its name is "zee" , analogous to the names for B, C, D, etc., and deriving from a late 17th-century English dialectal form. Another English dialectal form


title: Zebra content: Zebra Zebras ( , ) are several species of African equids (horse family) united by their distinctive black and white striped coats. Their stripes come in different patterns, unique to each individual. They are generally social animals that live in small harems to large herds. Unlike their closest relatives, horses and donkeys, zebras have never been truly domesticated. There are three species of zebras: the plains zebra, the mountain zebra and the Grévy's zebra. The plains zebra and the mountain zebra belong to the subgenus "Hippotigris", but Grévy's zebra is the sole species of subgenus "Dolichohippus". The latter resembles an


title: Zebra content: ass, to which zebras are closely related, while the former two look more horse-like. All three belong to the genus "Equus", along with other living equids. The unique stripes of zebras make them one of the animals most familiar to people. They occur in a variety of habitats, such as grasslands, savannas, woodlands, thorny scrublands, mountains, and coastal hills. However, various anthropogenic factors have had a severe impact on zebra populations, in particular hunting for skins and habitat destruction. Grévy's zebra and the mountain zebra are endangered. While plains zebras are much more plentiful, one subspecies, the quagga, became extinct


title: Zebra content: in the late 19th century – though there is currently a plan, called the Quagga Project, that aims to breed zebras that are phenotypically similar to the quagga in a process called breeding back. The name "zebra" in English dates back to c. 1600, from Italian "zebra", perhaps from Portuguese, which in turn is said to be Congolese (as stated in the "Oxford English Dictionary"). The "Encarta Dictionary" says its ultimate origin is uncertain, but perhaps it may come from Latin "equiferus" meaning "wild horse"; from "equus" ("horse") and "ferus" ("wild, untamed"). The word was traditionally pronounced with a long


title: Zebra content: initial vowel, but over the course of the 20th century, the pronunciation with the short initial vowel became the usual one in the UK and Commonwealth. The pronunciation with a long initial vowel remains standard in the United States. A group of zebras are referred to as a herd, dazzle, or zeal. Zebras evolved among the Old World horses within the last 4 million years. It has been suggested that zebras are paraphyletic and that striped equids evolved more than once. Extensive stripes are posited to have been of little use to equids that live in low densities in deserts


title: Zebra content: (like asses and some horses) or ones that live in colder climates with shaggy coats and annual shading (like some horses). However, molecular evidence supports zebras as a monophyletic lineage. The zebra has between 32 and 46 chromosomes, depending on the species. There are three extant species. Collectively, two of the species have eight subspecies (seven extant). Zebra populations are diverse, and the relationships between, and the taxonomic status of, several of the subspecies are not well known. The plains zebra ("Equus quagga", formerly "Equus burchelli") is the most common, and has or had about six subspecies distributed across much


title: Zebra content: of southern and eastern Africa. It, or particular subspecies of it, have also been known as the common zebra, the dauw, Burchell's zebra (actually the subspecies "Equus quagga burchellii"), Chapman's zebra, Wahlberg's zebra, Selous' zebra, Grant's zebra, Boehm's zebra and the quagga (another extinct subspecies, "Equus quagga quagga"). The mountain zebra ("Equus zebra") of southwest Africa tends to have a sleek coat with a white belly and narrower stripes than the plains zebra. It has two subspecies and is classified as vulnerable. Grévy's zebra ("Equus grevyi") is the largest type, with a long, narrow head, making it appear rather mule-like.


title: Zebra content: It is an inhabitant of the semi-arid grasslands of Ethiopia and northern Kenya. Grévy's zebra is the rarest species, and is classified as endangered. Although zebra species may have overlapping ranges, they do not interbreed. In captivity, plains zebras have been crossed with mountain zebras. The hybrid foals lacked a dewlap and resembled the plains zebra apart from their larger ears and their hindquarters pattern. Attempts to breed a Grévy's zebra stallion to mountain zebra mares resulted in a high rate of miscarriage. In captivity, crosses between zebras and other (non-zebra) equines have produced several distinct hybrids, including the zebroid,


title: Zebra content: zeedonk, zony, and zorse. In certain regions of Kenya, plains zebras and Grévy's zebra coexist, and fertile hybrids occur. The Hagerman horse ("Equus simplicidens") is sometimes referred to as the American zebra due to perceived similarities to the plains zebra, and sometimes depicted as striped. However, consensus appears to be that it wasn't particularly closely related to either "Hippotigiris" nor "Dolichohippus", nor is there unambiguous evidence that it had stripes. The common plains zebra is about at the shoulder with a body ranging from long with a tail. It can weigh up to , males being slightly bigger than females.


title: Zebra content: Grévy's zebra is considerably larger, while the mountain zebra is somewhat smaller. It was previously believed that zebras were white animals with black stripes, since some zebras have white underbellies. Embryological evidence, however, shows that the animal's background color is black and the white stripes and bellies are additions. It is likely that the stripes are caused by a combination of factors. The stripes are typically vertical on the head, neck, forequarters, and main body, with horizontal stripes at the rear and on the legs of the animal. A wide variety of hypotheses have been proposed to account for the


title: Zebra content: evolution of the striking stripes of zebras. The more traditional of these (1 and 2, below) relate to camouflage. Zebras have four gaits: walk, trot, canter and gallop. They are generally slower than horses, but their great stamina helps them outrun predators. When chased, a zebra will zig-zag from side to side, making it more difficult for the predator to attack. When cornered, the zebra will rear up and kick or bite its attacker. Zebras have excellent eyesight. Like most ungulates, the zebra's eyes are on the sides of its head, giving it a wide field of view. Zebras also


title: Zebra content: have night vision, although not as advanced as that of most of their predators. Zebras have excellent hearing and have larger, rounder ears than horses; like other ungulates, zebras can turn their ears in almost any direction. In addition to superb eyesight and hearing, zebras also have acute senses of smell and taste. Being an equid, zebras are subject to many of the same common infections and diseases of the domestic horse. Two Grévy's zebras were poisoned in 1995 by leaves from a hybrid red maple tree ("acer rubrum") at the St. Louis Zoo. Horses were first reported in 1981


title: Zebra content: to be susceptible and even a small amount of the leaves can be toxic to ponies. In 2000, a zebra was reported to be infected with a nematode, "halicephalobus", usually associated with decaying plant material. Like most members of the horse family, zebras are highly social. Their social structure, however, depends on the species. Mountain zebras and plains zebras live in groups, known as 'harems', consisting of one stallion with up to six mares and their foals. Bachelor males either live alone or with groups of other bachelors until they are old enough to challenge a breeding stallion. When attacked


title: Zebra content: by packs of hyenas or wild dogs a zebra group will huddle together with the foals in the middle while the stallion tries to ward them off. Unlike the other zebra species, Grévy's zebras do not have permanent social bonds. A group of these zebras rarely stays together for more than a few months. The foals stay with their mothers, while adult males live alone. Like the other two zebra species, bachelor male zebras will organize in groups. Like horses, zebras sleep standing up, and only sleep when neighbors are around to warn them of predators. Zebras communicate with each


title: Zebra content: other with high-pitched barks and whinnying. Grévy's zebras make mulelike brays. A zebra's ears signify its mood. When a zebra is in a calm, tense or friendly mood, its ears stand erect. When it is frightened, its ears are pushed forward. When angry, the ears are pulled backward. When surveying an area for predators, zebras will stand in an alert posture with ears erect, head held high, and staring. When tense, they will also snort. When a predator is spotted or sensed, a zebra will bark (or bray) loudly. Zebras feed almost entirely on grasses, but may occasionally eat shrubs,


