NUTRITION PROCESSES OF EXCRETION - the blood, as has been more than once said, is subject to continual additions and subtractions on the part of the tissues. The subtractions effected by the tissues are made good in part by the importation of fresh material of food from the alimentary canal. The analogous counterbalancing operations which serve to check the accumulation of used-up tissue-substance in the blood take place in certain organs called excretory. In a strict sense, all organs which cast out material from the body are excretory; the digestive glands, for example, pour their secretions into the alimentary canal, i.e., outside the strict limits of the body. But so much of their constituents are reabsorbed before the alimentary canal is traversed that they may, for practical purposes, be regarded as never having left the body. Even the constituents of tears, when these do not fall over the cheeks or escape at the nose, are reabsorbed from the nasal mucous membrane. With respect to the alimentary secretion, it is merely a small proportion of the bile which remains in the forces that is to be regarded as truly excretory in the sense of being utterly lost to the organism. If excreta are those substances which the body rejects utterly, then we may reckon as excreta (1) the urine, (2) the sweat and oily secretions of the skin, (3) the milk, (4) certain elements of bile contained in the faeces, (5) the gaseous and watery losses at the lungs, (6) the exuviated horny scales of skin and nails and hair, (7) the products of the generative organs, but not those constituents of faeces which are but the undigested remains of food. Among these (if we except the milk and the generative secretions whose elaboration is so peculiar and exceptional) the urine and the excretory products of the lungs and skin are those of paramount importance. The excretion from the lungs will be treated under RESPIRATION ; we shall therefore here concern ourselves with the excretion of kidneys and skin.

Kidneys and their Excretion. - Urine is a clear amber-coloured fluid, somewhat acid in reaction, with a peculiar aromatic odour and bitter saline taste. Its specific gravity varies, consistently with health, within wide limits, being affected very greatly (a) by the quantity of liquid consumed by the individual in a given time, (b) by the greater or less activity of the secretion of sweat. Whilst the average specific gravity may be stated to be about 1.020, it is often temporarily much lower and occasionally considerably higher. As a rule the specific gravity is higher in summer than in winter. The average quantity of urine passed by a healthy adult may be reckoned at 52 fluid ounces, though it is affected by the same causes as those which influence the specific gravity no less than by individual peculiarity and other circumstances.

The urine is essentially a watery solution of certain organic matters, of which much the most abundant and important is urea, and of mineral salts, of which the most abundant is common salt (sodium chloride).

Urea. - A well-nourished man passes, on an average, about 33 grammes (500 grains) of urea in twenty-four hours. This body, which has the composition CII4N20, is looked upon by chemists as carbamide, i.e., it is the amide of carbonic acid, and on this view may be written CO(NI12)2. It is isomeric with ammonium cyanate. This body represents the chief product of the metamorphosis of albuminous or proteid substances in the body.

Even during starvation the destruction of proteid matter continues, and the amount of urea may reach about 230 to 310 grains. The quantity is, however, specially and directly affected by the amount of proteid mattihr contained in the food; so that, for instance, by consuming a sufficient quantity of meat certain persons have excreted as much as 1540 grains of urea. The excretion of urea, which is a measure of the metamorphosis of proteids in the body, is not, as was formerly supposed, directly influenced by the amount of mechanical work done by the body, the greater activity of the muscles not being necessarily accompanied by their greater waste ; indirectly, however, the excretion of urea is increased by bodily exertion, inasmuch as man and other animals appear instinctively to desire and to need a larger quantity of proteid food when much work is to be done than when the body is comparatively at rest.

It was formerly believed that urea was a direct result of the oxidation of proteids in the body. The view actually held is, that it is the product of a synthesis occurring in the body in which a body or bodies resulting from the oxidation of the proteids take part. It has recently been shown that when salts of ammonia are introduced into the system the nitrogen of the ammonia is excreted as urea ; in this case it is obvious that a synthesis must have occurred in which the ammoniacal salt introduced has taken a part no less than some other body. It has for instance, been surmised that in the oxidation of the proteid molecule cyanic acid arises, and that in the presence of water, gives rise to urea, carbonic acid being separated in the reaction.

Uric acid combined with bases exists in small quantities in the urine of man, about 7 to 10 grains being passed by an adult in twenty-four hours, though individual peculiarities appear to affect in a remarkable manner the excretion of this constituent (maximum limits of quantity •3086 to 15.43 grains). In some animals, as in snakes and birds, uric acid and its salts represent the chief nitrogenous excretory product, i.e., take the place of the urea of the urine of man and carnivorous animals.

