Chemical Composition of the Body.—It has been estimated by various writers that the human body has an approximate average chemical composition[51] of—

Dependence of the body upon Food.—The human body, like any other piece of machinery, undergoes a constant wear and tear incidental to the work it performs, but in the human machine this is not all that must be included in its upkeep. The replacing of the dead and cast out cells with new ones, the repairing of the worn cells, the furnishing of heat, not only for the running of the engine but for the maintenance of the body temperature,—all of these must be considered and cared for if life is to continue. In man-made machinery, the renewal of the worn parts, and the replacing of those no longer useful must be accomplished by an outside agency. But in the body this work is performed by the organism; and the material used for the purpose, as well as that which is used to furnish the heat necessary for the internal and external activities of the body is food.

Exceptions to this Rule.—Under normal conditions the body never uses its own structure either for fuel or to replace tissue losses. In starvation the body rebuilds its important tissues, such as the nerves and glands, at the expense of the less important ones, such as the connective tissues and the skeletal muscles (Taylor).

Science has proved that for the most part the body does not use the food materials in their original form, but carries them through a series of transformations into substances more easily handled by the organism.

Roughly speaking, we may say that the body carries the foodstuffs through practically four processes on the pathway through the body, namely, digestion, absorption, metabolism, elimination.

Processes Included in Digestion.—There are several processes concerned in this transformation of the food materials. Some are purely mechanical and have to do with the movement of the food mass through the digestive tract: others are of a chemical character and bring about distinct changes in the food materials themselves. These mechanical and chemical processes with the retarding and stimulating agents that influence them are called digestion.

Absorption.—After the food materials have undergone digestion, or simplification into more available substances, these substances are absorbed, that is, they are passed through the membranes lining the walls of the intestinal tract, and thence to the blood.

Metabolism.—The utilization of the transformed food materials and their final fate in the human body is included under the term Metabolism. Elimination.—After the food materials have been utilized to the extent of the body’s ability to handle them, their waste products are cast out of the organism by way of the skin, the lungs, the intestines and the kidneys.

A brief description of these processes seems desirable here.

Digestion.—Mechanical digestion begins in the mouth, where through the action of the teeth, the tongue and the muscles of the jaw, the food material is ground up and liquefied to a certain extent and made ready for the chemical action which takes place, to a limited degree only, as a result of the salivary enzyme in the mouth. The eating of food causes a flow of saliva from the three pairs of large salivary glands, and from the numerous secretory cells situated in the membranes of the mouth. As a rule the food stays for too short a time in this organ for any appreciable amount of chemical action to take place, but the liquefaction of the food mass with the salivary juices which contain the ferment (ptyalin), prepares for its passage into the gastric organ, and allows the digestion of the starch (the only foodstuff affected by salivary digestant), to continue in that part of the stomach until its action is checked by the hydrochloric acid in the gastric juice.

Arrangement of Food in the Stomach.—To simplify the study of the gastric organs it may be well to think of the stomach as being divided into three regions, i.e., “the fundus, the middle region, and the pyloric end,”[52] each of which differs slightly from the other. After being swallowed, the food enters the region situated at the cardiac end, known as the fundus.

Motor Processes in the Stomach.—There are no peristaltic waves in the fundus of the stomach, and the movement of the food mass is accomplished through the stretching and contraction of the muscular walls of the organ which tends to churn and further mix it with the salivary juices as it is gently pushed out into the middle region. In this region the peristaltic waves begin and travel toward the pylorus and increase in force as digestion progresses, ceasing only with the emptying of the organ. When the first stratum of food reaches the middle of the stomach it is caught by these oscillating peristaltic waves and forced forward through the pyloric region and against the pylorus, from whence it is returned back through rings of constriction. This forward and backward movement continues as long as there is food in the stomach, thus thoroughly mixing the mass with the gastric juice and allowing the enzymes existing in the juices to have an opportunity for action (chemical digestion).

Passage of Food from Stomach.—The material prepared in the stomach, known as chyme, is passed into the duodenum through the pylorus. The opening of this sphincter is controlled, according to Cannon, to a certain extent by the liquefication of the chyme, but more especially by the presence of free acid in the stomach side of the pyloric orifice.

Behavior of Food in the Intestines.—The food does not pass at once along the canal, but waits in the duodenum until several portions have passed through. As the food mass is made alkaline in the presence of the intestinal juices, the pyloric valve closes, opening again as the contents nearest it on the stomach side are acidified.