title: Zebra content: herbs, twigs, leaves and bark. Their digestive systems allow them to subsist on diets of lower nutritional quality than that necessary for other herbivores. Female zebras mature earlier than the males, and a mare may have her first foal by the age of three. Males are not able to breed until the age of five or six. Mares may give birth to one foal every twelve months. She nurses the foal for up to a year. Like horses, zebras are able to stand, walk and suckle shortly after they are born. A zebra foal is brown and white instead of


title: Zebra content: black and white at birth. Plains and mountain zebra foals are protected by their mothers, as well as the head stallion and the other mares in their group. Grévy's zebra foals have only their mother as a regular protector, since, as noted above, Grévy's zebra groups often disband after a few months. Attempts have been made to train zebras for riding, since they have better resistance than horses to African diseases. While occasionally successful, most of these attempts failed due to the zebra's more unpredictable nature and tendency to panic under stress. For this reason, zebra-mules or zebroids (crosses between


title: Zebra content: any species of zebra and a horse, pony, donkey or ass) are preferred over purebred zebras. In England, the zoological collector Walter Rothschild frequently used zebras to draw a carriage. In 1907, Rosendo Ribeiro, the first doctor in Nairobi, Kenya, used a riding zebra for house calls. In the mid-19th century, Governor George Grey imported zebras to New Zealand from his previous posting in South Africa, and used them to pull his carriage on his privately owned Kawau Island. Captain Horace Hayes, in "Points of the Horse" (circa 1893), compared the usefulness of different zebra species. In 1891, Hayes broke


title: Zebra content: a mature, intact mountain zebra stallion to ride in two days' time, and the animal was quiet enough for his wife to ride and be photographed upon. He found the Burchell's zebra easy to break, and considered it ideal for domestication, as it was immune to the bite of the tsetse fly. He considered the quagga (now extinct) well-suited to domestication due to being easy to train to saddle and harness. Modern man has had great impact on the zebra population. Zebras were, and still are, hunted for their skins, and for meat. They also compete with livestock for forage


title: Zebra content: and are sometimes culled. The Cape mountain zebra was hunted to near extinction, with less than 100 individuals by the 1930s. The population has since increased to about 700 due to conservation efforts. Both mountain zebra subspecies are currently protected in national parks, but are still endangered. The Grévy's zebra is also endangered. Hunting and competition from livestock have greatly decreased their population. Because of the population's small size, environmental hazards, such as drought, are capable of affecting the entire species. Plains zebras are much more numerous and have a healthy population. Nevertheless, they too have been reduced by hunting


title: Zebra content: and loss of habitat to farming. One subspecies, the quagga, is now extinct. Zebras have been the subject of African folk tales which tell how they got their stripes. According to a San folk tale of Namibia, the zebra was once all white, but acquired its black stripes after a fight with a baboon over a waterhole. After kicking the baboon so hard, the zebra lost his balance and tripped over a fire, and the fire sticks left scorch marks all over his white coat. In the film "Fantasia", two centaurs are depicted being half human and half zebra, instead


title: Zebra content: of the typical half human and half horse. Zebras are a popular subject in art. The fourth Mughal emperor Jahangir (r.1605–24), commissioned a painting of the zebra, which was completed by Ustad Mansur. Zebra stripes are also a popular style for furniture, carpets and fashion. When depicted in movies and cartoons, zebras are most often miscellaneous characters, but have had some starring roles, notably in "Madagascar", "Racing Stripes" and "Khumba". One of the recurring characters in "" is a zebra named Zecora. Zebras also serve as mascots and symbols for products and corporations, notably Zebra Technologies and Fruit Stripe gum


title: Zebra content: as well as Investec. Zebras are featured on the coat of arms of Botswana. Some on-field officials in various sports, especially American football, basketball, and ice hockey, may be nicknamed "zebras" due to the resemblance of their uniforms (which feature alternating black and white stripes) to the animal's markings. Recent research has shown that TU-103, a strain of "Clostridium" bacteria found in zebra feces, can convert nearly any form of cellulose into butanol fuel. Zebra Zebras ( , ) are several species of African equids (horse family) united by their distinctive black and white striped coats. Their stripes come in


title: X86 content: 80387, abbreviated x87. This was also known as the NPX ("Numeric Processor eXtension"), an apt name since the coprocessors, while used mainly for floating-point calculations, also performed integer operations on both binary and decimal formats. With very few exceptions, the 80486 and subsequent x86 processors then integrated this x87 functionality on chip which made the x87 instructions a de facto integral part of the x86 instruction set. Each x87 register, known as ST(0) through ST(7), is 80 bits wide and stores numbers in the IEEE floating-point standard double extended precision format. These registers are organized as a stack with ST(0)


title: Zope content: To control what appears inside an element, use the tal:content attribute like this: Finally, to introduce or replace values of attributes use the tal:attributes attribute as below. You can use Python to alter the href at runtime. This is a very cursory explanation of Zope Page Templates. The behavior of Zope Page Templates is almost completely described by a template language, fixed on TAL, TALES, and METAL specifications: SchoolTool is an open source student information system that uses Zope. Zope Zope is a family of free and open-source web application servers written in Python, and their associated online community. Zope


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: Leonard P. Zakim Bunker Hill Memorial Bridge The Leonard P. Zakim () Bunker Hill Memorial Bridge (or Zakim Bridge) is a cable-stayed bridge across the Charles River in Boston, Massachusetts. It is a replacement for the Charlestown High Bridge, an older truss bridge constructed in the 1950s. Of ten lanes, using the harp-style system of nearly-parallel cable layout, coupled with the use of "cradles" through each pylon for the cables, the main portion of the Zakim Bridge carries four lanes each way (northbound and southbound) of the Interstate 93 and U.S. Route 1 concurrency between the Thomas P. "Tip" O'Neill


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: Jr. Tunnel and the elevated highway to the north. Two additional lanes are cantilevered outside the cables, which carry northbound traffic from the Sumner Tunnel and North End on-ramp. These lanes merge with the main highway north of the bridge. I-93 heads toward New Hampshire as the "Northern Expressway", and US 1 splits from the Interstate and travels northeast toward Massachusetts' North Shore communities, crossing the Mystic River via the Tobin Bridge. The bridge and connecting tunnel were built as part of the Big Dig, the largest highway construction project in the United States. The northbound lanes were finished in


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: March 2003, and the southbound lanes in December. The bridge's unique styling quickly became an icon for Boston, often featured in the backdrop of national news channels, to establish location, and included on tourist souvenirs. The bridge is commonly referred to as the "Zakim Bridge" or "Bunker Hill Bridge" by residents of nearby Charlestown. The Leverett Circle Connector Bridge was constructed in conjunction with the Zakim Bridge, allowing some traffic to bypass it. In a cable-stayed bridge, instead of hanging the roadbed from cables slung between towers, the cables run directly between the roadbed and the towers. Although cable-stayed bridges


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: have been common in Europe since World War II, they are relatively new to North America. The bridge concept was developed by Swiss civil engineer Christian Menn and its design was engineered by American civil engineer Ruchu Hsu with Parsons Brinckerhoff. Wallace Floyd Associates, sub-consultants to Bechtel/Parsons Brinckerhoff, was the lead architect/urban designer and facilitated community participation during the design process. The engineer of record is HNTB/FIGG. The lead designers were Theodore Zoli (from HNTB), Sajal Banerjee and W. Denney Pate (from FIGG). The bridge follows a new design in which, besides having its eight primary lanes running through the


