Absorption, Storage and Assimilation : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 13 of 25)

Why Alcohol is not a Food. - If the passage of alcohol through the body is followed, it is seen, in the first place, that it is a simple liquid and undergoes no digestive change; and in the second place, that it is rapidly absorbed from the stomach in both weak and concentrated solutions. This introduces it quickly into the blood, and once there, it diffuses rapidly into the lymph and then into the cells. Since the body cannot store alcohol or convert it into some nutrient that can be stored, there is no way of regulating the amount that should be present in the blood, or of supplying it to the cells as their needs require. They must take it in excess of their needs, regardless of the effect, at least until the organs of excretion can throw off the surplus as waste. Compared with protein, carbohydrates, or fats, alcohol is an unmanageable substance in the body. Attempting to use it as a food is as foolish as trying to burn gasoline or kerosene in an ordinary wood stove. It may be done to a limited extent, but is an exceedingly hazardous experiment. Not being adapted to the body method of using materials, alcohol cannot be classed as a food.

Assimilation. - Digestion, absorption, circulation, and storage of foods are the processes that finally make them available to the cells in the different parts of the body. There still remains another process for these materials to undergo before they serve their final purposes. This last process, known as assimilation, is the appropriation of the food material by the cell protoplasm. In a sense the storage of fat by connective tissue cells and of glycogen by the liver cells is assimilation. The term is limited, however, to the disposition of material with reference to its final use. Whether all the materials used by the cells actually become a part of the protoplasm is not known. It is known, however, that the cells are the places where most of the oxidations of the body occur and that materials taking part in these oxidations must, at least, come in close contact with the protoplasm. Assimilation, then, is the last event in a series of processes by which oxygen, food materials, and cell protoplasm are brought into close and active relations. The steps leading up to assimilation are shown in Table II.

Table Ii. The Passage Of Materials To The Cells Materials Digestion Absorption Route To Storage Condition The General In The Circulation Blood proteins Changed In passing Through the Become a As proteins into the portal vein part of the in proteoses capillaries, to the protoplasm colloidal and the liver and of all the solution. peptones by proteoses from there cells. the action and through the of the peptones hepatic gastric and change into veins into pancreatic the juices. proteins of inferior the blood. vena cava. Fat Changed In passing Through the As fat in Chiefly as into fatty into the lacteals to the cells minute oil acid, lacteals, the of droplets. glycerin, the thoracic collective and glycerin duct, by tissue. soluble unites with which it is soap by the soluble emptied bile and soap and into the pancreatic fatty acid left juice. to form the subclavian oil droplets vein. of the blood. Starch Reduced to Enters the Through the As glycogen As dextrose some of the capillaries portal chiefly by in different as dextrose. vein, the liver, solution. forms of liver, but to some sugar, as hepatic extent by maltose, veins, into muscle dextrose, inferior cells. etc. vena cava. Water Undergoes Taken up by Both Is not As the no change. both the routes, but stored in water which lacteals and mostly by the sense serves as a capillaries, way of the that energy carrier of but to the liver. foods are. all the greater other extent by constituents the of the capillaries. blood. Common salt Undergoes Taken up by By way of Not stored. In solution. no change. the portal capillaries vein, without liver, and undergoing hepatic apparent veins into change. inferior vena cava. Oxygen Taken up by Already in Is not United with the general stored. the capillaries circulation. hemoglobin at the and to a lungs. small extent in solution in the Plasma.

Tissue Enzymes. - The important part played by enzymes in the digestion of the food has suggested other uses for them in the body. It has been recently shown that many of the chemical changes in the tissues are in all probability due to the presence of enzymes. An illustration of what a tissue enzyme may do is seen in the changes which fat undergoes. In order for the body to use up its reserve fat, it must be transferred from the connective tissue cells, where it is stored, to the cells of the active tissues where it is to be used. This requires that it be reduced to the form of a solution and that it reenter the blood. In other words, it must be digested again. For bringing about these changes a substance identical in function with the steapsin of the pancreatic juice has been shown to exist in several of the tissues.

Although this subject is still under investigation, it may be stated with certainty that there are present in the tissues, enzymes that change dextrose to glycogen and vice versa, that break down and build up the proteins, and that aid in the oxidations at the cells. The necessity for such enzymes is quite apparent.

Summary. - The digested nutrients are taken up by the capillaries and the lymph vessels and transferred by two routes to the circulation. In passing from the alimentary canal into the circulation the more important of the foods undergo changes which adapt them to the needs of the body. Since materials are absorbed more rapidly than they are used, means are provided for storing them and for supplying them to the cells as their needs require. Capability of storage is an essential quality of energy-yielding foods; and substances, such as alcohol, which lack this quality are not adapted to the needs of the body. For causing the chemical changes that occur in the storage of foods, as well as the oxidations at the cells, the presence of active agents, or enzymes, is necessary.

Exercises. - 1. In what respects does the absorption of food materials from the alimentary canal differ from the absorption of a simple liquid by a solid?

2. In what different ways is the small intestine especially adapted to the work of absorption?

3. What are the parts of a villus? What are the lacteals? Account for the name.

4. What part is played by the capillaries and the lacteals in the work of absorption? How does their work differ?

5. What changes, if any, take place in water, common salt, fat, proteins, and carbohydrates during absorption?

6. What double purpose is served by the processes of digestion?

7. Trace the passage of proteins, fats, and carbohydrates from the small intestine into the general circulation.

8. What is the necessity for storing nutrients in the body? Why is it not also necessary to store up oxygen?

9. In what form and at what places is each of the principal nutrients stored?

10. How is the rate of supply of food to the cells regulated? Why is the body unable to regulate the supply of alcohol to the cells when this substance is taken?

11. Explain Fig. 80, page 181. What becomes of the alcohol if this is taken in any but very small quantities?

12. State the general purpose of enzymes in the body. Name the enzymes found in each of the digestive fluids. What ones are found in the tissues?

Practical Work

Illustrate the ordinary meaning of the term "absorption" by bringing the end of a piece of crayon in contact with water, or a piece of blotting paper in contact with ink, noting the passage of the liquid into the crayon or the paper. Show how absorption from the food canal differs from this kind of absorption.

Show by a diagram similar to Fig. 77 the two routes by which the foods pass from the alimentary canal into the blood stream.

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