Intestinal Movements.—The peristaltic waves in the small intestines begin in the upper part and start a course ever downward. These waves in the intestines are two-fold in character; the quick shallow wave which forces the food string forward, breaking it up into segments, and backward joining the segments together again, and the strong deep wave which carries the entire mass forward after each segmentation. This method of movement in the small intestines is the best one possible under the conditions which prevail in this region of the digestional apparatus, since it not only mixes the food material with the juices necessary for its digestion, but likewise spreads it out over a wide space, insuring a greater contact with the absorbing walls of the small intestines.

The Effect of Muscular Constrictions.—The muscular constrictions occurring in the intestines producing segmentation of the food string have, according to Sherman, the effect of “(1) further mixing of the food and digestive juices, (2) bringing the digested food into contact with the absorbing membrane, (3) emptying the venous and lymphatic radicles in the membrane, the material which they have absorbed being forced into the veins and lymph vessels by the compression of the intestinal walls.”[53]

Movements in the Large Intestine.—The movements in the large intestine or colon are much like those in other parts of the digestive tract. The small and large intestine are divided by a valve known as the ileocecal valve, and any food which passes through it cannot return, since the valve is a competent one. The cells in the walls of the larger intestine secrete fluids of a lubricating character, containing no enzymes of digestion but aiding in moving the fecal matter toward the rectum.

Distribution of Secretory Cells.—Secretory cells are distributed in each of the three regions of the stomach, but are more numerous in the middle region than at either end. The third region includes the pyloric vestibule through which all foods must pass before they can enter the small intestine, and terminates in the pylorus, the valve which shuts off the stomach from the duodenum and the rest of the intestinal canal.

Chemical Digestion.—The chemical changes in the food materials, after they are eaten, are brought about through the action of certain substances known as soluble ferments or “enzymes.” These enzymes exist in every tissue of the body, and their province is first to break down the food materials themselves into simpler compounds, and then to reconstruct the simpler substances into those of a more complex character, which are more available for the various uses of the organism.

Action of the Enzymes.—As Sherman has stated, “all fermentation is brought about either directly or indirectly by the activity of animal or vegetable organisms or cells. When the organisms or cells act directly and the chemical changes occur only in their presence, the fermentation is said to be due to an organized ferment. When the action is not brought about directly by the cell itself, but by means of a substance secreted by the cell but acting apart from it, this substance is called a soluble or unorganized ferment or ‘enzyme.’” The enzymes concerned in digestion and metabolism, their source and their action, may be found in the following table:

TABLE OF ENZYMES[54]

Source and Action of Enzymes

Classification of Enzymes.—Sherman classifies the enzymes of the body according to their effects:

1. The hydrolytic enzymes:

(a) Proteolytic or protein-splitting enzymes.
(b) Lipolytic or fat-splitting enzymes.
(c) Amylolytic or starch-splitting enzymes.
(d) Sugar-splitting enzymes.

2. The coagulating enzymes, such as thrombin or thrombase (the fibrin ferment) and rennin, which causes the clotting of milk.

3. The oxidizing enzymes or oxidases (which, if the oxidation be accompanied by a splitting off of amino groups, may be called “deaminizing” enzymes).

4. The reducing enzymes or “reductases.”

5. Those which produce carbon dioxide without the use of free “deamidizing” oxygen, such as zymase of yeast.

6. Enzymes causing the breaking down of a larger into a smaller molecule of the same composition, as in the production of lactic acid from glucose.

DIGESTION (CHEMICAL)

Salivary Digestion.—The table shows that enzymic action begins in the mouth. Saliva, the characteristic secretion of this region, contains the enzyme ptyalin which exerts its influence upon the starches and dextrins. The food mass remains in the mouth for so short a time, however, that a very small percentage of the starch is changed to maltose under salivary digestion. The action of ptyalin, however, continues in the fundus of the stomach until stopped by the acid in the gastric juice.