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: towers, a pair of northbound lanes are cantilevered outside of the cable-stays. It has a striking, graceful appearance that is meant to echo the tower of the Bunker Hill Monument, which is within view of the bridge, and the white cables evoke imagery of the rigging of the USS "Constitution", docked nearby. The 1975-built MBTA Orange Line's Haymarket North Extension tunnel lies beneath the bridge. The bridge's full name commemorates Boston civic leader and civil rights activist Leonard P. Zakim who championed "building bridges between peoples", and the Battle of Bunker Hill. Originally, Massachusetts Governor A. Paul Cellucci sought to


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: name it the "Freedom Bridge". In 2000, however, local clergy and religious leaders, including Cardinal Bernard Francis Law, requested the Zakim name shortly after Zakim's death from myeloma. Although Cellucci agreed to the naming, community leaders from Charlestown objected to the name as they felt that since the design reflected the nearby Bunker Hill memorial, it should be named the "Bunker Hill Freedom bridge". Allegations of antisemitism were leveled against members of the mostly white, Irish Catholic community as reasons for resistance to the Zakim name, based on some comments quoted in the "Boston Globe". Several local neo-Nazis also complained


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: about the honor for Zakim and launched an unsuccessful petition drive to drop his name from the Bunker Hill one (the petition needed 100 signatures to be reviewed by the Massachusetts State Legislature and only 20 people signed it). In response, several community leaders spoke out against the allegations in a press conference, stating that the claims, made by Professor Jonathan Sarna, were his alone and did not reflect the community's historical (not racial) basis of favoring the "Bunker Hill" name, though they dodged questions about the false claim that no Jews had fought in the battle of Bunker Hill.


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: Eventually a compromise between the Boston City Council, the Massachusetts State Legislature and community activists brought about the current name. As with the Hoover Dam, different communities call the bridge by different colloquial names. Many people in the Charlestown area refer to it as the "Bunker Hill Bridge", while most, including the local press and traffic monitoring services, refer to it as the "Zakim Bridge". Placement of footings for the Zakim Bridge required environmental permits to relocate areas of open water surface, changing the contour of the Charles River shoreline. The process of landscape design and environmental mitigation under the


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: bridge deck and around the bridge supports allowed for the creation of a new and accessible public landscape designed by Carol R. Johnson Associates. This under bridge landscape contains a series of perforated stainless steel lighting-based public artworks, entitled, "Five Beacons for the Lost Half Mile." Pedestrians and cyclists are able to travel from Charlestown toward Cambridge over the adjacent North Bank Pedestrian Bridge to North Point Park (Cambridge, Massachusetts). This bridge is a link in the Charles River Bike Path. The bridge was dedicated on October 4, 2002, in a ceremony held on the new span. The dedication speakers


title: "Leonard P. Zakim Bunker Hill Memorial Bridge" content: included members of Zakim's family, government officials, and a performance of the song Thunder Road by Bruce Springsteen. Introducing the song, Springsteen said about Zakim, "... I knew him a little bit during the last year of his life, he was one of those people whose, intensity, inner spirit you could feel even when he was very ill and uh. ... I guess, you know, we honor his memory obviously not with this beautiful bridge, very lovely, but by continuing on in his fight for social justice." Leonard P. Zakim Bunker Hill Memorial Bridge The Leonard P. Zakim () Bunker


title: 1929 content: memories many harbored of the horrors of World War I. A major seller was "All Quiet on the Western Front" by Erich Maria Remarque. Remarque was a German who had fought in the war at age eighteen and been wounded in the Third Battle of Ypres. He stated that he intended the book to tell the story "of a generation of men who, even though they may have escaped its shells, were destroyed by the war." Another 1929 book reflecting on World War I was Ernest Hemingway's "A Farewell to Arms", as well as "Good-Bye to All That" by Robert


title: "Thirteen Colonies" content: continued to be more ethnically diverse than the other colonies. Numerous settlers immigrated from Ireland, both Catholic and Protestant—particularly "New Light" Ulster Presbyterians. Protestant Germans also migrated in large numbers, particularly to Pennsylvania. In the 1740s, the Thirteen Colonies underwent the First Great Awakening. In 1738, an incident involving a Welsh mariner named Robert Jenkins sparked the War of Jenkins' Ear between Britain and Spain. Hundreds of North Americans volunteered for Admiral Edward Vernon's assault on Cartegena de Indias, a Spanish city in South America. The war against Spain merged into a broader conflict known as the War of the


title: Glycerius content: the invasion of both the Visigoths and the Ostrogoths were repelled, through a mixture of diplomatic and military acts. In 473, the Visigoth King Euric ordered an invasion of Italy, but his commander, Vincentius, was killed by the armies of the "comites" Alla and Sindila. After Vincentius was killed, Euric chose instead to invade Gaul, occupying both Arles and Marseilles. The Ostrogoth King Videmir proposed to invade Italy, but Glycerius was able to dissuade him through gifts, and divertied them from Italy to Gaul, where they were later attacked by surrounding groups. These actions to defend Rome may be the


title: Heart content: Heart The heart is a muscular organ in most animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, as well as assists in the removal of metabolic wastes. In humans, the heart is located between the lungs, in the middle compartment of the chest. In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the "right heart" and their left counterparts as the "left heart".


title: Heart content: Fish, in contrast, have two chambers, an atrium and a ventricle, while reptiles have three chambers. In a healthy heart blood flows one way through the heart due to heart valves, which prevent backflow. The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium. The heart pumps blood with a rhythm determined by a group of pacemaking cells in the sinoatrial node. These generate a current that causes contraction of the heart, traveling through the atrioventricular node


title: Heart content: and along the conduction system of the heart. The heart receives blood low in oxygen from the systemic circulation, which enters the right atrium from the superior and inferior venae cavae and passes to the right ventricle. From here it is pumped into the pulmonary circulation, through the lungs where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta to the systemic circulation−where the oxygen is used and metabolized to carbon dioxide. The heart beats at a resting rate close to


title: Heart content: 72 beats per minute. Exercise temporarily increases the rate, but lowers resting heart rate in the long term, and is good for heart health. Cardiovascular diseases (CVD) are the most common cause of death globally as of 2008, accounting for 30% of deaths. Of these more than three quarters are a result of coronary artery disease and stroke. Risk factors include: smoking, being overweight, little exercise, high cholesterol, high blood pressure, and poorly controlled diabetes, among others. Cardiovascular diseases frequently do not have symptoms or may cause chest pain or shortness of breath. Diagnosis of heart disease is often done


title: Heart content: by the taking of a medical history, listening to the heart-sounds with a stethoscope, ECG, and ultrasound. Specialists who focus on diseases of the heart are called cardiologists, although many specialties of medicine may be involved in treatment. The human heart is situated in the middle mediastinum, at the level of thoracic vertebrae T5-T8. A double-membraned sac called the pericardium surrounds the heart and attaches to the mediastinum. The back surface of the heart lies near the vertebral column, and the front surface sits behind the sternum and rib cartilages. The upper part of the heart is the attachment point


title: Heart content: for several large blood vessels – the venae cavae, aorta and pulmonary trunk. The upper part of the heart is located at the level of the third costal cartilage. The lower tip of the heart, the apex, lies to the left of the sternum (8 to 9 cm from the midsternal line) between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The largest part of the heart is usually slightly offset to the left side of the chest (though occasionally it may be offset to the right) and is felt to be on


