CHAPTER XI - ABSORPTION, STORAGE, AND ASSIMILATION
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 (A, Fig. 76), and contains the following essential parts:
1. An outer layer of epithelial cells, resting upon a connective tissue support.
[pg 174]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 (B, Fig. 76).
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.
Fig. 76—The villi. A. Diagram of a small section of mucous membrane of small intestine. 1. Villi. 2. Small glands, called crypts.
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,66 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[pg 175] 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. Thus, to reach the general circulation, the fat has to pass through the villi, the lacteals, the receptacle of the chyle, and the thoracic duct (Fig. 77). Its passage through these places, like the movements in all lymph vessels, is slow, and it is only gradually admitted to the blood stream.
Fig. 77—Diagram of routes from food canal to general circulation. See text.
2. Route of All the Nutrients except Fat.—Water and salts and the digested proteids 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[pg 176] 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 (Fig. 77). 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 (page 177).
Absorption Changes.—During digestion the insoluble foods are converted into certain soluble materials, such as peptones, maltose, and glycerine,—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. (See Composition of the Blood, page 30.) There are present in the blood, however, substances closely related to the peptones, maltose, glycerine, 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. Thus 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 glycerine, 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.67 They are changed, therefore, into the forms which the body can use.
[pg 177]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. (See Methods of Storage, below.) 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 proteids and carbohydrates are reduced to a simpler form than is necessary for dissolving them.68
The Storage of Nutriment.—For some time after the taking of a meal, food materials are being absorbed more rapidly than they can be used by the cells. Following this is an interval when the body is taking no food, but during which the cells must be supplied with nourishment. It also happens that the total amount of food absorbed during a long interval may be in excess of the needs of the cells during that time; and it is always possible, as in disease, that the quantity absorbed is not equal to that consumed. To provide against emergencies, and to keep up a uniform supply of food to the cells, it is necessary that the body store up nutrients in excess of its needs.
Methods of Storage.—The general plan of storage varies with the different nutrients as follows:
1. The carbohydrates are stored in the form of glycogen. This, as already stated (page 120), is a substance closely resembling starch. It is stored in the cells of both the[pg 178] liver and the muscles, but mainly in the liver (Fig. 78). It is a chief function of the liver to collect the excess of dextrose from the blood passing through it, and to convert it into glycogen, which it then stores within its cells. It does not, however, separate all of the dextrose from the blood, a small amount being left for supplying the immediate needs of the tissues. As this is used, the glycogen in the liver is changed back to dextrose and, dissolving, again finds its way into the blood. In this way, the amount of dextrose in the blood is kept practically constant. The carbohydrates are stored also by converting them into fat.
Fig. 78—Liver cells where is stored the glycogen. C. Capillaries.
Fig. 79—Stored-up fat. The figure shows four connective tissue cells containing small particles of fat. 1. Nucleus. 2. Protoplasm. 3. Fat. 4. Connective tissue fibers.
2. The fat is stored for the most part in the connective tissue. Certain of the connective tissue cells have the property of taking fat from the blood and of depositing it within their inclosing membranes (Fig. 79). When this is done to excess, and the cells become filled with fat, they form the so-called adipose tissue. Most of this tissue is found under the skin, between the muscles, and among the organs occupying the abdominal cavity. If one readily takes on fat, it may also collect in the[pg 179] connective tissue around the heart. The stored-up fat is redissolved as needed, and enters the blood, where it again becomes available to the active cells.
3. The proteids form a part of all the tissues, and for this reason are stored in larger quantities than any of the other food substances. The large amount of proteid found in the blood may also be looked upon as storage material. The proteids in the various tissues are spoken of as tissue proteids, and those in the blood as circulating proteids. The proteids of the tissues serve the double purpose of forming a working part of the cell protoplasm, and of supplying reserve food material. That they are available for supplying energy, and are properly regarded as storage material, is shown by the rapid loss of proteid in starving animals. When the proteids are eaten in excess of the body's need for rebuilding the tissues, they are supposed to be broken up in such a manner as to form glycogen and fat, which may then be stored in ways already described.
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 space. These solids are of such a nature that they can be changed back into their former condition and, by dissolving, reënter 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 Canal.—Not until three or four hours have elapsed are all the nutrients, eaten at a single meal, digested and passed into the body proper. The undigested food is held in reserve, awaiting digestion, and [pg 180]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 canal, 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 canal 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 reënter 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[pg 181] 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—Diagrams illustrating 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 quantity 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 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 (Fig. 80), there is no way of[pg 182] 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 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.
