Absorption, Storage and Assimilation : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 11 of 25)

The dissolved nutrients, to reach the cells, must be transferred from the alimentary canal to the blood stream. This process is known as absorption. In general, absorption means the penetration of a liquid into the pores of a solid, and takes place according to the simple laws of molecular movements. The absorption of food is, however, not a simple process, and the passage takes place through an active (living) membrane. Another difference is that certain foods undergo chemical change while being absorbed.

Small Intestine as an Organ of Absorption. - While absorption may occur to a greater or less extent along the entire length of the alimentary canal, most of it takes place at the small intestine. Its great length, its small diameter, and its numerous blood vessels all adapt the small intestine to the work of absorption. The transverse folds in the mucous membrane, by retarding the food in its passage and by increasing the absorbing surface, also aid in the process. But of greatest importance are the minute elevations that cover the surface of the mucous membrane, known as

The Villi. - Each single elevation, or villus, has a length of about one fiftieth of an inch and a diameter about half as great, and contains the following essential parts:

1. An outer layer of epithelial cells, resting upon a connective tissue support.

2. A small lymph tube, called a lacteal, which occupies the center of the villus and connects at the base with other lymph tubes, also called lacteals.

3. A network of capillaries.

The villi are structures especially adapted to the work of absorption, and they are found only in the small intestine. The mucous membrane in all parts of the canal, however, is capable of taking up some of the digested materials.

B. Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3. Villus showing termination of the lacteal. 4. Villus showing capillaries. 5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7. Layer of epithelial cells.

Work of Capillaries and Lacteals. - The capillaries and lacteals act as receivers of material as it passes through the layer of epithelial cells covering the mucous membrane. The lacteals take up the digested fats, and the capillaries receive all the other kinds of nutrients. These vessels do not, of course, retain the absorbed materials, but pass them on. Their final destination is the general circulation, which they reach by two well-defined channels, or routes.

Routes to the Circulation. - The two routes from the place of absorption to the general circulation are as follows:

1. Route taken by the Fat. - The fat is conveyed by the lacteals from the villi to the receptacle of the chyle. At this place it mingles with the lymph from the lower parts of the body, and with it passes through the thoracic duct to the left subclavian vein. Here it enters the general circulation. therefore, to reach the general circulation, the fat has to pass through the villi, the lacteals, the receptacle of the chyle, and the thoracic duct. Its passage through these places, like the movements in all lymph vessels, is slow, and it is only gradually admitted to the blood stream.

2. Route of All the Nutrients except Fat. - Water and salts and the digested proteins and carbohydrates, in passing into the capillaries, mix there with the blood. But this blood, instead of flowing directly to the heart, is passed through the portal vein to the liver, where it enters a second set of capillaries and is brought very near the liver cells. From the liver it is passed through the hepatic veins into the inferior vena cava, and by these it is emptied into the right auricle. This route then includes the capillaries in the mucous membrane of the stomach and intestines, the branches of the portal vein, the portal vein proper, the liver, and the hepatic veins. In passing through the liver, a large portion of the food material is temporarily retained for a purpose and in a manner to be described later.

Absorption Changes. - During digestion the insoluble foods are converted into certain soluble materials, such as peptones, maltose, and glycerin, - the conversion being necessary to their solution. A natural supposition is that these materials enter and become a part of the blood, but examination shows them to be absent from this liquid. There are present in the blood, however, substances closely related to the peptones, maltose, glycerin, etc.; substances which have in fact been formed from them. During their transfer from the food canal, the dissolved nutrients undergo changes, giving rise to the materials in the blood. Therefore are the serum albumin and serum globulin of the blood derived from the peptones and proteoses; the dextrose, from the maltose and other forms of sugar; and the fat droplets, from the glycerin, fatty acid, and soluble soap.

While considerable doubt exists as to the cause of these changes and as to the places also where some of them occur, their purpose is quite apparent. The materials forming the dissolved foods, although adapted to absorption, are not suited to the needs of the body, and if introduced in this form are likely to interfere with its work. They are changed, therefore, into the forms which the body can use.

A Second Purpose of Digestion. - Comparing the digestive changes with those of absorption, it is found that they are of a directly opposite nature; that while digestion is a process of tearing down, or separating, - one which reduces the food to a more finely divided condition - there is in absorption a process of building up. From the comparatively simple compounds formed by digestion, there are formed during absorption the more complex compounds of the blood. The one exception is dextrose, which is a simple sugar; but even this is combined in the liver and the muscles to form the more complex compound known as glycogen. These facts have suggested a second purpose of digestion - that of reducing foods to forms sufficiently simple to enable the body to construct out of them the more complex materials that it needs. Evidence that digestion serves such a purpose is found in the fact that both proteins and carbohydrates are reduced to a simpler form than is necessary for dissolving them.

D.C. Heath and Co. - Publishers
Original copyright 1909