title: Heart content: the left because the left heart is stronger and larger, since it pumps to all body parts. Because the heart is between the lungs, the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart. The heart is cone-shaped, with its base positioned upwards and tapering down to the apex. An adult heart has a mass of 250–350 grams (9–12 oz). The heart is often described as the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness,


title: Heart content: although this description is disputed, as the heart is likely to be slightly larger. Well-trained athletes can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle. The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum. This distinction is visible also on the surface of the heart as the coronary sulcus. There is an ear-shaped structure in the upper right atrium called the right atrial


title: Heart content: appendage, or auricle, and another in the upper left atrium, the left atrial appendage. The right atrium and the right ventricle together are sometimes referred to as the "right heart". Similarly, the left atrium and the left ventricle together are sometimes referred to as the "left heart". The ventricles are separated from each other by the interventricular septum, visible on the surface of the heart as the anterior longitudinal sulcus and the posterior interventricular sulcus. The cardiac skeleton is made of dense connective tissue and this gives structure to the heart. It forms the atrioventricular septum which separates the atria


title: Heart content: from the ventricles, and the fibrous rings which serve as bases for the four heart valves. The cardiac skeleton also provides an important boundary in the heart's electrical conduction system since collagen cannot conduct electricity. The interatrial septum separates the atria and the interventricular septum separates the ventricles. The interventricular septum is much thicker than the interatrial septum, since the ventricles need to generate greater pressure when they contract. The heart has four valves, which separate its chambers. One valve lies between each atrium and ventricle, and one valve rests at the exit of each ventricle. The valves between the


title: Heart content: atria and ventricles are called the atrioventricular valves. Between the right atrium and the right ventricle is the tricuspid valve. The tricuspid valve has three cusps, which connect to chordae tendinae and three papillary muscles named the anterior, posterior, and septal muscles, after their relative positions. The mitral valve lies between the left atrium and left ventricle. It is also known as the bicuspid valve due to its having two cusps, an anterior and a posterior cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall. The papillary muscles extend from the


title: Heart content: walls of the heart to valves by cartilaginous connections called chordae tendinae. These muscles prevent the valves from falling too far back when they close. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. As the heart chambers contract, so do the papillary muscles. This creates tension on the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria. Two additional semilunar valves sit at the exit of each of the ventricles. The


title: Heart content: pulmonary valve is located at the base of the pulmonary artery. This has three cusps which are not attached to any papillary muscles. When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket-like valve, pressing against the cusps which close to seal the valve. The semilunar aortic valve is at the base of the aorta and also is not attached to papillary muscles. This too has three cusps which close with the pressure of the blood flowing back from the aorta. The right heart consists of two chambers, the


title: Heart content: right atrium and the right ventricle, separated by a valve, the tricuspid valve. The right atrium receives blood almost continuously from the body's two major veins, the superior and inferior venae cavae. A small amount of blood from the coronary circulation also drains into the right atrium via the coronary sinus, which is immediately above and to the middle of the opening of the inferior vena cava. In the wall of the right atrium is an oval-shaped depression known as the fossa ovalis, which is a remnant of an opening in the fetal heart known as the foramen ovale. Most


title: Heart content: of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface has prominent ridges of pectinate muscles, which are also present in the right atrial appendage. The right atrium is connected to the right ventricle by the tricuspid valve. The walls of the right ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band reinforces the thin walls of the right ventricle and plays a crucial


title: Heart content: role in cardiac conduction. It arises from the lower part of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle. The right ventricle tapers into the pulmonary trunk, into which it ejects blood when contracting. The pulmonary trunk branches into the left and right pulmonary arteries that carry the blood to each lung. The pulmonary valve lies between the right heart and the pulmonary trunk. The left heart has two chambers: the left atrium, and the left ventricle, separated by the mitral valve. The left atrium receives oxygenated blood back


title: Heart content: from the lungs via one of the four pulmonary veins. The left atrium has an outpouching called the left atrial appendage. Like the right atrium, the left atrium is lined by pectinate muscles. The left atrium is connected to the left ventricle by the mitral valve. The left ventricle is much thicker as compared with the right, due to the greater force needed to pump blood to the entire body. Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The left ventricle pumps blood to the body through the aortic valve and into


title: Heart content: the aorta. Two small openings above the aortic valve carry blood to the heart itself, the left main coronary artery and the right coronary artery. The heart wall is made up of three layers: the inner endocardium, middle myocardium and outer epicardium. These are surrounded by a double-membraned sac called the pericardium. The innermost layer of the heart is called the endocardium. It is made up of a lining of simple squamous epithelium, and covers heart chambers and valves. It is continuous with the endothelium of the veins and arteries of the heart, and is joined to the myocardium with


title: Heart content: a thin layer of connective tissue. The endocardium, by secreting endothelins, may also play a role in regulating the contraction of the myocardium. The middle layer of the heart wall is the myocardium, which is the cardiac muscle – a layer of involuntary striated muscle tissue surrounded by a framework of collagen. The cardiac muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart, with the outer muscles forming a figure 8 pattern around the atria and around the bases of the great vessels and the inner muscles forming a figure


title: Heart content: 8 around the two ventricles and proceeding toward the apex. This complex swirling pattern allows the heart to pump blood more effectively. There are two types of cells in cardiac muscle: muscle cells which have the ability to contract easily, and pacemaker cells of the conducting system. The muscle cells make up the bulk (99%) of cells in the atria and ventricles. These contractile cells are connected by intercalated discs which allow a rapid response to impulses of action potential from the pacemaker cells. The intercalated discs allow the cells to act as a syncytium and enable the contractions that


title: Heart content: pump blood through the heart and into the major arteries. The pacemaker cells make up 1% of cells and form the conduction system of the heart. They are generally much smaller than the contractile cells and have few myofibrils which gives them limited contractibility. Their function is similar in many respects to neurons. Cardiac muscle tissue has autorhythmicity, the unique ability to initiate a cardiac action potential at a fixed rate – spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart. There are specific proteins expressed in cardiac muscle cells. These are mostly


title: Heart content: associated with muscle contraction, and bind with actin, myosin, tropomyosin, and troponin. They include MYH6, ACTC1, TNNI3, CDH2 and PKP2. Other proteins expressed are MYH7 and LDB3 that are also expressed in skeletal muscle. The pericardium is the sack that surrounds the heart. The tough outer surface of the pericardium is called the fibrous membrane. This is lined by a double inner membrane called the serous membrane that produces pericardial fluid to lubricate the surface of the heart. The part of the serous membrane attached to the fibrous membrane is called the parietal pericardium, while the part of the serous


title: Heart content: membrane attached to the heart is known as the visceral pericardium. The pericardium is present in order to lubricate its movement against other structures within the chest, to keep the heart's position stabilised within the chest, and to protect the heart from infection. Heart tissue, like all cells in the body, needs to be supplied with oxygen, nutrients and a way of removing metabolic wastes. This is achieved by the coronary circulation, which includes arteries, veins, and lymphatic vessels. Blood flow through the coronary vessels occurs in peaks and troughs relating to the heart muscle's relaxation or contraction. Heart tissue


title: Heart content: receives blood from two arteries which arise just above the aortic valve. These are the left main coronary artery and the right coronary artery. The left main coronary artery splits shortly after leaving the aorta into two vessels, the left anterior descending and the left circumflex artery. The left anterior descending artery supplies heart tissue and the front, outer side, and the septum of the left ventricle. It does this by branching into smaller arteries – diagonal and septal branches. The left circumflex supplies the back and underneath of the left ventricle. The right coronary artery supplies the right atrium,


