EXCRETION is the elimination from the body of the waste products which would otherwise accumulate within it, and, in the case of many of these substances, would poison it. These sub stances come from many sources. Some are formed in the process of digestion, and are absorbed from the alimentary canal into the blood, changed to some extent by the liver but never built into the tissue. Other waste products are produced by the breakdown of body tissue, muscle, nervous tissue, glandular tissue, blood pig ments and the like. To this latter class of waste products the term endogenous has been applied, to the former the term exogenous.
I. The Skin.—The skin was probably the most primitive form of excretory organ. In forms of life which have no blood system, waste material is shed from the skin or from cells derived from it. In man there are two fluids excreted by the skin, namely, sweat (or perspiration) and sebum—the oily excrement of the glands attached to the hair roots.
Sweat is credited with being composed of about 99% water, .5% sodium chloride and traces of organic material of which urea is the principal. About 700 cu.cm. of water leave the skin per day in ordinary weather. Some of this is insensible evapora tion from the surface of the cells rather than definite sweat secreted by the sweat glands. But taking it all as sweat—not more than 3.5 grams of urea per day (and probably much less) could be accounted for as cutaneous secretion—about one tenth of the amount secreted by the kidneys.
Nevertheless this potentiality of the skin to serve as an excre tory organ can be turned to account in cases of kidney disease in order to relieve the kidney. Thus residence in Egypt or some place of similar climate attains this end.
The secretion of sweat, apparently unlike that of urine, is under the direct ordering of the central nervous system (see SYMPA THETIC SYSTEM) . Many foreign substances such as guanine, iodine, arsenic, mercury and certain organic acids are capable of being excreted from the body in the sweat. The main function of the perspiration is however concerned with the regulation of tem perature (see ANIMAL HEAT).
The liver pours bile into the intestine, this fluid containing among its constituents bile salts, bile pigments, lecithin and choles trin. Bile-pigments (bilirubin and biliverdin) represent the break down of the haemoglobin of the blood. They consist substantially of the haemoglobin minus the globin and the iron. Factors which increase the breakdown of blood will therefore increase the quan tity of bile pigment eliminated. In the intestine, these pigments undergo reduction so that the actual yellowish colour of the faeces is due to the reduced product stercobilin. If the normal path of excretion of bile pigment be interfered with bilirubin becomes deposited in the skin, the condition being that of jaundice.
Certain metals, iron and possibly calcium appear to be excreted through the bowel and of course in abnormal cases water in large quantities may be so excreted.
Much of the solid portion of the faecal matter consists not of the residues of food but of actual material produced in the bowel— the dead bodies of bacteria, the residue of cells which have been shed by the mucosa and so forth. This fact perhaps accounts for the large amount of purine bodies found in the faeces as com pared with the urine.
Taking, however, the example of a person not engaged in active employment and on a standard diet—such for instance as an acci dent case in hospital—the composition of the urine secreted in 24 hours would be somewhat as follows: In examining the particulars given it will be observed that the kidney does not merely abstract certain substances such as urea, which are present only in small quantities in the blood, but it re frains from abstracting other substances such as protein and sugar which though present in considerable quantities in the blood are absent or nearly so from normal urine.
Of the materials enumerated in table i two only are known to be produced in the kidney itself, hippuric acid and ammonia salts. The other nitrogenous bodies are brought to the kidney in the blood.
Urea is by far the most important nitrogenous constituent of normal urine. It is made in the liver from amino bodies or am monium salts and represents the final breakdown of protein, whether of the food or of the tissues. Normally more than half the urea is formed directly from the breakdown of proteins in food. In birds there is little urea, the principal nitrogenous excre ment being uric acid. This gives the avian droppings their white solid nature.
Uric acid in mammals represents only the waste products of the nuclei of cells.
Creatinine is probably the constituent in the daily output which is most constant in amount, being 1-2 grams per day and is be lieved to be entirely of endogenous origin though the precise sub stances from which it is formed are by no means certain.
The actual process of urinary excretion consists of an act by which urine is derived from blood plasma. The average composi tion of the two may be compared. The following are some of the more bulky constituents of each, expressed in percentages :— For any one component the actual work is calculated in the fol lowing formula. If W be the work in calories : T the absolute temperature : C the molecular concentration of the substance in the arterial blood and C' that in the urine and V the volume of urine secreted in litres— Take for instance the case of urea secretion per day. V= 1.5 litres: T = 3 z o° absolute: C= .004 gram molecules per litre (.024%) : C'=4 gram molecules per litre (2.4%).
As the efficiency of the kidney is not likely to be less than that of other organs, which means that at least three times as much energy must be expended by the organ as is actually represented by work accomplished, therefore the expenditure of energy in se creting this one constituent of the urine is about 4,00o calories which represents about • t 5 of the total energy of the food eaten per day.