Uric acid has the composition C5114N403. Its constitution is yet involved in great doubt, in spite of the most profound and extensive researches which have been made on its more or less immediate relations, and in spite of the fact that its synthesis has recently been effected. It may be looked upon in all probability as a derivative of cyanic acid, CNOH, and glycoein, 021-15NO2, and there are many facts which seem conclusively to show that in the system it does not arise directly from a decomposition of proteids, but is the result of a synthesis in which derivatives of these bodies take a part.

Xanthiu, C5H4N 402, hypoxanthin, C5H4N40, and guava, C,II5II50, are immediate derivatives of uric acid, of which the first is a constant constituent of urine, and the second occurs in certain states of disease.

Hippuric acid, C9H9NO2, benzo-amido-acetic acid, is present in small quantities in the urine of man (about 15 grains a day), but is the chief nitrogenous constituent of the urine of herbivorous animals. It is formed whenever benzoic acid, or a body which can yield benzoic acid in the organism, is ingested.

In addition to the organic matters already enumerated the urine contains small quantities of other bodies, amongst which may be mentioned (1) colouring matters, of which one alone is known with any degree of accuracy, and is termed urobilin, a derivative proximately of the biliary colouring matters, and more remotely of the blood-colouring matter, and (2) minute traces of ferments.

As has already been said, the chief salt of the urine is common salt ; there are, however, also excreted in the urine considerable quantities of phosphates and sulphates. The two latter salts represent in part similar salts introduced as such into the system, but in part they are the products of oxidation of organic bodies which contain sulphur and phosphorus respectively; to the former belong the proteids, to the latter certain phosphorus-containing fatty bodies of great complexity, which occur widely diffused throughout the body, but particularly in the white matter of the great nerve-centres and in the nerves.

The changes which urine undergoes after it is passed may be briefly referred to. The faint acid reaction proper to freshly-drawn urine is increased, probably in consequence of an acid-fermentation process started by the mucus accidentally present. The visible effect of this is the deposit of acid urates or free uric acid. In a short time, especially if the urine be kept at a warm temperature, the acid-fermentation is overwhelmed by a fermentation for which organic germs are needed, and which leads to the resolution of urea into carbonate of ammonia. The reaction becomes alkaline, and the smell strongly ammoniacal. Precipitates of ammonia, urate, and ammonio-magnesian phosphate are deposited, the odour becomes putrefactive and multitudes of micro-organisms develop whose germs have been derived from the surrounding air.

Kidney. - The urine is excreted continuously in the kidneys, two organs situated at the back of the abdominal cavity. The fluid is continuously poured by two ducts called ureters into a common reservoir situated in the pelvis, and known as the urinary bladder. From this reservoir the urine is intermittently ejected by the urethra. The two kidneys never secrete symmetrically ; they exhibit an alternation of vascular and secretory activity. Similar variations have been observed in the different portions of one kidney, - first one and then another region of the kidney will be found to be in full activity. Nevertheless, when one kidney is extirpated or unfitted for its function, the other may be capable of the whole work of excretion.

The excretory portion of the kidney, like the secretory portion of all glandular organs, consists of tubes of basement membrane lined with cells of peculiar attributes and surrounded by capillaries for blood and lymph, which allow their fluids to come into close communication with the secreting cells. The complex disposition of the tubes, of which there are hundreds in each kidney, has been traced after an infinite amount of patient research. An account of the arrangement of the tubuli uriniferi falls beyond the scope of the present article.

Excretion of Urine. - A review of the constituents of the urine discloses that the function of the kidneys is to separate from the blood chiefly (1) nitrogenous crystalline bodies which are undoubtedly the end-products of the oxidation of nitrogenous bodies, and (2) inorganic salts and water. We have now to describe the probable manner in which this separation is effected. Ever since the whole course and form of the renal tubules became mapped out, and the existence of a double system of capillaries was established, it has been the habit of physiologists to regard the excretion of urine as a twofold operation. Sir William Bowman, so long ago as 1842, in the course of a histological investigation of the structure of the kidney, came to the conclusion that the watery portions of the urine are excreted in the capsule, while the solid parts are removed from the blood surrounding the lower parts of the renal tubule. Ludwig, about the same time, advanced the theory that the whole of the urine 'is separated from the blood in the glomeruli, but that it is separated in an extremely watery condition, the object of the complicated renal tubules being to permit of the reabsorption of the water, and to bring the urine into a suitable state of concentration for removal from the body. The progress of investigation has completely vindicated the theory proposed by Bowman, which now rests not merely on inference from anatomical structure, but upon a sound basis of physiological facts.