title: Heart content: right ventricle, and lower posterior sections of the left ventricle. The right coronary artery also supplies blood to the atrioventricular node (in about 90% of people) and the sinoatrial node (in about 60% of people). The right coronary artery runs in a groove at the back of the heart and the left anterior descending artery runs in a groove at the front. There is significant variation between people in the anatomy of the arteries that supply the heart The arteries divide at their furtherst reaches into smaller branches that join together at the edges of each arterial distribution. The coronary


title: Heart content: sinus is a large vein that drains into the right atrium, and receives most of the venous drainage of the heart. It receives blood from the great cardiac vein (receiving the left atrium and both ventricles), the posterior cardiac vein (draining the back of the left ventricle), the middle cardiac vein (draining the bottom of the left and right ventricles), and small cardiac veins. The anterior cardiac veins drain the front of the right ventricle and drain directly into the right atrium. Small lymphatic networks called plexuses exist beneath each of the three layers of the heart. These networks collect


title: Heart content: into a main left and a main right trunk, which travel up the groove between the ventricles that exists on the heart's surface, receiving smaller vessels as they travel up. These vessels then travel into the atrioventricular groove, and receive a third vessel which drains the section of the left ventricle sitting on the diaphragm. The left vessel joins with this third vessel, and travels along the pulmonary artery and left atrium, ending in the inferior tracheobronchial node. The right vessel travels along the right atrium and the part of the right ventricle sitting on the diaphragm. It usually then


title: Heart content: travels in front of the ascending aorta and then ends in a brachiocephalic node. The heart receives nerve signals from the vagus nerve and from nerves arising from the sympathetic trunk. These nerves act to influence, but not control, the heart rate. Sympathetic nerves also influence the force of heart contraction. Signals that travel along these nerves arise from two paired cardiovascular centres in the medulla oblongata. The vagus nerve of the parasympathetic nervous system acts to decrease the heart rate, and nerves from the sympathetic trunk act to increase the heart rate. These nerves form a network of nerves


title: Heart content: that lies over the heart called the cardiac plexus. The vagus nerve is a long, wandering nerve that emerges from the brainstem and provides parasympathetic stimulation to a large number of organs in the thorax and abdomen, including the heart. The nerves from the sympathetic trunk emerge through the T1-T4 thoracic ganglia and travel to both the sinoatrial and atrioventricular nodes, as well as to the atria and ventricles. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter norepinephrine (also known as noradrenaline) at the neuromuscular junction of the


title: Heart content: cardiac nerves. This shortens the repolarization period, thus speeding the rate of depolarization and contraction, which results in an increased heart rate. It opens chemical or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions. Norepinephrine binds to the beta–1 receptor. The heart is the first functional organ to develop and starts to beat and pump blood at about three weeks into embryogenesis. This early start is crucial for subsequent embryonic and prenatal development. The heart derives from splanchnopleuric mesenchyme in the neural plate which forms the cardiogenic region. Two endocardial tubes form here that fuse


title: Heart content: to form a primitive heart tube known as the tubular heart. Between the third and fourth week, the heart tube lengthens, and begins to fold to form an S-shape within the pericardium. This places the chambers and major vessels into the correct alignment for the developed heart. Further development will include the septa and valves formation and remodelling of the heart chambers. By the end of the fifth week the septa are complete and the heart valves are completed by the ninth week. Before the fifth week, there is an opening in the fetal heart known as the foramen ovale.


title: Heart content: The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the lungs. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern. A depression in the surface of the right atrium remains where the foramen ovale was, called the fossa ovalis. The embryonic heart begins beating at around 22 days after conception (5 weeks after the last normal menstrual period, LMP). It starts to beat


title: Heart content: at a rate near to the mother's which is about 75–80 beats per minute (bpm). The embryonic heart rate then accelerates and reaches a peak rate of 165–185 bpm early in the early 7th week (early 9th week after the LMP). After 9 weeks (start of the fetal stage) it starts to decelerate, slowing to around 145 (±25) bpm at birth. There is no difference in female and male heart rates before birth. The heart functions as a pump in the circulatory system to provide a continuous flow of blood throughout the body. This circulation consists of the systemic circulation


title: Heart content: to and from the body and the pulmonary circulation to and from the lungs. Blood in the pulmonary circulation exchanges carbon dioxide for oxygen in the lungs through the process of respiration. The systemic circulation then transports oxygen to the body and returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the lungs. The right heart collects deoxygenated blood from two large veins, the superior and inferior venae cavae. Blood collects in the right and left atrium continuously. The superior vena cava drains blood from above the diaphragm and empties into the upper back part of


title: Heart content: the right atrium. The inferior vena cava drains the blood from below the diaphragm and empties into the back part of the atrium below the opening for the superior vena cava. Immediately above and to the middle of the opening of the inferior vena cava is the opening of the thin-walled coronary sinus. Additionally, the coronary sinus returns deoxygenated blood from the myocardium to the right atrium. The blood collects in the right atrium. When the right atrium contracts, the blood is pumped through the tricuspid valve into the right ventricle. As the right ventricle contracts, the tricuspid valve closes


title: Heart content: and the blood is pumped into the pulmonary trunk through the pulmonary valve. The pulmonary trunk divides into pulmonary arteries and progressively smaller arteries throughout the lungs, until it reaches capillaries. As these pass by alveoli carbon dioxide is exchanged for oxygen. This happens through the passive process of diffusion. In the left heart, oxygenated blood is returned to the left atrium via the pulmonary veins. It is then pumped into the left ventricle through the mitral valve and into the aorta through the aortic valve for systemic circulation. The aorta is a large artery that branches into many smaller


title: Heart content: arteries, arterioles, and ultimately capillaries. In the capillaries, oxygen and nutrients from blood are supplied to body cells for metabolism, and exchanged for carbon dioxide and waste products. Capillary blood, now deoxygenated, travels into venules and veins that ultimately collect in the superior and inferior vena cavae, and into the right heart. The cardiac cycle refers to the sequence of events in which the heart contracts and relaxes with every heartbeat. The period of time during which the ventricles contract, forcing blood out into the aorta and main pulmonary artery, is known as systole, while the period during which the


title: Heart content: ventricles relax and refill with blood is known as diastole. The atria and ventricles work in concert, so in systole when the ventricles are contracting, the atria are relaxed and collecting blood. When the ventricles are relaxed in diastole, the atria contract to pump blood to the ventricles. This coordination ensures blood is pumped efficiently to the body. At the beginning of the cardiac cycle, the ventricles are relaxing. As they do so, they are filled by blood passing through the open mitral and tricuspid valves. After the ventricles have completed most of their filling, the atria contract, forcing further


title: Heart content: blood into the ventricles and priming the pump. Next, the ventricles start to contract. As the pressure rises within the cavities of the ventricles, the mitral and tricuspid valves are forced shut. As the pressure within the ventricles rises further, exceeding the pressure with the aorta and pulmonary arteries, the aortic and pulmonary valves open. Blood is ejected from the heart, causing the pressure within the ventricles to fall. Simultaneously, the atria refill as blood flows into the right atrium through the superior and inferior vena cavae, and into the left atrium through the pulmonary veins. Finally, when the pressure