One of the simplest forms of kidney is that of the earth-worm, though possibly the word "kidney" is scarcely applicable here, inasmuch as the multitudinous tubules of which the renal ap paratus is composed are not bound up into a single organ but exist separately—a pair being situated in each typical segment of the worm. Each tubule is distinct from its neighbours. The tubule is open at each end, one orifice leads from the interior of the body cavity of the worm, the other to the outer air. The body cavity of the worm contains fluid ; this fluid passes in greater or less quantities down the tubule. Its passage is promoted by the movement of the ciliary coat which lines the tubule. But that is not all: the tubule, which is a much coiled affair, is not uniform either in calibre or in the structure of its walls. In cer tain parts the so-called "glandular" portions, the tubule is of greater diameter than elsewhere, the wall is thicker and the cells of which the wall is composed, are made of granular protoplasm— protoplasm of the type which we are accustomed to associate with glandular secretion. These portions of the tubule also are well supplied with blood vessels, and it is clear that in some way there is an exchange of material between the blood and the fluid passing down the tubule. But what is the nature of this exchange? Is material transferred from the tubule fluid to the blood ; or is it transferred from the blood to the interior of the tubule ; or do both operations take place? Does the cell of the tubule say in effect, "Here is all this body fluid running to waste ; let me reclaim such of its water and other constituents as may be useful to the body and restore them to the blood"? Or does it say, "Here is a water-course running to the exterior; let me pick out such things from the blood as are superfluous and cast them into this channel"? Or can the tubule perform both these functions? These are ques tions difficult to answer, but they are among those asked con cerning all the higher forms of kidney.
Passing to the vertebrates, the kidney of the embyronic dog-fish is not entirely unlike the excretory system of the worm. It, too, consists of a series of tubes, passing down a portion of the body. A typical tube in this embryo opens by a funnel into the body cavity ; the other end of this tube does not, like that of the worm, open directly into the external air, but into a longitudinal duct which gathers up all the urine from all the tubules on its side of the body. Something new, however, appears on the tubule of the dogfish : a sort of hollow shaped expansion or cup grows out in one place. Into the cavity of the cup grows a knot of blood vessels called the glomerulus. The cup is called the Malpighian Body. Such primitive tubules may develop into sperm ducts or kidney tubules; in the latter case the opening into the body cavity be comes closed. There remains, therefore, a series of closed tubules which form the kidney, each of which has two contacts with the blood (I) along the general length of the tubule, this being sur rounded with capillaries and (2) at the Malpighian Bodies, one or more of which occurs on each tubule.
The same question arises with regard to the function of the human kidney as in the case of the kidney of the earth-worm. Does this tubule cell abstract material from within the tubule and transfer it to the blood, thus saving it for the organism? Or does the tubule cell select material from the blood and transfer it to the tubule? Or do both processes occur? But in addition there is another question not met with in the tubule of the worm—what is the function of the glomerulus? Physiologists would agree that in a sense the function of the glomerulus has much in common with that of the funnel of the earth-worm ; that is to say, the glomerulus is the source of the stream of fluid which passes down the tubule. As, however, this fluid is derived from the blood and not from the coelom, a series of open tubes would allow the animal to bleed to death. Some permeable filter must be placed between the blood vessel and the tubule ; and such a filter is the combined capillary wall and sur face of the Bowman's capsule. Through this filter, so far as is known, neither red blood corpuscles nor plasma proteins normally pass.
As to precisely what other materials pass through the glomerular filter there is no real agreement. Some authorities believe that all the constituents of plasma, other than their proteins are so filtered and that the urine attains its ultimate composition by the reclama tion of some of the constituents as the fluid passes down the tubule. If this theory is correct, all the sugar which filters into the cavity of the Bowman's capsule would have to be either reabsorbed or oxidised in the tubular passage, for the urine is normally free from sugar. Moreover, since the concentration of urea in urine is seventy (or more) times that in plasma, it follows that if no urea is added to the urine in its way down the tubule, seventy times as much water must be filtered through the glomerulus as appears in the urine, i.e., about ioo litres per day.
At the other end of the scale is the view that only water and certain crystalline bodies such as salt pass into the tubules, through the glomerulus but that the remainder, including the nitrogenous bodies, the phosphates, the sulphates, etc., leave the blood vessels which surround the tubules and are secreted into the fluid which is passing down the tubules. On this theory both the functions of the glomerulus and of the tubules would be acts of secretion involving the transformation of potential energy stored in the cells. Between the two views put forward there are many grades of opinion. Thus it is quite possible to believe that the glomerulus is simply a filter and the tubule an apparatus which is capable both of secretion and reabsorption. If such a view seems somewhat more complicated than either of the two extreme views, it must be borne in mind that the histology of the renal tubule is also very complicated and it seems probable that different histological units should have different functions.
It is a matter of common experience that much more urine is secreted at some times than at others. Exposure to cold, for instance, causes increased flow (diuresis). The cause of such changes is to be sought frequently in the regulation of the blood supply to the kidney. So far as may be judged from observation of the frog's kidney, only a fraction of the whole number of renal tubules are in use ; the remainder are usually out of action be cause blood is withheld from them. When the skin is cold the blood flow through it becomes reduced and by way of compensa tion the kidney vessels open out, increasing probably not only the amount of urine which passes down each tubule but also the whole number of tubules in action. Some articles of diet, tea, coffee, etc., increase the flow of urine. This is in part simply due to the amount of water which they contain, but in part to chemical ingredients which stimulate them, e.g., theobromine, caffeine, etc., such as are known as diuretics.
While some diuretics may act directly on the kidney-cells, others undoubtedly act on the renal vessels, in either case promot ing the flow of urine.
Among the paradoxes presented by the kidney there is none greater than its relation to the central nervous system. Apparently the cells of the kidney are not themselves directly supplied with nerves, yet their activity responds with great delicacy to the emo tions, shock producing suppression of urine, while fear or excite ment causes untoward production. Here also there is little doubt that in the regulation of the blood-flow is to be found the key to this paradox.