Blood performs various essential functions as it circulates through the body. It delivers oxygen and essential nutrients (such as fats, sugars, minerals, and vitamins) to the body's tissues. It carries carbon dioxide to the lungs and other waste products to the kidneys for elimination from the body. It transports hormones (chemical messengers) to allow various parts of the body to communicate with each other. Also, it carries components that fight infection and stop bleeding.
Blood Disorders Sections (A-Z)
Biology of Blood
Blood is a complex mixture of plasma (the liquid component), white blood cells, red blood cells, and platelets. The body contains about 5 to 6 quarts (about 5 liters) of blood. Once blood is pumped out of the heart, it takes 20 to 30 seconds to make a complete trip through the circulation and return to the heart.
Bleeding Due to Abnormal Blood Vessels
Blood vessel disorders that cause bleeding result from blood vessel defects, typically causing red to purple discoloration of the skin or bruising but seldom causing serious blood loss. Bleeding may be a result of connective tissue disorders, vitamin deficiencies, blood protein abnormalities, defective blood vessels, or conditions that cause blood vessel inflammation. Bleeding under the skin may also be more common among older people ( senile purpura).
Bleeding Due to Clotting Disorders
(See also How Blood Clots.)
Blood Clotting Process
Hemostasis is the body's way of stopping injured blood vessels from bleeding. Hemostasis includes clotting of the blood. Too much clotting can block blood vessels that are not bleeding. Consequently, the body has control mechanisms to limit clotting and dissolve clots that are no longer needed. An abnormality in any part of this system that controls bleeding can lead to excessive bleeding or excessive clotting, both of which can be dangerous. When clotting is poor, even a slight injury to a blood vessel may lead to severe blood loss. When clotting is excessive, small blood vessels in critical places can become clogged with clots. Clogged vessels in the brain can cause strokes, and clogged vessels leading to the heart can cause heart attacks. Pieces of clots from veins in the legs, pelvis, or abdomen can travel through the bloodstream to the lungs and block major arteries there (pulmonary embolism).
Iron is essential for life, so the body usually tightly controls iron absorption from food and recycles the iron from red blood cells. People lose small amounts of iron every day, and even a healthy diet contains only a small amount of iron. Thus, people rarely have too much iron in their body. Causes of excess iron in the body (iron overload) include the following:
In myeloproliferative disorders (myelo = bone marrow, proliferative = rapid multiplication), the blood-producing cells in the bone marrow (precursor cells) develop and reproduce excessively or are crowded out by an overgrowth of fibrous tissue. Typically, these disorders are acquired and not inherited, although rarely there are families in which several members have these disorders.
Plasma Cell Disorders
Plasma cell disorders are uncommon. They begin when a single plasma cell multiplies excessively. The resulting group of genetically identical cells (called a clone) produces a large quantity of a single type of antibody (immunoglobulin). Plasma cells develop from B cells (B lymphocytes), a type of white blood cell that normally produces antibodies. Antibodies help the body fight infection. Plasma cells are present mainly in bone marrow and lymph nodes. Every plasma cell divides repeatedly to form a clone. The cells of a clone produce only one specific type of antibody. Because thousands of different clones exist, the body can produce a vast number of different antibodies (see Acquired Immunity : Antibodies) to fight the numerous infectious microorganisms to which the body is exposed.
Platelets (sometimes called thrombocytes) are cell fragments that circulate in the bloodstream and help blood clot (see How Blood Clots). Thrombopoietin, primarily produced in the liver, stimulates the bone marrow to make large cells (megakaryocytes), which in turn make platelets from their cytoplasm. Platelets that are not used in clots circulate for 7 to 10 days and are then destroyed. About one third are always stored in the spleen. The platelet count (number of platelets circulating in the bloodstream) can decrease near the end of pregnancy (gestational thrombocytopenia) and increase in response to inflammation (secondary, or reactive, thrombocytosis). Neither of these conditions is serious, and most affected people have no problems resulting from either one.
The spleen, a spongy, soft organ about as big as a person’s fist, is located in the upper left part of the abdomen, just under the rib cage. The splenic artery brings blood to the spleen from the heart. Blood leaves the spleen through the splenic vein, which drains into a larger vein (the portal vein) that carries the blood to the liver. The spleen has a covering of fibrous tissue (the splenic capsule) that supports its blood vessels and lymphatic vessels.
Symptoms and Diagnosis of Blood Disorders
Disorders that affect the cells in the blood (blood cells) or proteins in the blood clotting or immune systems are called blood disorders or hematologic disorders. Laboratory tests to detect blood disorders generally begin with examination of the blood, which is easily obtained from a vein with a needle and syringe or sometimes from the fingertip by a needle prick. However, evaluation may require examination of the bone marrow, because that is where blood cells develop.
White Blood Cell Disorders
White blood cells (leukocytes) are an important part of the body’s defense against infectious organisms and foreign substances. To defend the body adequately, a sufficient number of white blood cells must receive a message that an infectious organism or foreign substance has invaded the body, get to where they are needed, and then kill and digest the harmful organism or substance (see White blood cells and see Figure: Lymphatic System: Helping Defend Against Infection).