title: Heart content: within the ventricles falls below the pressure within the aorta and pulmonary arteries, the aortic and pulmonary valves close. The ventricles start to relax, the mitral and tricuspid valves open, and the cycle begins again. Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume) in one minute. This is calculated by multiplying the stroke volume (SV) by the beats per minute of the heart rate (HR). So that: CO = SV x HR. The cardiac output is normalized to body size through body surface area and is called the cardiac index. The


title: Heart content: average cardiac output, using an average stroke volume of about 70mL, is 5.25 L/min, with a normal range of 4.0–8.0 L/min. The stroke volume is normally measured using an echocardiogram and can be influenced by the size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload and afterload. Preload refers to the filling pressure of the atria at the end of diastole, when they are at their fullest. A main factor is how long it takes the ventricles to fill: if the ventricles contract more frequently, then there is less time to fill


title: Heart content: and the preload will be less. Preload can also be affected by a person's blood volume. The force of each contraction of the heart muscle is proportional to the preload, described as the Frank-Starling mechanism. This states that the force of contraction is directly proportional to the initial length of muscle fiber, meaning a ventricle will contract more forcefully, the more it is stretched. Afterload, or how much pressure the heart must generate to eject blood at systole, is influenced by vascular resistance. It can be influenced by narrowing of the heart valves (stenosis) or contraction or relaxation of the


title: Heart content: peripheral blood vessels. The strength of heart muscle contractions controls the stroke volume. This can be influenced positively or negatively by agents termed inotropes. These agents can be a result of changes within the body, or be given as drugs as part of treatment for a medical disorder, or as a form of life support, particularly in intensive care units. Inotropes that increase the force of contraction are "positive" inotropes, and include sympathetic agents such as adrenaline, noradrenaline and dopamine. "Negative" inotropes decrease the force of contraction and include calcium channel blockers. The normal rhythmical heart beat, called sinus rhythm,


title: Heart content: is established by the sinoatrial node, the heart's pacemaker. Here an electrical signal is created that travels through the heart, causing the heart muscle to contract. The sinoatrial node is found in the upper part of the right atrium near to the junction with the superior vena cava. The electrical signal generated by the sinoatrial node travels through the right atrium in a radial way that is not completely understood. It travels to the left atrium via Bachmann's bundle, such that the muscles of the left and right atria contract together. The signal then travels to the atrioventricular node. This


title: Heart content: is found at the bottom of the right atrium in the atrioventricular septum—the boundary between the right atrium and the left ventricle. The septum is part of the cardiac skeleton, tissue within the heart that the electrical signal cannot pass through, which forces the signal to pass through the atrioventricular node only. The signal then travels along the bundle of His to left and right bundle branches through to the ventricles of the heart. In the ventricles the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the heart muscle. The normal


title: Heart content: resting heart rate is called the sinus rhythm, created and sustained by the sinoatrial node, a group of pacemaking cells found in the wall of the right atrium. Cells in the sinoatrial node do this by creating an action potential. The cardiac action potential is created by the movement of specific electrolytes into and out of the pacemaker cells. The action potential then spreads to nearby cells. When the sinoatrial cells are resting, they have a negative charge on their membranes. However a rapid influx of sodium ions causes the membrane's charge to become positive. This is called depolarisation and


title: Heart content: occurs spontaneously. Once the cell has a sufficiently high charge, the sodium channels close and calcium ions then begin to enter the cell, shortly after which potassium begins to leave it. All the ions travel through ion channels in the membrane of the sinoatrial cells. The potassium and calcium start to move out of and into the cell only once it has a sufficiently high charge, and so are called voltage-gated. Shortly after this, the calcium channels close and potassium channels open, allowing potassium to leave the cell. This causes the cell to have a negative resting charge and is


title: Heart content: called repolarization. When the membrane potential reaches approximately −60 mV, the potassium channels close and the process may begin again. The ions move from areas where they are concentrated to where they are not. For this reason sodium moves into the cell from outside, and potassium moves from within the cell to outside the cell. Calcium also plays a critical role. Their influx through slow channels means that the sinoatrial cells have a prolonged "plateau" phase when they have a positive charge. A part of this is called the absolute refractory period. Calcium ions also combine with the regulatory protein


title: Heart content: troponin C in the troponin complex to enable contraction of the cardiac muscle, and separate from the protein to allow relaxation. The adult resting heart rate ranges from 60 to 100 bpm. The resting heart rate of a newborn can be 129 beats per minute (bpm) and this gradually decreases until maturity. An athlete's heart rate can be lower than 60 bpm. During exercise the rate can be 150 bpm with maximum rates reaching from 200 to 220 bpm. The normal sinus rhythm of the heart, giving the resting heart rate, is influenced a number of factors. The cardiovascular centres


title: Heart content: in the brainstem that control the sympathetic and parasympathetic influences to the heart through the vagus nerve and sympathetic trunk. These cardiovascular centres receive input from a series of receptors including baroreceptors, sensing stretch the stretching of blood vessels and chemoreceptors, sensing the amount of oxygen and carbon dioxide in the blood and its pH. Through a series of reflexes these help regulate and sustain blood flow. Baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Baroreceptors fire at a rate


title: Heart content: determined by how much they are stretched, which is influenced by blood pressure, level of physical activity, and the relative distribution of blood. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation . As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation. There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where


title: Heart content: specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase heart rate. The opposite is also true. Chemoreceptors present in the carotid body or adjacent to the aorta in an aortic body respond to the blood's oxygen, carbon dioxide levels. Low oxygen or high carbon dioxide will stimulate firing of the receptors. Exercise and fitness levels, age, body temperature, basal metabolic rate, and even a person's emotional state can all affect the


title: Heart content: heart rate. High levels of the hormones epinephrine, norepinephrine, and thyroid hormones can increase the heart rate. The levels of electrolytes including calcium, potassium, and sodium can also influence the speed and regularity of the heart rate; low blood oxygen, low blood pressure and dehydration may increase it. Cardiovascular diseases, which include diseases of the heart, are the leading cause of death worldwide. The majority of cardiovascular disease is noncommunicable and related to lifestyle and other factors, becoming more prevalent with ageing. Heart disease is a major cause of death, accounting for an average of 30% of all deaths in


title: Heart content: 2008, globally. This rate varies from a lower 28% to a high 40% in high-income countries. Doctors that specialise in the heart are called cardiologists. Many other medical professionals are involved in treating diseases of the heart, including doctors such as general practitioners, cardiothoracic surgeons and intensivists, and allied health practitioners including physiotherapists and dieticians. Coronary artery disease, also known as ischaemic heart disease, is caused by atherosclerosis – a build-up of fatty material along the inner walls of the arteries. These fatty deposits known as atherosclerotic plaques narrow the coronary arteries, and if severe may reduce blood flow to


title: Heart content: the heart. If a narrowing (or stenosis) is relatively minor then the patient may not experience any symptoms. Severe narrowings may cause chest pain (angina) or breathlessness during exercise or even at rest. The thin covering of an atherosclerotic plaque can rupture, exposing the fatty centre to the circulating blood. In this case a clot or thrombus can form, blocking the artery, and restricting blood flow to an area of heart muscle causing a myocardial infarction (a heart attack) or unstable angina. In the worst case this may cause cardiac arrest, a sudden and utter loss of output from the


