Absorption of Carbohydrates.—The carbohydrates are chiefly absorbed in the form of monosaccharids. This has been proved by introducing cane sugar or lactose into the blood vessels and getting the greater portion of it back unchanged by way of the urine. As the monosaccharids are taken up by the capillaries lining the walls of the small intestines, they are passed on to the portal vein and carried by the portal blood into the liver, where they are stored temporarily as glycogen, and given out to the blood in the form of glucose as needed. After a meal rich in carbohydrates, the portal blood will be rich in glucose, while the blood in general circulation contains about the same amount as usual, about 0.1%.
Absorption of Proteins.—The absorption of the products of protein digestion occurs through the capillary blood vessels and passes on to the portal vein. The metabolism of protein is more complex than that of any of the foodstuffs. It is probable that each living cell contains enzymes which are capable of breaking down the body proteins with the production of amino acids just as the proteins of the food are broken down by enzymes of digestion, and according to Sherman “it is not improbable that protein synthesis also may be brought about by every living cell.”
The Absorption of Water.—This does not occur in the stomach, as was formerly believed, but in the small intestines.
The Absorption of the Mineral Salts.—This occurs in conjunction with the other food material. Some of the mineral salts are much more soluble than others and are more readily absorbed. The function of the mineral salts in the body has already been described, and since they form a part of every tissue and fluid in the body their absorption and fate in metabolism must be studied with that of the other chemical combinations.
ABSORPTION IN THE LARGE INTESTINES
The digestion of the food as it is passed into the last portion of the alimentary canal has been largely completed. However, that part which has escaped digestion in the small intestine is finished and absorbed here. The important peristaltic waves occurring in this region are antiperistaltic in character and have the property of churning the food thoroughly and bringing a larger portion of it in contact with the absorbing walls. The water which is left in the food mass together with the products of the digestion of the foodstuffs is absorbed in the first part of the large intestine, leaving the remainder more solid. This residue is known as feces or fecal matter.
The diagram on page [179] shows the various processes through which the foodstuffs pass after absorption.[56]
Bacterial Action in the Alimentary Canal.—The changes in the foods so far mentioned have been chiefly the result of the activity of the enzymes existing in the various digestive processes throughout the body. But there are other changes which occur in the foods during their sojourn in the digestive tract which are not accountable to enzymic action, but which, in fact, modify to a certain degree the changes wrought by the enzymes. These are the result of the activity of certain specific bacteria which inhabit the entire digestive tract of the individual from a few hours after birth until death. Some of these have so adapted themselves to the existing conditions that, unless present in overwhelming numbers, they are not only harmless, but they actually assist in protecting the organism from the inroads of more harmful species. Many experiments have been made to find whether or not bacteria are essential to human nutrition, and the results of these experiments prove that they are not. However, since they are so firmly established in the body it is well to study the various types and learn as much as possible of the products of their activity and the influence which they exert in human nutrition.