[pg 183]| TABLE II. THE PASSAGE OF MATERIALS TO THE CELLS | |||||
|---|---|---|---|---|---|
| MATERIALS | DIGESTION | ABSORPTION | ROUTE TO THE GENERAL CIRCULATION | STORAGE | CONDITION IN THE BLOOD |
| Proteids | Changed into proteoses and peptones by the action of the gastric and pancreatic juices. | In passing into the capillaries, the proteoses and peptones change into the proteids of the blood. | Through the portal vein to the liver and from there through the hepatic veins into the inferior vena cava. | Become a part of the protoplasm of all the cells. | As proteids in colloidal solution. |
| Fat | Changed into fatty acid, glycerine, and soluable soap by the bile and pancreatic juice. | In passing into the lacteals, the glycerine unites with the soluable soap and fatty acid to form the oil droplets of the blood. | Through the lacteals to the thoracic duct, by which it is emptied into the left subclavian vein. | As fat in the cells of collective tissue. | Chiefly as minute oil droplets. |
| Starch | Reduced to some of the different forms of sugar, as maltose, dextrose, etc. | Enters the capillaries as dextrose. | Through the portal vein, liver, hepatic veins, into inferior vena cava. | As glycogen chiefly by the liver, but to some extent by muscle cells. | As dextrose in solution. |
| Water | Undergoes no change. | Taken up by both the lacteals and capillaries, but to the greater extent by the capilaries. | Both routes, but mostly by way of the liver. | Is not stored in the sense that energy foods are. | As the water which serves as a carrier of all the other constituents of the blood. |
| Common salt | Undergoes no change. | Taken up by the capillaries without undergoing apparent change. | By way of portal vein, liver, and hepatic veins into inferior vena cava. | Not stored. | In solution. |
| Oxygen | Taken up by the capillaries at the lungs. | Already in the general circulation. | Is not stored. | United with the hemoglobin and to a small extent in solution in the plasma. | |
[pg 184]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 reënter 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 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?
[pg 185]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 passage 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 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.
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 energy69 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.
[pg 187]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—Simple device for storing energy through gravity.
Simple Methods of Storing Energy.—Energy is stored by converting the kinetic into the potential form. Two of the simplest ways of doing this are the following:
1. Storing of Energy through Gravity.—On account of the attraction between the earth and all bodies upon the earth, the mere lifting of a weight puts it in a position where gravity can cause it to move (Fig. 81). As a consequence the raising of bodies above the earth's surface is a means of storing energy—the energy remaining stored until the[pg 188] bodies fall. As they fall, the stored-up (potential) energy becomes kinetic and can be made to do work.
2. Storing of Energy through Elasticity.—Energy is stored also by doing work in opposition to elasticity, as in bending a bow or in winding a clock spring. The bending, twisting, stretching, or compressing of elastic substances puts them in a condition of strain which causes them to exert a pressure (called elastic force) that tends to restore them to their former condition. Energy stored by this means becomes active as the distorted or compressed substance returns to its former shape or volume.
These simple methods of storing energy will serve to illustrate the general principles upon which such storage depends:
1. To store energy, energy must be expended, or work done.
2. The work must be against some force, such as gravity or elasticity, which can undo the work, i.e., bring about an effect opposite to that of the work.
3. The stored energy becomes active (kinetic) as the force through which the energy was stored undoes the work, or puts the substance upon which the work was done into its former condition (gravity causing bodies to fall, etc.).
These principles are further illustrated by the
Storing of Energy through Chemical Means.—A good example of storing energy by chemical means is that of decomposing water with electricity. If a current of electricity is passed through acidulated water in a suitable apparatus (Fig. 82), the water separates into its component gases, oxygen and hydrogen. These gases now have power (energy) which they did not possess before they were separated. The hydrogen will burn in the oxygen,[pg 189] giving heat; and if the two gases are mixed in the right proportions and then ignited, they explode with violence. This energy was derived from the electricity. It was stored by decomposing the water.
Fig. 82—Storing energy by chemical means. Apparatus for decomposing water with electricity.