Gastric Digestion.—The conditions existing in this region of the gastric organ of digestion are particularly favorable to the continuance of salivary digestion on account of the neutral character of the juices secreted by the cells there, and because there is so little movement taking place. The cells in the middle region, however, secrete a fluid rich in acid, and as the food mass is gradually pushed forward by the contraction of the stomach walls into this portion of the stomach, further conversion of starch and dextrin to maltose is checked. Gastric juice is secreted by cells situated in all parts of the stomach. The character of the secretions differs in different parts of the organ. However, that in the fundus is neutral in character or even slightly alkaline, according to Howell, while that in the middle region is highly acid. The pyloric end of the stomach exhibits strong peptonizing powers and much of the hydrolysis of protein takes place here. As the food is pushed out of the fundus it is caught by the waves of peristaltic action and swept toward the pylorus. This movement of the food mass to and from the pylorus under the influence of the muscular constriction in the stomach tends to mix it thoroughly with the juices in all parts of the stomach, and in a measure to liquefy it to the “souplike” mixture known as chyme.

Rate of Carbohydrates, Proteins, and Fats.—Carbohydrates, for example, do not require any acid for their digestion, hence all of the acid with which they come in contact can go toward acidifying them, while the proteins require hydrochloric acid before the enzymes can begin to exert their activities. Consequently they leave the stomach much more slowly than the carbohydrates. The fats leave more slowly than any of the other food combinations. If carbohydrates and proteins are taken together they leave the stomach more slowly than if the carbohydrates were fed alone, but more quickly than they would if the meal consisted of protein alone. When the meal consists of fats and proteins, the stomach is emptied more slowly than is the case when either is fed alone.

Intestinal Digestion.—Digestion proceeds in an orderly manner throughout the intestinal canal. The pancreatic juice, bile, and intestinal juice are poured upon the food mass on its entrance into the duodenum. The enzymes work simultaneously. Trypsin in the pancreatic juice takes up the hydrolysis of the proteoses and peptones and those proteins which have escaped gastric digestion. The amylopsin likewise in the pancreatic secretion acts upon the starch and dextrin, changing them to maltose. The lipases split the fats to fatty acids and glycerol.

The erepsin in the intestinal juice, “succus entericus,” brings about further change in the proteins, with the production of amino acids. The bulk of the carbohydrates are converted into monosaccharids in the small intestines. The lactose, maltose, and sucrose are changed through the activity of the lactase, maltase, and invertase into glucose. Sherman states that “it is possible that the splitting of the lactose (milk sugar) may occur in the intestinal wall rather than in the food mass.”[55]

Bile.—Human bile, the secretion most actively concerned in the digestion and absorption of the fats, contains water, bile salts, bile acids, bile pigments, cholesterin, lecithin, and a peculiar protein derived from the mucous membranes of the bile ducts and gall bladder.

Stimulation of Intestinal Secretions.—The flow of the intestinal juices is stimulated by a substance or hormone known as “secretin.” This hormone is the result of the action of hydrochloric acid upon some substance in the intestinal wall. Starling claims that the formation of hormones and their circulation through the blood to the reactive tissues is sufficient to account for the activity of the pancreas; he doubts if the nervous system plays any part in the activity of that organ.

Secretion of Water in the Stomach.—The secretion of water by the cells of the stomach is such, according to Taylor, as to produce chyme of quite constant consistency, the solid particles being held in suspension in the fluid medium.

Factors Influencing Gastric Digestion.—The factors influencing digestion in the stomach constitute all those mechanical, electrical, chemical, and psychical factors which stimulate or retard the action of the gastric juices. The movements in the stomach are involuntary, but their activities may be stimulated by the flow of gastric juice. Sleep retards digestion in the stomach by retarding the movements in the organ itself.

Stimuli to Gastric Flow.—The division and liquefaction of the food in the mouth hastens gastric digestion by making the food better fitted for the action of the enzymes in the gastric juice. The type as well as the character of the food acts as a stimulus to the gastric secretion.

Water is probably the best of all the agents for stimulating the secretion of gastric juice, while dextrin (toast, zwieback) and the extractives of meat likewise exert similar powers.

Retarding the Gastric Flow.—The nervous system, on the other hand, at times checks or entirely inhibits a flow of these juices. Worry, excitement, anger, fatigue, chill; each plays its part in promoting poor digestion in the gastric organ. As chemical factors, water and salts are the two necessary substances for gastric digestion, since the enzymes in the juices cannot act except in their presence.

