Absorption Of Carbohydrates

The end products of carbohydrate digestion pass to the blood by way of the portal system. The monosaccharides on reaching the liver enter the general circulation as dextrose if required for immediate use; otherwise they are stored in the liver in the form of glycogen, which can be again transformed into sugar when demanded by the system to furnish energy. Sugars are so readily absorbed that if taken in large quantities the body may be unable to utilize them, and the excess is excreted in the urine. This is especially true in case of an excess of cane sugar or milk sugar. They may be absorbed before digestion, and there are no enzymes in the blood capable of converting them into dextrose so that they can be burned or stored. Under such circumstances they are useless and must be gotten rid of. If maltose is absorbed too rapidly, it may still be changed to dextrose by enzymes in the blood or tissues, and so be utilized.

Absorption Of Mineral Matter

As salts are not digested in the ordinary sense of the word, they require nothing more than solution in the digestive fluids before absorption. The mineral matter is set free from food and diffuses more or less rapidly into the surrounding fluids. Common salt is most quickly absorbed. The chief locality for absorption is the small intestines. The older text books taught that salts were absorbed only by diffusion through the intestinal wall, which required that another solution must at the same time pass from the blood to the intestines until both fluids were of the same degree of concentration. It appears, however, that in the main, mineral matter is taken up in the intestinal wall without diffusion, and as fast as it can be incorporated with certain peculiar solvents is absorbed into the blood.

Absorption Of Water

"Water is not absorbed by the stomach, but passes quickly in little spurts (if taken alone) into the intestines. Here it is very rapidly absorbed; it may be excreted through the kidneys within twenty minutes after ingestion. The rapidity of absorption may be considerably influenced by mineral matter dissolved in the water.

Summary

Absorption is an active, not a passive process. It takes place mainly in the intestines, the larger part before the food reaches the ileo-cecal valve.

Proteins are absorbed by the villi, chiefly in the form of amino acids and pass through the portal vein to the liver.

Fats are absorbed by the lacteals, chiefly in the form of fatty acids and glycerine, but pass to the lymphatics as neutral fat, and enter the blood stream from the thoracic duct.

Carbohydrates are absorbed by the villi in the form of sugar (chiefly dextrose) and pass through the portal vein to the liver.

Unabsorbed material forms the feces (see Excretion, page 53). Undigested proteins, fats and carbohydrates are liable to attack by putrefactive and fermentative organisms in the intestines. The products of such bacterial action are absorbed to a considerable extent. Carbohydrates which escape digestion tend to produce lactic and other acids. This fermentation is not altogether an evil, for it checks the putrefaction of undigested meat. The latter is undesirable, as the products are likely to prove toxic. It is for this reason that lactic acid, especially in the form of buttermilk and artificially fermented sour milk, is largely employed as a remedial agent in intestinal indigestion.