Energy is stored by chemical means by causing it to do work in opposition to the force of chemism, or chemical affinity. Instead of changing the form of bodies or moving them against gravity, it overcomes the force that causes atoms to unite and to hold together after they have united. Since in most cases the atoms on separating from any given combination unite at once to form other combinations, we may say that energy is stored when strong chemical combinations are broken up and weak ones formed. Energy stored by this means becomes active when the atoms of weak combinations unite to form combinations that are strong.70
How Plants store the Sun's Energy.—The earth's supply of energy comes from the sun. While much of this, after warming and lighting the earth's surface, is lost by radiation, a portion of it is stored up and retained. The sun's energy is stored both through the force of gravity71[pg 190] and by chemical means, the latter being the more important of the two methods. Plants supply the means for storing it chemically (Fig. 83). Attention has already been called to the fact (page 112) that growing plants are continually taking carbon dioxide into their leaves from the air. This they decompose, adding the carbon to compounds in their tissues and returning the oxygen to the air. It is found, however, that this process does not occur unless the plants are exposed to sunlight. The sunlight supplies the energy for overcoming the attraction between the atoms of oxygen and the atoms of carbon, while the plant itself serves as the instrument through which the sunlight acts. The energy for decomposing the carbon dioxide then comes from the sun, and through the decomposition of the carbon dioxide the sun's energy is stored—becomes potential. It remains stored until the carbon of the plant again unites with the oxygen of the air, as in combustion.
Fig. 83—Nature's device for storing energy from the sun. See text.
The Sun's Energy in Food and Oxygen.—Food is derived directly or indirectly from plants and sustains the same relation to the oxygen of the air as do the plants themselves. (The elements in the food have an attraction for[pg 191] the oxygen, but are separated chemically from it.) On account of this relation they have potential energy—the energy derived through the plant from the sun. When a person eats the food and breathes the oxygen, this energy becomes the possession of the body. It is then converted into kinetic energy as the needs of the body require.
Fig. 84—Simple apparatus for illustrating transformation of energy. Potential energy is converted into heat and heat into motion.
From the Sun to the Cells.—It thus appears that the body comes into possession of energy, and is able to use it, through a series of transferences and transformations that can be traced back to the sun.72 Coming to the earth as kinetic energy, it is transformed into potential energy and stored in the compounds of plants and in the oxygen of the air. Through the food and the oxygen the potential energy is transferred to the cells of the body. Then by the uniting of the food and the oxygen at the cells (oxidation), the potential becomes kinetic energy and is[pg 192] used by the body in doing its work. The phrase "Child of the Sun" has sometimes been applied to man to express his dependence upon the sun for his supply of energy.
Why Oxygen and Food are Both Necessary.—The necessity for introducing both oxygen and food into the body for the purpose of supplying energy is now apparent. The energy which is used in the body is not the energy of food alone. Nor is it the energy of oxygen alone. It belongs to both. It is due to their attraction for each other and their condition of separation. It cannot, therefore, become kinetic except through their union. To introduce one of these substances into the body without the other, would neither introduce the energy nor set it free. They must both be introduced into the body and there caused to unite.
Bodily Control of Energy.—A fact of importance in the supply of energy to the body is that the rate of transformation (changing of potential to kinetic) is just sufficient for its needs. It is easily seen that too rapid or too slow a rate would prove injurious. The oxidations at the cells are, therefore, under such control that the quantity of kinetic energy supplied to the body as a whole, and to the different organs, is proportional to the work that is done. This is attained, in part at least, through the ability of the body to store up the food materials and hold them in reserve until they are to be oxidized (page 180).
Animal Heat and Motion.—Most of the body's energy is expended as heat in keeping warm. It is estimated that as much as five sixths of the whole amount is used in this way. The proportion, however, varies with different persons and is not constant in the same individual during different seasons of the year. This heat is used in keeping the body at that temperature which is best suited to [pg 193]carrying on the vital processes. All parts of the body, through oxidation, furnish heat. Active organs, however, such as the muscles, the brain, and the glands (especially the liver), furnish the larger share. The blood in its circulation serves as a heat distributer for the body and keeps the temperature about the same in all its parts (page 33).
Next to the production of heat, in the consumption of the body's energy, is the production of motion. This topic will be considered in the study of the muscular system (Chapter XV).
Some Questions of Hygiene.—The heat-producing capacity of the body sustains a very important relation to the general health. A sudden chill may result in a number of derangements and is supposed to be a predisposing cause of colds. One's capacity for producing heat may be so low that he is unable to respond to a sudden demand for heat, as in going from a warm room into a cold one. As a consequence, the body is unable to protect itself against unavoidable exposures.
Impairment of the heat-producing capacity is brought about in many ways. Several diseases do this directly, or indirectly, to quite an extent. In health too great care in protecting the body from cold is the most potent cause of its impairment. Staying in rooms heated above a temperature of 70° F., wearing clothing unnecessarily heavy, and sleeping under an excess of bed clothes, all diminish the power of the body to produce heat. They accustom it to producing only a small amount, so that it does not receive sufficient of what might be called heat-producing exercise. Lack of physical exercise in the open air, as well as too much time spent in poorly lighted and ventilated rooms, tends also to reduce one's ability to produce heat. Moreover, since most of the heat of the body comes[pg 194] from the union of oxygen and food materials at the cells, a lack of either of these will interfere with the production of heat.