Alkaline carbonates and fatty foods both check the flow of gastric juice, and retard digestion. The psychic factors which result in a stimulation of the secretory cells in the stomach are exerted through the sight, smell, and taste. One often hears the expression: “The food looked, smelled, or tasted so good that it made my mouth water.” This actually occurs; hence the secretion has been named appetite juice. This appetite juice acts as a direct stimulant to the cells in the mucous lining of the stomach, causing a flow of gastric juice. It cannot be said to cause digestion, but it certainly institutes that process, thus starting the whole digestional procedure.

Digestion in the Larger Intestine.—Science has proved that most of the nourishing part of the food ingested is digested and absorbed before it reaches the larger intestine. The two portions of the alimentary canal known as the small and large intestine are separated by the ileocecal valve. Cannon claims “that this valve is competent, that is, under normal conditions the food mass which passes through into the colon cannot be forced back into the small intestine.” The food mass sometimes contains materials which have escaped digestion, likewise some of the active enzymes which bring about their hydrolysis, in which case a certain amount of their digestion may continue in the large intestine.

So far, investigators have found no enzymes in the fluids secreted by the cells in the walls of the large intestine, but they have found an alkaline fluid which assists in completing the digestion of the foods which has started in other parts of the intestinal tract.

Absorption.—Absorption of food occurs in all parts of the intestinal canal, but the major portion of it occurs in the small intestines, the mucous membrane lining of which seems particularly adapted for this purpose. According to Taylor there is no absorption of fats, carbohydrates, or proteins in the stomach. Other investigators believe that some of the protein is absorbed and also some glucose in concentrated solution. However, the stomach cannot be considered of great value as an absorbing organ. Physiology teaches that the absorption of the products of digestion occurs by means of the millions of small projections or villi with which the intestinal wall is lined. These villi contain numerous capillary blood vessels and spaces known as lacteals. The former converge into the portal vein, the latter into the lymphatic vessels and thence into the thoracic ducts.

The Absorption of Fat.—The fats, as has been described, are split into their two constituents, fatty acid and glycerol. The former is dissolved by the bile to form soap; the latter is readily soluble in water. These constituents thus dissolved pass through the walls and recombine in the form of neutral fat droplets, probably during the passage through the walls, since they appear in this form in the cells. They pass into the thoracic duct and thence into the blood stream.

Absorption of Carbohydrates.—The carbohydrates are chiefly absorbed in the form of monosaccharids. This has been proved by introducing cane sugar or lactose into the blood vessels and getting the greater portion of it back unchanged by way of the urine. As the monosaccharids are taken up by the capillaries lining the walls of the small intestines, they are passed on to the portal vein and carried by the portal blood into the liver, where they are stored temporarily as glycogen, and given out to the blood in the form of glucose as needed. After a meal rich in carbohydrates, the portal blood will be rich in glucose, while the blood in general circulation contains about the same amount as usual, about 0.1%.

Absorption of Proteins.—The absorption of the products of protein digestion occurs through the capillary blood vessels and passes on to the portal vein. The metabolism of protein is more complex than that of any of the foodstuffs. It is probable that each living cell contains enzymes which are capable of breaking down the body proteins with the production of amino acids just as the proteins of the food are broken down by enzymes of digestion, and according to Sherman “it is not improbable that protein synthesis also may be brought about by every living cell.”

The Absorption of Water.—This does not occur in the stomach, as was formerly believed, but in the small intestines.

The Absorption of the Mineral Salts.—This occurs in conjunction with the other food material. Some of the mineral salts are much more soluble than others and are more readily absorbed. The function of the mineral salts in the body has already been described, and since they form a part of every tissue and fluid in the body their absorption and fate in metabolism must be studied with that of the other chemical combinations.

ABSORPTION IN THE LARGE INTESTINES

The digestion of the food as it is passed into the last portion of the alimentary canal has been largely completed. However, that part which has escaped digestion in the small intestine is finished and absorbed here. The important peristaltic waves occurring in this region are antiperistaltic in character and have the property of churning the food thoroughly and bringing a larger portion of it in contact with the absorbing walls. The water which is left in the food mass together with the products of the digestion of the foodstuffs is absorbed in the first part of the large intestine, leaving the remainder more solid. This residue is known as feces or fecal matter.

