*General Facts Relating to Storage.*-The form into which the food is converted for storage in the body is that of _solids_-the form that takes up the least amount of s.p.a.ce. These solids are of such a nature that they can be changed back into their former condition and, by dissolving, reenter the blood.

Only energy-yielding foods are stored. Water and salts, though they may be absorbed in excess of the needs of the body, are not converted into other substances and stored away. Oxygen, as already stated (page 108), is not stored. The interval of storage may be long or short, depending upon the needs of the body. In the consumption of stored material the glycogen is used first, then as a rule the fat, and last of all the proteids.

*Storage in the Food Ca.n.a.l.*-Not until three or four hours have elapsed are all the nutrients, eaten at a single meal, digested and pa.s.sed into the body proper. The undigested food is held in reserve, awaiting digestion, and is only gradually absorbed as this process takes place. It may properly, on this account, be regarded as _stored material_. That such storage is of advantage is shown by the observed fact that substances which digest quickly (sugar, dextrin, "predigested foods," etc.) do not supply the needs of the body so well as do substances which, like starch and proteids, digest slowly. Even substances digesting quite slowly (greasy foods and pastry), since they can be stored longer in the food ca.n.a.l, may be of real advantage where, from hard work or exposure, the body requires a large supply of energy for some time. These "stay by" the laborer, giving him strength after the more easily digested foods have been used up. Storage by the food ca.n.a.l is limited chiefly to the stomach.

*Regulation of the Food Supply to the Cells.*-The storage of food materials is made to serve a second purpose in the plan of the body which is even more important than that of supplying nourishment to the cells during the intervals when no food is being taken. It is largely the means whereby the rate of supply of materials to the cells is regulated. The cells obtain their materials from the lymph, and the lymph is supplied from the blood. Should food substances, such as sugar, increase in the blood beyond a low per cent, they are converted into a form, like glycogen, in which they are held in reserve, or, for the time being, placed beyond the reach of the cells. When, however, the supply is reduced, the stored-up materials reenter the blood and again become available to the cells. By this means their rate of supply to the cells is practically constant.

We are now in a position to understand why carbohydrates, fats, and proteids are so well adapted to the needs of the body, while other substances, like alcohol, which may also liberate energy, prove injurious.

It is because foods are of such a chemical nature that they are adapted in all respects to the body plan of taking up and using materials, while the other substances are lacking in some particular.

[Fig. 80]

Fig. 80-*Diagrams ill.u.s.trating the relation of nutrients* and the non-relation of these to alcohol. _A._ Inter-relation and convertibility of proteids, fats, and carbohydrates (after Hall).

_B._ Diagram showing disposition of alcohol if this substance is taken in quant.i.ty corresponding to that of the nutrients (F.M.W.). The alcohol thrown off as waste is unoxidized and yields no energy.

*Why Alcohol is not a Food.*-If the pa.s.sage 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 (Fig. 80), _there is no way of_ _regulating the amount that shall 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 proteid, 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 gasolene 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 cla.s.sed as a food.

*a.s.similation.*-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 _a.s.similation_, 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 a.s.similation. 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. a.s.similation, 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 a.s.similation are shown in Table II.

TABLE II. THE Pa.s.sAGE OF MATERIALS TO THE CELLS MATERIALS DIGESTION ABSORPTION ROUTE TO STORAGE CONDITION THE GENERAL IN THE CIRCULATION BLOOD Proteids Changed In pa.s.sing Through the Become a As proteids into 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 the juices. proteids of inferior the blood. vena cava.

Fat Changed In pa.s.sing Through the As fat in Chiefly as into fatty into the lacteals to the cells minute oil acid, lacteals, the of droplets.

glycerine, the thoracic collective and glycerine duct, by tissue.

soluable unites with which it is soap by the the soluable 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 const.i.tuents the of the capilaries. 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 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 ill.u.s.tration 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 _redigested_. 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 proteids, 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 pa.s.sing from the alimentary ca.n.a.l 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 ca.n.a.l 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, proteids, and carbohydrates during absorption?

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

7. Trace the pa.s.sage of proteids, 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 princ.i.p.al 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 quant.i.ties?

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

Ill.u.s.trate 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 pa.s.sage of the liquid into the crayon or the paper. Show how absorption from the food ca.n.a.l differs from this kind of absorption.

Show by a diagram similar to Fig. 77 the two routes by which the foods pa.s.s from the alimentary ca.n.a.l into the blood stream.

CHAPTER XII - ENERGY SUPPLY OF THE BODY

If one stops taking food, it becomes difficult after a time for him to move about and to keep warm. These results show that food has some relation to the energy of the body, for motion and heat are forms of energy. The relation of oxygen to the supply of energy has already been discussed (Chapter VIII). We are now to inquire more fully into the energy supply of the body, and to consider those conditions which make necessary the introduction of both food and oxygen for this purpose.

*Kinds of Bodily Energy.*-The healthy body has at any time a considerable amount of _potential_, or reserve, energy,-energy which it is not using at the time, but which it is able to use as its needs require. When put to use, this energy is converted into such forms of _kinetic_ energy(69) as are indicated by the different kinds of bodily power. These are as follows:

1. _Power of Motion._-The body can move itself from place to place and it can give motion to things about it.

2. _Heat Power._-The body keeps itself warm and is able to communicate warmth to its surroundings.

3. _Nervous Power._-Through the nervous system the body exercises the power of control over its different parts.

As motion, heat, and nervous power the body uses most of its energy.

*The Source of Bodily Energy.*-As already indicated, the energy of the body is supplied through the food and the oxygen. These contain energy in the potential form, which becomes kinetic (active) through their uniting with each other in the body. Somewhat as the power of the steam engine is derived from the combustion of fuel in the furnaces, the energy of the body is supplied through the oxidations at the cells. How the food and oxygen come to possess energy is seen by a study of the general methods by which energy is stored up and used.

[Fig. 81]