The digestion stage (fig 2.13)
Food is prepared for digestion and digested by the digestive organs in the following manner: food is broken up into smaller particles by the action of chewing, and becomes warm and moist. The chemical action of the digestive juices splits these particles into simple substances which are soluble in water, allowing them to be taken up more easily into the body. The action of the digestive juices is helped by the mixing and churning movements of the involuntary muscles of the stomach and intestines (also called plain or smooth muscles because they are not striped as are the skeletal or voluntary muscles).
In the mouth the food is broken up by chewing, and moistened by saliva, a digestive juice poured into the mouth from several glands (salivary glands) situated near it.
The act of swallowing passes the moistened food down the gullet into the stomach. The stomach acts as a reservoir in which the food is retained until it is thoroughly warmed and reduced to a fluid condition. The walls of the stomach pour out a second juice (gastric juice) which also acts upon the food. Upon leaving the stomach the partially digested food enters the small intestine in a liquid state, and is there acted upon by the digestive juice of the pancreas (pancreatic juice). The bile also acts to some extent upon the food. At the same time the walls of the intestine also secrete a digestive juice (the intestinal juice). The small intestine is the main area in which digestion takes place.
Thus, in its passage through the digestive canal, food is continuously undergoing change. Finally, after passing through the large intestine, the undigested and unabsorbed remainder of the food is expelled from the body. The passing of the food through the digestive canal takes about 24 hours.
A normal diet contains, in proportions suited to the needs of the body, the following three main classes of nutrients:
· Nitrogenous nutrients found mainly in animal foods (meat, fish, etc), and which also form the main part of these foods. Several vegetable foods, however, also contain a considerable amount of nitrogenous nutrients.
· Starches and sugars, found mainly in vegetables, bread, potatoes, sugar, fruits, etc. Meats contain virtually no starch or sugar.
· Fats, found for example in butter, oil, meat, fat and cream.
While all these nutrients serve as sources of energy, it is chiefly starches, sugars and fats which supply energy. In a normal diet starches, sugars and fats amount to more than three-quarters of the total food intake. The main use of nitrogenous nutrients is to repair the break-down in the living structures of the body.
In addition to these nutrients a normal diet contains mineral salts and vitamins. These are necessary to the body, but they are not sources of energy, and the same is true of the water which we drink.
Digestive juices act upon the nutrients in different ways. Saliva only acts upon starches in the food, and gastric juice mainly upon the nitrogenous nutrients. Gastric juice is the only acid digestive juice, and for this reason it has an important action in checking the development of any infective germs which may have been swallowed.
Partially digested foodstuffs are acted upon in the small intestine by pancreatic juice, bile, and intestinal juice. All these are alkaline juices, and together they aid digestion, but the strongest is pancreatic juice. By the joint action of these juices the digestion of all three classes of nutrients is continued and completed in the small intestine where they are reduced to the simple forms in which they can be absorbed into the body.
In the lower part of the small intestine and in the large intestine a certain amount of putrefactive change also takes place. This is due not to the digestive juices, but to the action of germs or bacteria.