The diagram on page 179 shows the various processes through which the foodstuffs pass after absorption.[56]

Bacterial Action in the Alimentary Canal.—The changes in the foods so far mentioned have been chiefly the result of the activity of the enzymes existing in the various digestive processes throughout the body. But there are other changes which occur in the foods during their sojourn in the digestive tract which are not accountable to enzymic action, but which, in fact, modify to a certain degree the changes wrought by the enzymes. These are the result of the activity of certain specific bacteria which inhabit the entire digestive tract of the individual from a few hours after birth until death. Some of these have so adapted themselves to the existing conditions that, unless present in overwhelming numbers, they are not only harmless, but they actually assist in protecting the organism from the inroads of more harmful species. Many experiments have been made to find whether or not bacteria are essential to human nutrition, and the results of these experiments prove that they are not. However, since they are so firmly established in the body it is well to study the various types and learn as much as possible of the products of their activity and the influence which they exert in human nutrition.

Types of Bacteria.—It would be impossible and unnecessary to consider the action of all of the bacteria in the body in this text, but it is necessary to consider those which are prominent in bringing about decomposition of the foods in the digestive tract. Sherman holds that there are three main types having this property: “(1) the bacteria of fermentation, such, for example, as the lactic acid bacteria; (2) the putrefactive bacteria, such as the anaËrobic B. aËrogenes capsulatus; (3) bacteria of the B. coli type, showing the character of both the fermentative and putrefactive organisms but tending in general to antagonize the putrefactive anaËrobes.”[57]

Fermentation in the Stomach.—In the stomach, fermentation of the carbohydrates with the production of organic acids, and at times alcohol, occurs. The types of fermentation taking place in the stomach are alcoholic, lactic, butyric, acetic, formic, oxalic, and cellulose. The bacteria inhabiting the gastric organs are dependent upon air for existence, while those in the intestines are not.

Factors Influencing Excessive Fermentation.—The factors influencing excessive fermentation in the stomach are lack of “tone” and motility in the organ, insufficient amount or absence of free hydrochloric acid in the gastric secretion, dilatation of the stomach, and an excess of carbohydrate foods in the diet. Of the latter, sucrose and glucose are especially susceptible to the action of fermentative bacteria. Under normal conditions, that is, in health, the conditions prevailing in the stomach are very unfavorable to the development of bacteria of the putrefactive type, the gastric juice exhibiting decided germicidal properties. Then, too, the presence of air acts against their development. Much of the so-called gastric fermentation does not occur in the stomach but rather in the duodenum.

Bacterial Action in the Intestines.—In the lower part of the small and in the large intestines, the bacteria of the anaËrobic type increase, conditions more favorable to their development existing there than farther up in the intestinal tract. However, there are a great many bacteria in the whole of the small intestine. Those producing decomposition of the unabsorbed proteins are especially prominent in the colon.

Herter[58] states that “the presence in the colon of immense numbers of obligate micro-organisms of the B. coli type may be an important defense of the organism in the sense that they hinder the development of that putrefactive decomposition which, if prolonged, is so injurious to the organism as a whole. We have in this adaptation the most rational explanation of the meaning of the myriads of colon bacilli that inhabit the large intestine. This view is not inconsistent with the conception that under some conditions the colon bacilli multiply to such an extent as to prove harmful through the part they take in promoting fermentation and putrefaction.”

Effect of Bacterial Activity in the Body.—In summarizing the effects of bacterial action in the body it is found that with the exception of oxalic acid, which is exceedingly injurious, and which, according to Herter, results from the eating of large quantities of meat and sugar, the products of fermentation are simply irritating in character, while those resulting from putrefaction are distinctly toxic. Among the substances deserving mention under this head we have indol, skatol, cresol, and phenol. These substances are very soluble and upon absorption combine with the sulphuric acid formed in the body and are excreted by way of the kidneys where they appear in the urine as “conjugated sulphates,” the chief of which is indican. The amount of indican in the urine is taken as a measure of the intensity of the putrefaction taking place in the body.

Metabolism.—Under the term metabolism we understand the series of processes through the foodstuffs and carried (a) in the conservation of the tissues of the body and (b) in the maintenance of body temperature and physical work (Taylor). The processes concerned in metabolism are chiefly those of building up, “anabolism,” and breaking down, “catabolism.” In the processes of anabolism the products absorbed are built into the tissues and cells of the body. In catabolism, the worn particles from the cells, and the dead cells no longer useful are broken up and thrown out of the body. According to Taylor, “side by side with these processes are the reactions of combustion, whereby the temperature necessary for the life of the cells is maintained, and the energy needed for external work furnished.”