title: Heart content: heart. Obesity, high blood pressure, uncontrolled diabetes, smoking and high cholesterol can all increase the risk of developing atherosclerosis and coronary artery disease. Heart failure is defined as a condition in which the heart is unable to pump enough blood to meet the demands of the body. Patients with heart failure may experience breathlessness especially when lying flat, as well as ankle swelling, known as peripheral oedema. Heart failure is the end result of many diseases affecting the heart, but is most commonly associated with ischaemic heart disease, valvular heart disease, or high blood pressure. Less common causes include various


title: Heart content: cardiomyopathies. Heart failure is frequently associated with weakness of the heart muscle in the ventricles (systolic heart failure), but can also be seen in patients with heart muscle that is strong but stiff (diastolic heart failure). The condition may affect the left ventricle (causing predominantly breathlessness), the right ventricle (causing predominantly swelling of the legs and an elevated jugular venous pressure), or both ventricles. Patients with heart failure are at higher risk of developing dangerous heart rhythm disturbances or arrhythmias. Cardiomyopathies are diseases affecting the muscle of the heart. Some cause abnormal thickening of the heart muscle (hypertrophic cardiomyopathy), some


title: Heart content: cause the heart to abnormally expand and weaken (dilated cardiomyopathy), some cause the heart muscle to become stiff and unable to fully relax between contractions (restrictive cardiomyopathy) and some make the heart prone to abnormal heart rhythms (arrhythmogenic cardiomyopathy). These conditions are often genetic and can be inherited, but some such as dilated cardiomyopathy may be caused by damage from toxins such as alcohol. Some cardiomyopathies such as hypertrophic cardiomopathy are linked to a higher risk of sudden cardiac death, particularly in athletes. Many cardiomyopathies can lead to heart failure in the later stages of the disease. Healthy heart valves


title: Heart content: allow blood to flow easily in one direction, but prevent it from flowing in the other direction. Diseased heart valves may have a narrow opening and therefore restrict the flow of blood in the forward direction (referred to as a stenotic valve), or may allow blood to leak in the reverse direction (referred to as valvular regurgitation). Valvular heart disease may cause breathlessness, blackouts, or chest pain, but may be asymptomatic and only detected on a routine examination by hearing abnormal heart sounds or a heart murmur. In the developed world, valvular heart disease is most commonly caused by degeneration


title: Heart content: secondary to old age, but may also be caused by infection of the heart valves (endocarditis). In some parts of the world rheumatic heart disease is a major cause of valvular heart disease, typically leading to mitral or aortic stenosis and caused by the body's immune system reacting to a streptococcal throat infection. While in the healthy heart, waves of electrical impulses originate in the sinus node before spreading to the rest of the atria, the atrioventricular node, and finally the ventricles (referred to as a normal sinus rhythm), this normal rhythm can be disrupted. Abnormal heart rhythms or arrhythmias


title: Heart content: may be asymptomatic or may cause palpitations, blackouts, or breathlessness. Some types of arrhythmia such as atrial fibrillation increase the long term risk of stroke. Some arrhythmias cause the heart to beat abnormally slowly, referred to as a bradycardia or bradyarrhythmia. This may be caused by an abnormally slow sinus node or damage within the cardiac conduction system (heart block). In other arrhythmias the heart may beat abnormally rapidly, referred to as a tachycardia or tachyarrhythmia. These arrhythmias can take many forms and can originate from different structures within the heart – some arise from the atria (e.g. atrial flutter),


title: Heart content: some from the atrioventricular node (e.g. AV nodal re-entrant tachycardia) whilst others arise from the ventricles (e.g. ventricular tachycardia). Some tachyarrhythmias are caused by scarring within the heart (e.g. some forms of ventricular tachycardia), others by an irritable focus (e.g. focal atrial tachycardia), while others are caused by additional abnormal conduction tissue that has been present since birth (e.g. Wolff-Parkinson-White syndrome). The most dangerous form of heart racing is ventricular fibrillation, in which the ventricles quiver rather than contract, and which if untreated is rapidly fatal. The sack which surrounds the heart, called the pericardium, can become inflamed in a


title: Heart content: condition known as pericarditis. This condition typically causes chest pain that may spread to the back, and is often caused by a viral infection (glandular fever, cytomegalovirus, or coxsackievirus). Fluid can build up within the pericardial sack, referred to as a pericardial effusion. Pericardial effusions often occur secondary to pericarditis, kidney failure, or tumours, and frequently do not cause any symptoms. However, large effusions or effusions which accumulate rapidly can compress the heart in a condition known as cardiac tamponade, causing breathlessness and potentially fatal low blood pressure. Fluid can be removed from the pericardial space for diagnosis or to


title: Heart content: relieve tamponade using a syringe in a procedure called pericardiocentesis. Some people are born with hearts that are abnormal and these abnormalities are known as congenital heart defects. They may range from the relatively minor (e.g. patent foramen ovale, arguably a variant of normal) to serious life-threatening abnormalities (e.g. hypoplastic left heart syndrome). Common abnormalities include those that affect the heart muscle that separates the two side of the heart (a 'hole in the heart' e.g. ventricular septal defect). Other defects include those affecting the heart valves (e.g. congenital aortic stenosis), or the main blood vessels that lead from the


title: Heart content: heart (e.g. coarctation of the aorta). More complex syndromes are seen that affect more than one part of the heart (e.g. Tetralogy of Fallot). Some congenital heart defects allow blood that is low in oxygen that would normally be returned to the lungs to instead be pumped back to the rest of the body. These are known as cyanotic congenital heart defects and are often more serious. Major congenital heart defects are often picked up in childhood, shortly after birth, or even before a child is born (e.g. transposition of the great arteries), causing breathlessness and a lower rate of


title: Heart content: growth. More minor forms of congenital heart disease may remain undetected for many years and only reveal themselves in adult life (e.g. atrial septal defect). Heart disease is diagnosed by the taking of a medical history, a cardiac examination, and further investigations, including blood tests, echocardiograms, ECGs and imaging. Other invasive procedures such as cardiac catheterisation can also play a role. The cardiac examination includes inspection, feeling the chest with the hands (palpation) and listening with a stethoscope (auscultation). It involves assessment of signs that may be visible on a person's hands (such as splinter haemorrhages), joints and other areas.


title: Heart content: A person's pulse is taken, usually at the radial artery near the wrist, in order to assess for the rhythm and strength of the pulse. The blood pressure is taken, using either a manual or automatic sphygmomanometer or using a more invasive measurement from within the artery. Any elevation of the jugular venous pulse is noted. A person's chest is felt for any transmitted vibrations from the heart, and then listened to with a stethoscope. Typically, healthy hearts have only two audible heart sounds, called S1 and S2. The first heart sound S1, is the sound created by the closing


title: Heart content: of the atrioventricular valves during ventricular contraction and is normally described as "lub". The second heart sound, S2, is the sound of the semilunar valves closing during ventricular diastole and is described as "dub". Each sound consists of two components, reflecting the slight difference in time as the two valves close. S2 may split into two distinct sounds, either as a result of inspiration or different valvular or cardiac problems. Additional heart sounds may also be present and these give rise to gallop rhythms. A third heart sound, S3 usually indicates an increase in ventricular blood volume. A fourth heart


title: Heart content: sound S4 is referred to as an atrial gallop and is produced by the sound of blood being forced into a stiff ventricle. The combined presence of S3 and S4 give a quadruple gallop. Heart murmurs are abnormal heart sounds which can be either related to disease or benign, and there are several kinds. There are normally two heart sounds, and abnormal heart sounds can either be extra sounds, or "murmurs" related to the flow of blood between the sounds. Murmurs are graded by volume, from 1 (the quietest), to 6 (the loudest), and evaluated by their relationship to the


