The alimentary or digestive system of the human being is an extremely complicated apparatus and process. Fortunately, however, the process of digestion, turning food into nutrient, is practically automatic; man need be concerned only with getting food into his mouth and ejecting the waste products of that food about 10% of the total in a socially acceptable manner. The physiology of digestion is managed by the autonomic nervous system; it takes no conscious effort.
In its simplest terms the digestive system is a flexible mucus-lined muscular tube, 24 to 36 feet long, beginning in the mouth and ending at the anus (see figure, page 90.). Along the tube lie a series of glands and cells which produce the chemical substances — such as saliva and gastric juice-that act upon foodstuffs taken into the mouth and break them down for absorption into the cells of the body.
In the mouth the process of mastication takes place the chewing of food into small bits so that it can be efficiently handled by the rest of the digestive system. Mastication is the function of the teeth, which must be considered as part of the digestive system. We shall deal with the important subject of the teeth at the end of this chapter.
The Physiology of Digestion
The process of digestion begins in the mouth, where the teeth chew and grind food, and enzymes secreted by the salivary glands immediately go to work breaking down carbohydrates. About 7 seconds later the time it takes for swallowed food to traverse the esophagus the stomach takes over. Its 5 million tiny glands manufacture about 3 quarts of gastric juice a day. This includes hydrochloric acid in a weak solution ranging from 0.2 to 0.5%and the enzymes pepsin and rennin. The stomach itself is a pear shaped distensible pouch, capable of holding
2 to 3 pints of food. Very few substances, notably honey and alcohol, are absorbed into the
bloodstream directly from the stomach.
Food stays in the stomach about 2 to 4 hours sometimes longer. It is churned into
a mushy, semi-solid mass, called chyme, and discharged into the duodenum. There it is
acted on by more enzymes, arriving by way of a common duct (or channel) from the pan
creas, and by bile. Bile is not an enzyme; it is essentially a solvent and emulsifier of fats. It
is produced by the liver but stored and concentrated in the gall bladder. The pancreatic
juices contain a great number of powerful and important enzymes. Some absorption of nutrients takes place in the duodenum, but to a far greater extent in the lower reaches of the
Projecting from the inner lining of the small intestine are thousands of tiny absorptive organs called intestinal villi (singular, villus). Finger-shaped projections, about V25 of an inch long, the numerous villi give the lining of the small intestine the feel of velvet. Within each villus is a network of minute blood vessels (capillaries) surrounding a small tube called a lacteal?
When the broken-down food particles are finally absorbed by the villi and conveyed to the bloodstream for distribution to the rest of the body, the process of digestion is completed and the far more complicated processes of tissue nutrition and body metabolism may b e said to begin.