Behavior of Carbohydrates in Metabolism.—As Sherman[59] has said: “At least two kinds of enzymes are believed to be involved in the combustion of glucose in the tissue cells, (1) cleavage enzymes, which split the molecule into fragments more easily oxidized, and (2) oxidizing enzymes or oxidases which stimulate the oxidation of the cleavage products. Both kinds of enzymes are widely distributed through the body and are believed to be normal constituents of all active cells.”

Production of Energy.—It has been proved that the energy for external and internal work is produced largely from the glucose brought by the blood and oxidized in the muscles.

When a surplus amount of carbohydrate food is eaten, over and above the immediate needs of the body for fuel, it is stored in the liver and muscles as glycogen, which can be readily reconverted into glucose. When the supply of carbohydrate food is greatly in excess of the body’s needs, that is when the liver and muscles cease to store glycogen, it is built up into adipose tissue and furnishes a readily available source of emergency fuel.

Fate of the Carbohydrates.—After their oxidation the end products of carbohydrates, that is, the substances which are no longer available for use in the body, leave it in the form of carbon dioxide and water by way of the kidneys (urine), the skin, the lungs, and the intestines.

Fate of the Fats.—The fats upon absorption are taken up by the lymph vessels instead of the capillaries and enter the blood with the lymph. According to various investigators, the fat which causes the turbidity of the blood plasma at the height of absorption will, as a rule, disappear after a few hours, part of it being burned as fuel, producing energy for the internal and external work of the body, and at least a part of the fats eaten being rebuilt into body fat. The end-products of fat metabolism, like those of the carbohydrates, consist of carbon dioxide and water, and leave the body by the same excretory channels.

When the normal oxidation of the fatty acids is interfered with or is overtaxed, a different reaction from that which usually occurs may take place, and this results in an excretion of acetone in the urine (see Chapter on Diabetes).

Protein metabolism is certainly more complex than that of either of the other active organic food groups. The amino acids which are the products of protein digestion are taken up by the capillary blood vessels in the intestinal walls and are passed by them into the portal vein, soon to become available for the needs of the body.

Fate of the Proteins.—After utilization in the body, the proteins, like the other foods, leave certain waste products which indicate to a greater or lesser extent the completeness with which the organism has made use of the food materials. The end-products of protein metabolism are: urea, ammonium salts, purin bodies, and creatinin. These products leave the body chiefly in the urine. The chief end-product in man is urea. This substance represents from 82-88% of the total nitrogen excreted by the kidneys. However, the less highly oxidized products represent the incomplete products of protein metabolism and thus indicate the changes through which these products must pass before being changed into urea. If for any reason there is an impairment of the liver through which they must pass and where the change into urea is accomplished, there will be a rise of ammonia and a corresponding decrease in the output of urea in the urine. Thus, ammonia is formed at the expense of the urea. This occurs in fevers, diabetes, and certain structural diseases of the liver. According to Sherman:[60] “Normally about 2 to 6% of the total nitrogen eliminated is in the form of ammonium salts, the amount depending largely upon the relation between the acid-forming and base-forming elements in the food.”

Acid-forming and Base-forming Foods.—Mendel[61] states: “There are foods which act as potential acids and others which function as bases in the organism. When burned up either in the laboratory or in the body cells, they have a preponderance of acid or base, as the case may be, in their ash.” In this respect potatoes, apples, raisins, and cantaloupes, for example, are base-forming foods which depress the output of ammonia and increase the solubility of uric acid in the urine, whereas meal, cereals, and prunes (the latter with their content of benzoic acid) furnish acids in predominance.

Purin Bases.—These compounds are formed in the body as cleavage products of nucleoproteins or taken into the body in food. The chief of these products are adenin, guanin, hypoxanthin, xanthin, and uric acid. The latter is the most highly oxidized of all the purin bases and is the form in which they are chiefly eliminated in the urine.

Formation of Uric Acid.—The formation of uric acid can in a measure be controlled by attention to the diet, eliminating those foods known to be purin-bearing. Normally from 1 to 3% of the nitrogen eliminated will be in the form of uric acid. The normal human being oxidizes about half of the purins eaten and excretes about half, mainly in the form of uric acid. According to Mendel, the formation of uric acid takes place throughout the body, and its partial destruction is accomplished by the kidneys, muscles, and liver. The formation of purins in the body and their elimination in the form of uric acid is especially significant in certain pathological conditions, gout, for example, in which the body has difficulty in eliminating these compounds.