Not only is the process twofold in respect of the two classes of constituents secreted ; it may be twofold also in respect of the processes of the act. There is at least much reason for thinking that the water is separated from the blood mainly, though not entirely, by a physical process of filtration, while some, if not all, of the specific' elements of the urine are secreted by a peculiar selective or elaborative action of living epithelial cells. The reason for supposing that the excretion of water is mainly a process of filtration is the simple one that the flow of urine seems, in a general way, to obey the same laws of pressure as the flow of water through a filter. For example, whatever tends to increase the blood-pressure in the branches of the renal artery tends to increase the flow of urine in a given time. If the heart beats more quickly than usual more urine is excreted. If cold contracts the superficial vessels of the skin, and drives a larger quantity of blood upon the kidney, or if the same result ensues from moderate stimulation of the splanchnic nerves, the blood-pressure in the renal artery becomes raised, and an enlarged secretion follows. If the spinal cord be divided in the lower part of the cervical region, the great fall of blood-pressure which results is associated with suppression of the excretion of urine. Further, if the pressure in the ureters be allowed to reach the value of 10 to 40 mm. (•3937 to 1.5748 inches) of mercury the transfusion of fluid from the blood is prevented. Now the blood-pressure in the renal artery is about 120 to 140 mm. (4.7244 to 5.5118 inches) of mercury. Secretion stops, therefore, long before the pressure in the glandular ducts reaches that of the glandular vessels, - a relationship between pressure and secretion the very reverse of that which obtains in the case of the salivary gland. These facts sufficiently justify the conclusion that the secretion of water in the kidney is a process of filtration. But there are other facts which demand the assumption that even the separation of water is in part an act of living protoplasm. The free ingestion of water by drinking is one of the most certain ways of producing a copious flow of urine ; and yet there is no evidence to show that in most cases this effect is brought about by a raising of the blood-pressure. On the contrary, if one animal be bled into the veins of another there is no increase produced in the amount of urine excreted, and it is difficult to imagine that the blood-pressure may be more easily raised by the drinking of water, however freely, than by introducing into the blood-vessels of one animal the quantity of blood proper to two. In such cases we must assume that the outflow of water from the blood is due to the activity of living cells aiding the normal influence of pressure.

While the secretion of water cannot wholly be ascribed to physical processes, there is no doubt that the secretion of some other bodies is a selective act of living protoplasm taking place independently of, or it may be in opposition to, physical processes. The first substance whose secretion was experimentally proved to be due to the renal epithelium was sulphindigotate of soda, or indigo-carmine. If this substance be injected into the blood-vessels of an animal in which the blood-pressure has been so far lowered by division of the spinal cord below thy medulla that the flow of urine has stopped altogether, it can be traced in a short time into the epithelium of the renal tubules, and through them into the lumen of the duct. The absence of the usual flow of water has left the granules stranded at the place where they entered the renal system of tubes. No trace of the substance was found by the original observer, Heidenhain, in the glomeruli or capsules ; but it may be stated that later experimenters have succeeded, by injecting the drug freely and pursuing it over longer intervals, in tracing it into the glomerular cells also.

Sulphindigotate of soda is not a normal constituent of the urine ; but there is little doubt that what happens in the case of this body happens also in the case of some of the usual constituents. Thus, uric acid has been detected within the epithelium of the renal tubules; and by a comparatively new and beautiful observation it is placed beyond doubt that urea also enters the secretion at the same cells rather than through the glomeruli. This proof was rendered practicable by the discovery that in the frog the kidney is supplied with blood from two separate and distinct sources. The renal artery supplies the glomeruli with blood, while the so-called renal portal vein, a branch from the femoral vein which runs along the outer border of the kidney, supplies the capillary network surrounding the uriniferous tubules. If the renal artery be tied and the blood-supply shut off from the glomeruli, sugar when introduced into the circulation is not excreted with urine, although it readily is when the vascular supply of the kidney is untouched. But in the same circumstances, when the action of the glomeruli is eliminated, urea is readily excreted. Urea and sugar are therefore removed from the blood by different organs; or at least the renal epithelium of the tubules can excrete,urea, although it cannot excrete sugar. At the same time that urea is being excreted a. copious flow of water is determined, which most presumably escapes at the point where the urea is excreted. This fact is significant when we remember that ligature of the renal artery usually stops the flow of water, and that sulphindigotate of soda injected into the circulation when the glomeruli are tied out of it may be traced into the same epithelium, but gives rise to no flow into the bladder.