title: Heart content: heart sounds, position in the cardiac cycle, and additional features such as their radiation to other sites, changes with a person's position, the frequency of the sound as determined by the side of the stethoscope by which they are heard, and site at which they are heard loudest. Murmurs may be caused by damaged heart valves, congenital heart disease such as ventricular septal defects, or may be heard in normal hearts. A different type of sound, a pericardial friction rub can be heard in cases of pericarditis where the inflamed membranes can rub together. Blood tests play an important role


title: Heart content: in the diagnosis and treatment of many cardiovascular conditions. Troponin is a sensitive biomarker for a heart with insufficient blood supply. It is released 4–6 hours after injury, and usually peaks at about 12–24 hours. Two tests of troponin are often taken – one at the time of initial presentation, and another within 3–6 hours, with either a high level or a significant rise being diagnostic. A test for brain natriuretic peptide (BNP) can be used to evaluate for the presence of heart failure, and rises when there is increased demand on the left ventricle. These tests are considered biomarkers


title: Heart content: because they are highly specific for cardiac disease. Testing for the MB form of creatine kinase provides information about the heart's blood supply, but is used less frequently because it is less specific and sensitive. Other blood tests are often taken to help understand a person's general health and risk factors that may contribute to heart disease. These often include a full blood count investigating for anaemia, and basic metabolic panel that may reveal any disturbances in electrolytes. A coagulation screen is often required to ensure that the right level of anticoagulation is given. Fasting lipids and fasting blood glucose


title: Heart content: (or an HbA1c level) are often ordered to evaluate a person's cholesterol and diabetes status, respectively. Using surface electrodes on the body, it is possible to record the electrical activity of the heart. This tracing of the electrical signal is the electrocardiogram (ECG) or (EKG). An ECG is a bedside test and involves the placement of ten leads on the body. This produces a "12 lead" ECG (three extra leads are calculated mathematically, and one lead is a ground). There are five prominent features on the ECG: the P wave (atrial depolarisation), the QRS complex (ventricular depolarisation) and the T


title: Heart content: wave (ventricular repolarisation). As the heart cells contract, they create a current that travels through the heart. A downward deflection on the ECG implies cells are becoming more positive in charge ("depolarising") in the direction of that lead, whereas an upward inflection implies cells are becoming more negative ("repolarising") in the direction of the lead. This depends on the position of the lead, so if a wave of depolarising moved from left to right, a lead on the left would show a negative deflection, and a lead on the right would show a positive deflection. The ECG is a useful


title: Heart content: tool in detecting rhythm disturbances and in detecting insufficient blood supply to the heart. Sometimes abnormalities are suspected, but not immediately visible on the ECG. Testing when exercising can be used to provoke an abnormality, or an ECG can be worn for a longer period such as a 24-hour Holter monitor if a suspected rhythm abnormality is not present at the time of assessment. Several imaging methods can be used to assess the anatomy and function of the heart, including ultrasound (echocardiography), angiography, CT scans, MRI and PET. An echocardiogram is an ultrasound of the heart used to measure the


title: Heart content: heart's function, assess for valve disease, and look for any abnormalities. Echocardiography can be conducted by a probe on the chest ("transthoracic") or by a probe in the esophagus ("transoesophageal"). A typical echocardiography report will include information about the width of the valves noting any stenosis, whether there is any backflow of blood (regurgitation) and information about the blood volumes at the end of systole and diastole, including an ejection fraction, which describes how much blood is ejected from the left and right ventricles after systole. Ejection fraction can then be obtained by dividing the volume ejected by the heart


title: Heart content: (stroke volume) by the volume of the filled heart (end-diastolic volume). Echocardiograms can also be conducted under circumstances when the body is more stressed, in order to examine for signs of lack of blood supply. This cardiac stress test involves either direct exercise, or where this is not possible, injection of a drug such as dobutamine. CT scans, chest X-rays and other forms of imaging can help evaluate the heart's size, evaluate for signs of pulmonary oedema, and indicate whether there is fluid around the heart. They are also useful for evaluating the aorta, the major blood vessel which leaves


title: Heart content: the heart. Diseases affecting the heart can be treated by a variety of methods including lifestyle modification, drug treatment, and surgery. Narrowings of the coronary arteries (ischaemic heart disease) are treated to relieve symptoms of chest pain caused by a partially narrowed artery (angina pectoris), to minimise heart muscle damage when an artery is completely occluded (myocardial infarction), or to prevent a myocardial infarction from occurring. Medications to improve angina symptoms include nitroglycerin, beta blockers, and calcium channel blockers, while preventative treatments include antiplatelets such as aspirin and statins, lifestyle measures such as stopping smoking and weight loss, and treatment


title: Heart content: of risk factors such as high blood pressure and diabetes. In addition to using medications, narrowed heart arteries can be treated by expanding the narrowings or redirecting the flow of blood to bypass an obstruction. This may be performed using a percutaneous coronary intervention, during which narrowings can be expanded by passing small balloon-tipped wires into the coronary arteries, inflating the balloon to expand the narrowing, and sometimes leaving behind a metal scaffold known as a stent to keep the artery open. If the narrowings in coronary arteries are unsuitable for treatment with a percutaneous coronary intervention, open surgery may


title: Heart content: be required. A coronary artery bypass graft can be performed, whereby a blood vessel from another part of the body (the saphenous vein, radial artery, or internal mammary artery) is used to redirect blood from a point before the narrowing (typically the aorta) to a point beyond the obstruction. Diseased heart valves that have become abnormally narrow or abnormally leaky may require surgery. This is traditionally performed as an open surgical procedure to replace the damaged heart valve with a tissue or metallic prosthetic valve. In some circumstances, the tricuspid or mitral valves can be repaired surgically, avoiding the need


title: Heart content: for a valve replacement. Heart valves can also be treated percutaneously, using techniques that share many similarities with percutaneous coronary intervention. Transcatheter aortic valve replacement is increasingly used for patients consider very high risk for open valve replacement. Abnormal heart rhythms (arrhythmias) can be treated using antiarrhythmic drugs. These may work by manipulating the flow of electrolytes across the cell membrane (such as calcium channel blockers, sodium channel blockers, amiodarone, or digoxin), or modify the autonomic nervous system's effect on the heart (beta blockers and atropine). In some arrhythmias such as atrial fibrillation which increase the risk of stroke, this


title: Heart content: risk can be reduced using anticoagulants such as warfarin or novel oral anticoagualants. If medications fail to control an arrhythmia, another treatment option may be catheter ablation. In these procedures, wires are passed from a vein or artery in the leg to the heart to find the abnormal area of tissue that is causing the arrhythmia. The abnormal tissue can be intentionally damaged, or ablated, by heating or freezing to prevent further heart rhythm disturbances. Whilst the majority of arrhythmias can be treated using minimally invasive catheter techniques, some arrhythmias (particularly atrial fibrillation) can also be treated using open or


title: Heart content: thoracoscopic surgery, either at the time of other cardiac surgery or as a standalone procedure. A cardioversion, whereby an electric shock is used to stun the heart out of an abnormal rhythm, may also be used. Cardiac devices in the form of pacemakers or implantable defibrillators may also be required to treat arrhythmias. Pacemakers, comprising a small battery powered generator implanted under the skin and one or more leads that extend to the heart, are most commonly used to treat abnormally slow heart rhythms. Implantable defibrillators are used to treat serious life-threatening rapid heart rhythms. These devices monitor the heart,