The purin bodies are both endogenous and exogenous—that is, they may be brought into the body in food as such, or they may be formed as a result of the metabolism of the body tissues. For this reason the damage wrought by these substances may to a certain extent be controlled by eliminating the purin-bearing foods from the diet. Flesh-foods are high in purins, especially the highly nucleated glandular organs, liver, thymus (sweetbreads), etc., kidney, beef, mutton, veal, pork, chicken, turkey, goose, sardines, anchovies, all kinds of fish except cod. Among the vegetable foods asparagus, beans, peas, and spinach are highest in purins. Boiling extracts much of the purins from food. Meat especially should be prepared by this method, if used in the diet of individuals suffering from gout. Eggs and milk are purin free, and may be used freely. Certain substances increase the difficulty of eliminating uric acid. Alcoholic beverages for example are especially deleterious.

Creatinin.—This end-product of protein metabolism is, like uric acid, endogenous and exogenous. It is one of the normal constituents of the urine. The quantity is fairly constant for the individual, averaging about 0.02 gram per kilogram of body weight per day.

Mineral Metabolism.—A study of the organic foodstuffs reveals the fact that many of the mineral salts concerned in nutrition enter the body in organic combination with those constituents. Certain of the mineral salts, however, enter, exist in and leave the body in the same organic form in which they occur in the food materials. This is true of chlorine, which for the most part, functions in and leaves the body in the form of chlorides (chiefly sodium chloride). A small part of the chlorine is used in the production of the hydrochloric acid of the gastric juice.

Sulphur and iron, both enter the body as essential constituents of proteins, and their metabolism occurs with that of these foodstuffs; the sulphur being converted largely into sulphuric acid must be neutralized at once, and it leaves the body by way of the urine as inorganic sulphates. Part of the sulphates are excreted as ethereal (conjugated) sulphates; the amount excreted in this form depending largely upon the extent of purification in the intestinal tract.[62]

Phosphorus.—This mineral salt is found to be present as an essential constituent of certain proteins, fats and carbohydrates. It also enters the body in the form of inorganic phosphates. During the digestion and metabolism of the organic foodstuffs the phosphoric acid radical is split off and eventually nearly all of the phosphorus leaves the body in inorganic form (inorganic phosphates).

Calcium.—Being the chief constituent of the bones, large quantities of calcium salts are stored in the skeleton of the child both before and after birth. The functions of calcium have already been discussed. That part not stored, which has finished its work in the body is excreted through the intestinal wall and leaves the body by way of the feces, only a small part of the calcium concerned in metabolism being excreted in the urine.

The Process of Osmosis in the Body.—The influence exerted upon the process of osmosis in the body is one of the most important parts played by the mineral salts in metabolism. The fact that these chemical substances are indispensable to the metabolic processes makes it necessary for the nurse to know where they can be found in food and how best to use them.

Metabolism of Body Tissues.—The constant breaking down and building up of the tissues of the body and the evolution of heat as a by-product of the energy expended may be summed up in the term “metabolism.” The metabolism of the body is normally supported by the food ingested. However, it is a known fact that were no food eaten the processes would continue just the same, the difference only being the use of the body structure instead of food materials. According to Sherman, the chemical changes and energy transformations are of course inseparable. It has become customary to speak of the metabolism of matter and the metabolism of energy, and to regard the extent of the metabolism of any material substance as measured by the amount of its end products eliminated, and the extent of the energy metabolism as measured by the amount of heat or of heat and external muscular work which the body gives off.

In summarizing the important factors in the utilization of food by the body several distinct points are made evident:

1. The composition of the body, and the composition of food, are practically the same.

2. Food must make good the losses resulting from metabolic processes and physical activity.

3. Utilization of food by the body necessitates:

(a) Transformation through a series of processes:
(1) Digestion.
(2) Absorption.
(3) Metabolism.
(4) Elimination.
(b) Factors influencing use of food:
(1) Stimulation of secretory cells through appetite juice (stomach); hormone secretin (intestines).
(2) Factors retarding use of food:—Fear, worry, anger, nervous excitement, fatty foods and alkaline carbonates (bacterial action causing fermentation and putrefaction).

PROBLEMS