The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

If a kidney be split longitudinally, it will be noticed that its outer part, the cortex, is darker in colour than its inner part, the medulla ([Fig. 9]). The glomeruli already referred to occur in the cortex. The medulla is occupied by radiating tubules, collected into groups. Those of each group converge towards a common duct. From twelve to eighteen ducts open into the expanded end of the ureter, each at the apex of a pyramid. If the section of the kidney be examined with a lens, it will be seen that narrow rays from the medulla extend into the cortex. The cortex is therefore made up of interdigitating pyramids of dark substance, consisting of glomeruli and the contorted tubules, about to be described, and of lighter substance, consisting of straight tubules continuous with those of the medulla.

Fig. 9.—The Upper End of the Left Kidney, vertically divided, and magnified.

It is invested by a capsule with which, at the hilus, the dilated end of the ureter blends. A portion of a papilla (the end of a pyramid) is shown projecting into one of the calices into which the ureter dilates. The peripheral portion of the kidney containing glomeruli and contorted tubes is termed its cortex, the central portion medulla. At A is shown a single urinary tubule. Commencing at the third glomerulus, it winds in the cortex, descends into the medulla, turns in a loop of Henle, again winds in the cortex, and ends in a collecting tube, which joins a duct. The arrangement of the bloodvessels is shown at B. A straight artery and a straight vein lie side by side. The artery gives branches to the glomeruli. The venules from the glomeruli again divide into capillaries, which supply the contorted tubes and loops of Henle. The ducts are supplied by long arterial capillaries. C shows the structure (magnified) of a glomerular tuft of capillary vessels, invested by a capsule which closes into a contorted tube, ct; dH, a descending limb; aH, an ascending limb of a loop of Henle; d, a duct.

The urinary tubules are the separate pieces of apparatus of which the kidney consists. The problems connected with a single tubule are therefore the problems of the kidney as a whole. These structures are all exactly alike. The description of any one of them applies to all. Each begins as a capsule containing a glomerulus. The wall of the bulb—which is merely a thin basement membrane covered by epithelial scales—is involuted by the tuft of bloodvessels. The vessels do not penetrate its capsule. Between the tessellated epithelium which covers the tuft and the similar epithelium which lines the capsule there is a space communicating by a narrow aperture with the next portion of the tubule—termed its “contorted” part, because it is twisted about like a tangled thread in the cortex of the kidney. The contorted tubule is of relatively large calibre. The cells which line it are irregular in form and indistinct in outline. The basal half of each cell, between its nucleus and the basement membrane, is vertically striated, or “rodded,” as it is usually termed. Such an arrangement of the protoplasm of a cell is commonly associated with a habit of absorbing fluid. It would seem to indicate in this case that the cells take water and various substances dissolved in water from the direction of the basement membrane. After a time the contorted portion of the tubule, although still sinuous, becomes more nearly straight—the “spiral portion”—and assumes a radial direction. In the zone between the cortex and the medulla, the spiral portion tapers into an exceedingly slender tubule which, after running some distance in the direction of the hilus, turns back again towards the cortex, making a loop, known as the “loop of Henle.” The ascending limb of this loop is of larger calibre than the descending limb. The descending limb is lined by flattened epithelium, each cell so thin that (in microscopic sections as ordinarily prepared) its nucleus bulges into the lumen of the tube. The cells of the ascending limb are more nearly cubical in form. On reaching the cortex, the tubule again becomes contorted. The second contorted portion narrows into a “collecting portion,” which joins a ductule. The ductules unite together, until at last a single duct is formed which opens at the apex of a pyramid. The cells of the ductules are cubical or columnar. Their cell-substance is clear, whereas that of the cells lining other parts of the tubule is cloudy in appearance.

Such a tubule, viewed as a hydrostatic mechanism, presents three portions, evidently fitted for different functions: (1) The glomerulus is an apparatus which allows of the rapid exudation of water from blood. (2) The contorted portions of the tubule present the appearance of a secreting mechanism. The large soft, cloudy cells which line them are eminently fitted to take from the blood, or rather from the lymph which fills the tissue-spaces which intervene between the walls of the capillary bloodvessels and tubules, the various substances which they excrete. (3) The loop of Henle is a remarkable piece of apparatus, the purpose of which has been a subject of much controversy. Looking at it from the point of view of hydrostatics, it seems safe to conclude, from its extremely narrow bore, that it raises the pressure of the fluid in the glomerulus and first contorted portion; but it may have other functions also.

A consideration of the arrangement of the bloodvessels of the kidney bears out the conclusion that the secreting apparatus is divisible into at least two separate portions, possibly into three. The glomeruli are supplied by short and relatively wide arterioles. Each arteriole breaks up, as soon as it enters the capsule, into a bunch of capillary vessels, which, in the same abrupt manner, reunite to form a venule. On leaving the capsule, this little vein behaves in a fashion for which the only parallel is to be found in the portal system of the liver. Instead of uniting with a larger vein, it again breaks up into capillary vessels, which supply the contorted tubules and loops of Henle. The medulla of the kidney is supplied by long arterial capillaries of the usual type. The short arterioles of the glomeruli are controlled by nerves which, constricting them, or allowing them to dilate—possibly by actively causing them to dilate—rapidly diminish or increase the amount of blood passing through their tufts of capillary vessels. Here, therefore, is a mechanism by which the glomeruli can be suddenly flushed with blood—a condition favourable to exudation into the urinary tubules. The interposition of a second set of capillaries prevents this sudden flushing from unduly disturbing the pressure in the vascular system as a whole. In the renal-portal capillaries of the kidney the blood-pressure is fairly constant and, presumably, low. The use of the term “renal-portal” is justifiable, not only on the ground that the vessels of the kidney behave like those of the portal system of the liver, but also owing to the very significant fact that in fishes and amphibia the kidney actually has a double blood-supply. In such an animal as the frog the glomeruli are supplied with arterial, the tubules with venous, blood. The glomeruli receive branches from the renal artery, the tubules from a portal system derived from veins of the abdomen and hind-legs.

Sir William Bowman, who in 1842 gave the first detailed description of the microscopic structure of the kidney, concluded that, whereas “the tubes and their plexus of capillaries are probably the parts concerned in the secretion of that portion of the urine to which its characteristic properties are due (the urea, lithic acid, etc.), the Malpighian bodies [i.e., the glomeruli] may be an apparatus destined to separate from the blood the watery portion.”

All physiologists are in accord in regarding the glomeruli as the principal seat of exudation. There is great diversity of view as to the function of the tubules. In 1844 Ludwig advanced the opinion that all the constituents of the urine pass through the glomeruli in a large excess of water, and that in the course of the tubules this excess of water is reabsorbed. This theory was based, among other considerations, upon the extreme thinness of the epithelium which covers the glomerular tufts; he judged that water would filter through it very readily. A large amount of experimental work has been directed to the solution of these two problems—viz., (1) Do urea and other similar substances pass through the glomeruli? (2) Is water returned from the tubules to the venous system? Our views as to the functions of the kidney as a whole will not be greatly influenced by the answers that may eventually be given to these questions; yet their discussion is of very great interest, owing to the nature of the evidence which may be marshalled on either side.