The renal epithelium, then, has the power of attracting to itself and removing from the blood certain elements which the latter already contains. It may now be asked whether the kidney has the power of elaborating any of the components of its excretion from antecedent forms which it obtains from the blood. In other words, is the kidney a simple separating or straining mechanism, or are the constituents of the urine, like the constituents of bile or saliva, elaborated in the course of metabolic changes going on within the secreting epithelium ?

This question has already been touched in discussing the metabolism of liver-cells, There is little doubt that the kidney exhibits some metabolic activity. Inasmuch as the blood of herbivorous animals contains no trace of hippuric acid, this must presumably be formed within the kidney. If blood containing sodium benzoate and glycodin be passed through the vessels of a fresh kidney, hippuric acid arises. The kidney, therefore, must be assumed to be capable of elaborating hippuric acid out of simpler antecedent forms. But it is not certain that it has any further power. With regard to urea, it appears certain that the activity of the kidney is confined to straining it off from the blood. Normal blood contains about 1 part of urea in 4000, while the blood of the renal vein contains less than this. Urea, therefore, is separated from the blood in the kidney. If the kidneys be extirpated, or if their blood-vessels be ligatured so as to exclude them from the circulation, or if the ureters be tied (the effect of which is speedily to unfit the kidney-epithelium for the work of excretion), then the amount of urea increases in the blood up to 1 part in 300 or 400, while much urea is voided in the fluids ejected from the stomach and intestines (luring such experiments. Similar experiments in birds leave led to similar results in the case of uric acid. All these facts point to the conclusion that urea and the allied bodies in urine are derived proximately from the blood rather than elaborated in the kidney itself. We are therefore justified in regarding the kidneys as almost exclusively an apparatus for purifying the blood from the injurious products of the cell-metabolism of other organs and tissues.

Excretions of the Sweat-Clands: - The second great excretory system is that of the skin, which supplements in important particulars the excretory functions of the kidneys. This function of the skin is effected in great measure by certain glands, called sweat-glands, opening on the surface of the skin. There are, indeed, other glands besides the sweat-glands connected with the skin, viz., the sebaceous glands, which open chiefly into the sacs of hair-follicles and secrete an oily material which keeps the surface of the skin supple and water-tight. The sebaceous secretion resembles in its formation the secretion of milk. Inasmuch as it is not reabsorbed, it is a true excretion ; but there is reason to believe that the material removed from the blood is elaborated out of complex fat-yielding molecules contained in the blood very much as the milk is secreted. We know very little either of the nature of the bodies excreted or of the processes of their formation. The chief excretory products of the skin are furnished by the sweat-glands, and constitute sweat. In addition, however, there is constantly being thrown off from the skin a certain quantity of carbonic acid.

Kature of Sweat. - It is impossible to collect sweat for analysis under perfectly normal conditions ; either the body must be subjected to great heat to adduce a copious flow, or a part of the body must be enclosed in an airtight bag of india-rubber. In both cases the conditions are abnormal. So far as can be ascertained, sweat is a colourless clear fluid of acid reaction and characteristic odour. The odour varies with the part of the skin from which the sweat is obtained. It consists of water containing 1'81 per cent. of solids. The solids are (1) sodium chloride and other inorganic salts, (2) urea and other nitrogenous bodies, (3) fats and cholesterin (which are not altogether due to contamination with sebaceous matter), fatty acids (formic, acetic, butyric, &c., but not lactic), a trace of pigment. In addition to these bodies the skin excretes a small amount of carbon dioxide. Although the excretion of the skin is small in amount (if we except the water), if the escape of it be prevented by varnishing the skin death very quickly ensues. This is probably due to the retention of some poisonous substance, the nature and production of which are very little understood.

