Contributed by Yoav Givati
Please send any comments or contributions to the maintainer Roger Caffin
The process of digestion begins when food enters the mouth. In the mouth mechanical or physical breakdown of food occurs. There are many salivary glands found in humans. The largest pair is located just below and in front of the ears, the another pair is below the jaw, and the third is below the tongue. There are other smaller glands as well. These glands secrete different chemnicals which together make up saliva. They include fluid to soften and moisten food and to convert starch into sugar. In the cheeks there are many capillaries: these are tiny blood vessels that allow nutrients to be absorbed into the blood stream they are also used for gas exchange in the lungs. Sugars can be directly absorbed into the blood stream via the cheeks and begin to work almost instantly.
The tongue helps to move and manipulate food around in the mouth. It eventually positions the ball of moist food in the back of the throat. This triggers a swallowing action and the ball of food continues down the esophagus by peristaltic action (wave-like contractions, similar to a slinky). The food reaches the stomach quickly. The stomach has a lining of mucus which protects it from the hydrochloric acid contained there. The stomach is made up of four muscles and many folds and flaps which aids to mechanical breakdown. The juices in the stomach can only break down proteins, rather then do a full 'digestion'. However the acid kills many bacteria that may have found their way down to your stomach with the food.
After going through the stomach the food is passed into the small intestine. The first part of the small intestine, called the duodenum, is where the majority of chemical breakdown takes place. It is connected by ducts to the spleen (looks similar to a leaf, located behind the stomach) and the gall bladder (stores bile produced by the liver) by ducts. The bile breaks down fat to a molecular level which allows it to be absorbed. The spleen produces more then 28 enzymes and baking soda (which neutralizes the stomach acid). By now what was once your nicely presented dinner is just a liquid, with the nutrients and other components of the food floating around or dissolved in it. [This means that cooking all your dinner in one pot makes perfect sense. RNC] This liquid flows until it gets to the large intestine, at which point it is just water and indigestible fibre. That is, unless there were not enough enzymes excreted, not enough bile in the gall bladder, or you have gall bladder problems.
The large intestines have lots of folds to increase the service area so that the water can be absorbed back into the body. As the undigested matter moves toward the anus water is absorbed, forming hard masses which wait in the rectum until you are ready to release it. If you don't drink enough water or don't have a bowel movement for a while, too much water can be absorbed from the stool and you can become constipated. Bacteria in the large intestines plays an important role in producing some vitamins.
This was contributed by Yoav Givati from Canada. I was curious, and asked him who he was and where he got it from, and he replied:
"I'm 16. Most of it is from my high-school biology course, combined together with just talking to people in the medical field. I want to be a vet when I grow up so I'm fairly interested in this stuff."
© Yoav Givati and Roger Caffin 27/10/2004