Fun Facts About The Digestive System
Digestion Begins in the Mouth
Eating food is not enough. To actually provide our bodies with the carbohydrates, proteins and fats it needs, and the minerals and vitamins, food must be broken down and the nutrients absorbed. This is the function of the digestive system.
One little known fact about digestion is that it starts in the mouth. Not only does chewing physically break down the food, so it is far more manageable, but the enzymes in the saliva start to chemically breakdown starch into simpler sugars. These enzymes are active proteins known as amylases.
Saliva is produced by three pairs of salivary glands. It also moistens and softens the food so it is easy to swallow. We produce between 1 and 3 pints of saliva every day.
When Food is Swallowed it Enters the Oesophagus
Swallowed food enters the gullet, which is technically known as the oesophagus, from the Greek words for "entrance for eating". This is a narrow tube, connecting the mouth to the stomach. It is lined with smooth muscle, and their rhythmic contractions,known as peristalsis push the food into the stomach.
It is the peristaltic contraction, not gravity, that drive the food into the stomach. The process would still happen if you stood on your head after eating. Also astronauts have no problems with food going down the right way, in the absence of gravity.
Quick & Fun Facts
- Humans produce between 1 and 3 pints of saliva a day.
- An adult stomach holds up to 3 pints of content at a time.
- The stomach secretes hydrochloric acid, a very strong acid that can melt metals.
- Gall bladder stones are made from the cholesterol in the bile.
- In the absence of bile, fats are not digested and are excreted in white, oily faeces.
- Birds don't have teeth so they swallow small stones which break down their food in gizzards.
What Happens in the Stomach
Food is further physically and chemically broken down in the stomach. The stomach is a muscular pouch, with a ring of muscle (sphincters) at each end which open to allow the food to enter and exit, but close when the food is being digested.
Even before you start eating, gastric juices are secreted into the stomach. These are mostly made up of hydrochloric acid (HCL), a very strong acid which can burn through many materials. pH inside the stomach is between 1-3, very acidic!
It is quite amazing that we can tolerate such acidic conditions inside our bodies, but the stomach lining also secretes a lot of mucus, which protects the cells from the acid. In some cases this mucosal lining becomes damaged, leading to stomach ulcers, which are very painful, since that part of the stomach lining is no longer protected from the acid.
The hydrochloric acid kills many of the harmful bacteria that are ingested with the food. They also provide the optimal conditions for the enzyme pepsin to work on digesting proteins. The acid denatures proteins.
During this whole time the stomach muscles churn the food breaking it up and making sure it mixes well with the gastric juices. At the end of its time in the stomach (usually 1-2 hours in humans) the food is a homogeneous fluid mixture called chyme.
Billy Rubin in Silence of the Lambs
One of the components of bile is bilirubin, which is a product of the destruction of old red blood cells. It is one of the reason faeces are brown. In the book the Silence of the Lambs, the mad psychiatrists Lecter misdirects the FBI that the psychopathic kidnapper and mass murderer is a patient of his called Billy Rubin. If they remembered their biology, they would have realised faster that he was feeding them bull*****.
Digestion and Absorption in the Small Intestine
Once the stomach is done with the food, it can pass to the small intestine. This is a very long tube, about 23 foot in length on average, which is tightly coiled so it can actually fit in the abdomen. Most of the chemical digestion of food actually happens in the first part of the small intestine.
When the food first enters, it is mixed with 3 different fluids:
- Bile which is made in the liver, stored in the gall bladder and enters the small intestine through the bile duct. Bile contains salts which emulsify the fats so enzymes can act on them, without bile fats become indigestible. The liver is the biggest organ in the body, it performs many different functions, producing bile which is necessary for fat digestion is only one of them.
- Pancreatic Juices from the pancreas, contain many enzymes such as trypsinogen, which becomes trypsin which digests proteins, amylase, for starch breakdown, and lipase which digests fats. The pancreas also secretes a lot of bicarbonate which neutralises the acid from the stomach.
- The intestinal Juices produced by the lining of the small intestine contain more enzymes such as trypsin and chymotrypsin for proteins and maltase, sucrase and lactase that digest carbohydrates.
Absorbing Nutrients in the Small Intestine, the Role of the Microvilli
Other than the digestion, the main role of the small intestine is to absorb the amino acids, glucose and other simple sugars, and fat that the food has been turned into.
To help it do this efficiently the lining is covered with lots of small projections, known as villi, which in turn are covered with microscopic projections known as microvilli.
This hugely increases the surface area that is in contact with the digested food. Nutrients can pass across the mucosal lining of the intestine and into the blood capillaries inside the villi, they are then carried in the blood to all the cells of the body.
The fact that the intestine is so long also helps with the efficient absorption. In the modern day, when people increasingly face the problem of obesity, I bet many of us wish we didn't absorb all the nutrients from food so efficiently, but the digestive system evolved during prehistoric times when getting sufficient nutrition was difficult, so it is designed to make the most of what we eat.
What Happens in the Large Intestine?
The large intestine is the last fragment of the digestive system. By the time the contents get here, they have been digested, and most of the nutrients have been absorbed out of them.
The large intestine contains up to 400 different kinds of bacteria. These are the "good" bacteria that you hear about in probiotic yogurt advertisements. They ferment the remains of the chyme to break down the carbohydrates that didn't quite get fully digested, and the resulting simple sugars are absorbed.
All that is left to do is to excrete the indigestible fibre, and various impurities produced by the liver in the form of faeces. But before that can happen water has to be absorbed out of the contents. The body doesn't want to lose more water than is necessary, in case it becomes dehydrated, and that is the function of the lining of the large intestine.