In concluding these remarks on excretory organs it may be pointed out that the lungs (looked at as excretory organs), the kidneys, and the skin are all engaged in the great task of ridding the system of its superfluous matters, and that each supplements the action of the others. The lungs are the great excretors of carbonic acid, which is the chief oxidation product of the body, though they share with the kidneys and skin the task of getting rid of water. The kidneys have thrown upon them the task of removing from the system nearly the whole of the nitrogenous waste products and the superabundant salts, besides being the greatest excretors of water. The skin, on the other hand, looked upon as an excretory organ, is second in importance to the kidneys as a remover of water, and comes next to the lungs in separating carbonic acid. The skin, it must be remembered, however, has many functions besides those of an excretory organ, for, besides being an organ of sense, it takes the chief part in regulating the temperature of the animal body.

The chemical changes which occur in all the tissues and organs of the body are, it has been stated, in the main, processes of oxidation, in which energy that was potential in the organic compounds and the oxygen that takes part in them become in great part kinetic. This energy takes the form of mechanical work and heat ; the mechanical work is in part expended within the body itself and ultimately takes the form of heat; in part, however, it is expended upon the objects of the external world, and, though even then ultimately transformed into heat, this is not heat which is available for the purposes of the body. There can be no doubt, however, that a large portion of the total heat evolved in the body is the immediate result of chemical operations, and has not in the first instance taken the form of mechanical work.

In the article DIETETICS (q.v.) attention has been drawn to the amount of energy which is stored up in the organic matters constituting the food of animals, and which can approximately be estimated by determining the amount of heat which the organic matters evolve when burned in a calorimeter. An approximate estimate is thus formed of the energy which is at the disposal of the animal or man whose diet is subjected to study. Thus it has been calculated that the available energy derived from the oxidation of the organic matters of the food of a well-fed man amounts to about 2,700,000 units of heat, the unit chosen being the amount required to heat 1 gramme of water 1° C. When the man is doing no external work this energy is dissipated from the body almost entirely as heat, and, according to the calculations of Helmholtz, the losses of heat are approximately distributed as follows : - Employed in raising the temperature of matters introduced into the alimentary Unnietastof total 70,157 2.6 tion of water in the-lungs 397,536 14-7 Lost by radiation, conduction, and evaporation from the skin 2,162,275 80'1 2,700,000 100'0 The total income and expenditure of energy of an average man in twenty-four hours is thus calculated to correspond to the amount of heat required to raise 595 Is of water from the temperature of melting ice to that of boiling water.

Where, it will be asked, does this transformation of energy chiefly have its seat 1 The answer to this question is that it is firstly in the muscles, then in the glands of the body. At all times, whether• rest or activity, heat is evolved, but the quantity increases in the case both of glands and of muscles as they pass from the former into the latter condition. In the resting body, it has been remarked, the losses of energy are represented by the loss of heat, for the mechanical work done within it takes the form of heat within the body itself. It is difficult, nay, impossible, to calculate the amount of energy which in the first instance takes the form of mechanical work in the body, and which is always transformed into heat. The case of the heart is one in which, however, an approximate calculation can be made. Upon fairly reliable data it has been calculated that the work expended by the heart of a man in twenty-four hours amounts to not less than 627,768 foot-pounds, an amount of work which is equivalent to nearly 45 pound-units of heat, and which represents the energy evolved as heat in the complete combustion of about 386 grains of carbon.

These calculations enable one to form some idea of the magnitude of the nutritive processes which have their seat in the muscular substance of the heart; and to a less degree the same is the case with all the other muscles which are engaged in so-called opus vitale, that is, in the performance of internal work absolutely essential to the continuance of the life of the organism.

The proportion of the total energy of the body which takes the form of mechanical work varies within very wide limits. Assuming that the conversion of potential into kinetic energy in equal times were a constant quantity, it would follow that all external work done diminished the amount of heat set free from the body. To a certain extent it is probable that such a relation exists. It is, however, to be remembered that when external work has to be done there is invariably an increased consumption of organic constituents of food, and therefore an increased store of available energy. The working animal consumes more 0 and produces more CO, than the resting animal. Different animals, like steam-engines of different construction, vary in the proportion borne by the external work they are capable of performing to the total energy which becomes kinetic. Experiments made with the separate muscles of animals, no less than observations on the relation between the external mechanical work done by and the total heat evolved in the body of animals, have, however, shown that animals are more economical machines than the most perfect steam-engines. Whilst the latter cannot convert more than one-eighth of their available energy into work, the animal may yield as much as one-fifth of its energy in the form of available external work.