From an anatomical point of view, the urinary apparatus consists of two kidneys with their ducts, named the ureters, which open into a mus-culo-membranous sac, the bladder, and this again has a tube, short in the female, of considerable length in the male, by which the fluid collected in the bladder is discharged from the body at convenient intervals, and is named the urethra. In the male this tube terminates at the extremity of the penis, which it traverses along its whole length. From a physiological stand-point, the kidneys are organs by which the excess of water, as well as the salts and the nitrogen of the body, are got rid of, the latter substance being chiefly in the form of urea, of uric and hippuric acids, and creatinine, which represent the waste of the proteids or albuminous and albuminoid components of the tissues.
The Kidneys (fig. 131) are two in number, one being situated on each side of the lumbar vertebrae, partly under cover of the last ribs, and rest-ins; against the under surface of the loins, where they are embedded in much fat. The right kidney is somewhat heart-shaped and rather the larger of the two. It reaches to the level of the 16th rib and touches the liver in front; it weighs about 27 oz. The left is more bean-shaped, is situated rather farther backwards than the right, touching the spleen in front, and weighs about 25 ozs. The external surface of each kidney is smooth, and on its inferior surface is in great part covered by the peritoneum or lining membrane of the belly. The inner border is deeply notched to form the hilus of the kidney, wherein is lodged the pelvis - a small sac or receptacle into which the urine is first received. Here also the renal arteries enter, the renal veins emerge, and the ureter begins and continues its course from the pelvis to the bladder. Each kidney is enclosed in a dense membrane or capsule, which in health can be easily stripped off the proper substance of the organ, whilst in some forms of disease it is firmly adherent. If a kidney be divided by a horizontal cut (fig. 132), into an upper and a lower half, a difference in colour will be noticed between the outer cortical portion and the inner medullary portion. The outer or cortical portion is of dark reddish-brown colour and finely granular aspect; the inner or medullary portion is lighter in colour and presents a number of fine lines, converging towards the cavity in the centre of the kidney named the pelvis. Both the cortical and medullary zones are chiefly composed of delicate tubes - the urinary tubules,- - together with many blood-vessels, and the difference in their aspect is due to the difference in the form and arrangement of these tubuli uriniferi.
Fig. 131. - The Kidneys, Ureters, and Bladder.
RK, Right kidney. L K, Left kidney. A, Right ureter. B, Abdominal aorta, C, D, E, F, I, Arteries arising from the same. G, Bladder. H, Pelvis.
If we follow one of these tubuli (fig. 133) from its commencement in the outer or cortical substance of the organ to the point where it terminates by opening into the pelvis of the kidney, we find that it begins with a little dilatation or bulb termed the capsule of Malpighi (fig. 133, c), about 1/125th of an inch in diameter, from which proceeds a cylindrical and much-contorted tube that lies in the cortical zone and has a diameter of about 1/500th of an inch. The tube then enters the internal or medullary zone, where, becoming much narrowed, it forms a long loop, the loop of Henle, and having reascended towards the outer or cortical zone becomes once more coiled, and finally joins with others to form a collecting tube. These winding tubes are lined by a layer of cells which secrete or separate the urine from the blood, after which the collecting tubes convey it into the pelvis of the kidney, whence it passes into the ureter, and so on to the bladder. The cells by which the urine is secreted vary in form in different parts. Those lining the little bulb or capsule of Malpighi are flattened; those in both of the convoluted portions are columnar and striated or fibrillated; whilst those lining the loop of Henle are flattened and clear in the descending, and striated in the ascending portion. These differences in the character of the cells seem to be associated with differences in function, for, if certain colouring-matters are injected into the blood, they are not found in the cells lining the capsule, but they deeply stain those parts in which the striated cells are found.
Fig. 132. - Section through Kidney.
A, Ureter. B, Renal capsule. C, Cortex. D, Medulla. E, Renal vessels. F, Pelvis.
The arrangement of the blood-vessels of the kidney presents several points of great interest. In the first place they are very large for the size of the organ, and consequently the whole mass of the blood circulating the body traverses the kidneys in a comparatively short space of time. Now the constituents of the urine, being of a poisonous nature, are jealously removed from the blood by the cells of the kidney as soon as they enter it. In consequence of this rapid excretion we find that under healthy conditions very delicate chemical examination is requisite to demonstrate their presence in the blood at all.
Thus, for example, the quantity of urea discoverable in the blood passing to the kidney by the renal arteries does not exceed 0.016 part in 1000 in health, and is still less in the venous blood returning from them.
In the next place, there is a double system of capillary vessels which is not found in any other organ of the body. The renal arteries entering the kidney at the hilus break up into arches which are situated at the junction of the medullary with the cortical zone. From this region numerous large branches run to the cortex, where they break up into minute afferent branches, one of which runs to each little capsule of Malpighi, and, deeply indenting the wall, divides into a little ball of branching and intertwined capillaries (fig. 133, a), which unite together again to form an efferent vessel. This efferent vessel (fig. 133, v), which would elsewhere be termed a vein, after a short course again divides, as before, like an artery into a net-work of capillaries which are distributed over the outer side of the cortical and convoluted portion of the renal tubules. These, reuniting, form the proper renal veins by which the blood is conveyed out of the gland.
Fig. 133. - Uriniferous Tubules. a, a, Artery; v, », vein; c, Malpighian corpuscle. A, The cortex or cortical substance. B, Boundary layer. c, Papillary portion, showing the loop of Henle.
Mr. Bowman pointed out that the peculiar arrangements of the bloodvessels are well adapted for the secretion and excretion of such a fluid as the urine. The little balls or glomerules of capillaries which are formed by the first divisions of the renal arteries are well adapted to permit the escape of the watery parts of the blood, which, if the expression may be used, flushes the renal tubuli through their entire length, and in doing so dissolves and washes away the urea and hippuric acid and salts which are secreted by the cells lining the convolute portions of the tubules, and which have been separated by them from the blood coursing, under comparatively low pressure, through the second plexus of capillaries which surrounds them.