Results of Exhaustion.—Through overwork, or excesses in pleasurable pursuits, one may make greater demands upon the energy of his body than it can properly supply. The resulting condition, known as exhaustion, is not only a matter of temporary inconvenience, but may through repetition lead to a serious impairment of the health. It should be noted, in this connection, that the energy of the body is spent in two general ways: first, in carrying on the vital processes; and second, in the performance of voluntary activities. Since, in all cases, there is a limit to one's energy, it is easily possible to expend so much in the voluntary activities that the amount left is not sufficient for the vital processes. This leads to various disturbances and, among other things, renders the body less able to supply itself with energy.
The Problem of Increasing One's Energy.—Since the energy supply is kept up through the food and the oxygen, it might be inferred that the introduction of these substances into the body in larger amounts would increase the energy at one's disposal. This does not necessarily follow. Oxidation at the cells is preceded by digestion, absorption, circulation, and assimilation. It is followed and influenced by the removal of wastes from the body. A careful study of the problem leads to the conclusion that while the energy supply to the body does depend upon the introduction of the proper amounts of food and oxygen, it also depends upon the efficiency of the vital processes. The maximum amount of energy may, therefore, be expected when the body is in a condition of perfect health. Hence, one desiring to increase the amount of his energy must[pg 195] give attention to all those conditions that improve the health.
Effect of Stimulants on the Energy Supply.—In the effort to get out of the body as much as possible of work or of pleasure, various stimulants, such as alcohol, tobacco, and strong tea and coffee, have been used. Though these have the effect of giving a temporary feeling of strength and of enabling the individual in some instances to accomplish results which he could not otherwise have brought about, the general effect of their use is to lessen, rather than to increase, the sum total of bodily power. The student, for example, who drinks strong coffee in order to study late at night is able to command less energy on the day following. While enabling him to draw upon his reserve of nervous power for the time being, the coffee deprives him of sleep and needed rest.
The danger of stimulants, so far as energy is concerned, is this: they tend to exhaust the bodily reserve so that there is not sufficient left for properly running the vital processes. Evidences of their weakening effect are found in the feeling of discomfort and lassitude which result when stimulants to which the body has become accustomed are withdrawn. Not until one gets back his bodily reserve is he able to work normally and effectively. Increase in bodily energy comes through health and not through the use of stimulants.
Summary.—The body requires a continuous supply of energy. To obtain this supply, materials possessing potential, or stored-up, energy are introduced into it. The free oxygen of the air and the substances known as foods, on account of the chemical relations which they sustain to each other, contain potential energy and are utilized for supplying the body. So long as the foods are not oxidized, the energy remains in the potential form, but in the process of oxidation the potential energy is changed to kinetic energy and made to do the work of the body.
Exercises.—1. In what different ways does the body use energy?
2. Show that a stone lying against the earth has no energy, while the same stone above the earth has energy.
[pg 196]3. How does potential energy differ from kinetic energy?
4. What kind of energy is possessed by a bent bow? By a revolving wheel? By a coiled spring? By the wind? By gunpowder?
5. How does decomposing water with electricity store energy?
6. Account for the energy possessed by the oxygen of the air and food substances.
7. Trace the energy supply of the body back to the sun.
8. Why must both oxygen and food be introduced into the body in order to supply it with energy?
9. How may overwork and overexercise diminish the energy supply of the body?
10. How may one increase the amount of his energy?
PRACTICAL WORK
Suggested Experiments.—1. The change of kinetic into potential energy may be shown by stretching a piece of rubber, by lifting a weight, and by separating the armature from a magnet.
2. The change of potential into kinetic energy may be shown by letting weights fall to the ground, by releasing the end of a piece of stretched rubber, and by burning substances.
3. The change of one form of kinetic energy to another may be illustrated by rubbing together two pieces of wood until they are heated, by ringing a bell, and by causing motion in air or in water by heating them. If suitable apparatus is at hand, the transformation of electrical energy into heat, light, sound, and mechanical motion can easily be shown.
4. A weight connected by a cord with some small machine and made to run it, will help the pupil to grasp the general principles in the storage of energy through gravity. A vessel of water on a high support from which the water is siphoned on to a small water wheel will serve the same purpose.
5. The storing of energy by chemical means may be illustrated by decomposing potassium chlorate with heat or by decomposing water by means of a current of electricity.
6. Study the transfer of energy from the body to surrounding objects, as in moving substances and lifting weights.
Fill a half gallon jar two thirds full of water and carefully take the temperature with a chemical thermometer. Hold the hand in the water for four or five minutes and take the temperature again. Inference.