With respect to the shape of the paired kidneys the evolutionary tendency is towards compactness and consequently there is a certain parallel between their form and the general contour of the body. The kidneys of the primitive eel-like cyclostomes are long straplike bands, while in fishes generally they extend not only throughout the length of the body cavity but also they may penetrate even beyond into the tail musculature. Frequently too, the typical shape of the kidneys of fishes is modified to conform to the presence of the swim bladder, which in fishes is a bedfellow of these organs.
Among amphibians the wormlike Gymnophiona and the long-bodied urodeles have correspondingly elongated kidneys, narrower anteriorly and widening posteriorly, while in the squat anurans these organs become much more compact and rounded in shape.
Although lizards and alligators somewhat resemble urodeles superficially, the relation between the shape of the kidneys and the form of the body is less marked. The kidneys, however, are still somewhat elongated, but in turtles become decidedly compact, conforming to the rigid requirements of space imposed by the shell. The opposite extreme is shown by snakes that have the kidneys not only attenuated like the body but also entirely crowded out of the typical side by side position, so that they lie tandem-fashion one behind the other.
The concavities of the elaborate pelvis of birds, in which the kidneys are for the most part packed, form a restricting bony casket that determines their tabulated form. The highest degree of compactness is found among mammals.
The bear (Fig. 360), ox, seal, walrus, and porpoise have lobed kidneys, a condition appearing also in human embryos but which becomes obliterated soon after birth.
Fusion of the two kidneys occurs frequently in fishes, and in some lizards, at least at the posterior end, as well as in many birds. Posterior fusion of the kidneys may exceptionally appear even in man, when a so-called “horseshoe kidney” results.
If for any reason one of two kidneys is put out of commission, the other usually enlarges into a “compensating kidney,” taking over the work of its incapacitated mate in addition to its own.
The kidneys are closely associated with the dorsal wall of the body cavity where they lie outside of the peritoneum.
In fishes and birds they fit with intimate snugness along either side of the backbone, but in amphibians, reptiles, and mammals are less closely attached to the body wall, sometimes projecting into the body cavity (Fig. 361). In all of these classes the kidneys are usually retroperitoneal, behind the peritoneum, although in some mammals they may even hang free, enclosed in a peritoneal envelope which entirely surrounds them.
Symmetry of position is quite fixed in birds whose kidneys are rigidly held side by side in depressions of the pelvis, but it is less apparent in mammals where, being less restricted, one kidney is usually not exactly opposite the other. The left kidney in man is ordinarily situated at a somewhat higher level than the right, although many exceptions to this arrangement have been observed.
Each human kidney weighs about four and one-half ounces, and in its three major dimensions measures slightly more than four by two by one inches. It is shaped like a “kidney bean,” with a convex lateral and a concave medial margin. The depression of the concave margin is the hilus, where the renal artery and the nerves enter and the renal vein and ureter make their exit (Fig. 362).
Gross Structure of the Mammalian Kidney
When split lengthwise in the frontal plane the mammalian kidney is seen to be a hollow organ with walls of very unequal thickness on the convex and the concave sides. The excentric cavity within is known as the renal pelvis. This cavity is really a funnel-shaped expansion of the urinary duct, or ureter. The thicker walls are the solid substance of the kidney itself, made up of a mass of nephridia, blood vessels, and connective tissue, which even to the naked eye appears to be differentiated into a narrow, outer, rather uniform cortical zone, and a wider, inner, more diversified medullary zone, bordering immediately on the renal pelvis. The entire structure is surrounded by a tightly fitting capsule of connective tissue, the tunica fibrosa.
The medullary zone presents a number of cone-shaped segments, the Malpighian pyramids, the bases of which rest against the cortical layer while the apices, papillae, project into the cavity of the pelvis. There are seven to twenty pyramids in the human kidney. Each cup-shaped part of the pelvis that surrounds a papilla is called a calyx. Between the Malpighian pyramids are masses of blood vessels, forming the columns of Bertini, that are made up of subdivisions of the renal arteries and veins on their way to and from the renal tubules of excretion which are crowded together in the Malpighian pyramids. The narrow outer layer of cortex upon which the bases of the Malpighian pyramids rest, lies just underneath the tunica fibrosa. Due to the presence of the cortical rays, it has a striated appearance.
The kidneys of many animals, for example, Echidna, marsupials, insectivores, rodents, carnivores, perissodactyls, and apes, that are unipyramidal with only a single papilla (Fig. 363), are never lobed, since the number of pyramids determines the number of lobes. The true multilobed condition of the human kidneys, which is most plainly apparent at about the fourth fetal month, is masked by the growth of parts which eventually fill in the interstices between the lobes. The relation of all these parts is described in the following section dealing with the microscopic structure of a single urinary unit or nephridial apparatus, and its position in the Malpighian pyramid.
A Urinary Unit
A urinary unit is a transformed nephridium that has gained an intimate connection with the blood system and established an avenue of drainage to the outside. The various parts of such a unit are pictured in Figure 364.
The junction where the nephridial tube makes contact with the blood stream is called the renal corpuscle (Fig. 365). It consists of a spherical tuft or knot of arterial capillaries, the glomerulus, enveloped by a double cup of epithelial cells, Bowman’s capsule, between the double layers of which is the cavity of the renal tubule, or nephridium.
The formation of Bowman’s capsule around the glomerulus at the tip of the renal tubule is brought about when the glomerulus comes into contact with the tip of the tubule and pushes it in from the outside, like the finger tip of an empty glove. The delicate inner cup of Bowman’s capsule is thus closely adherent to the glomerulus so that the blood in the glomerular capillaries is separated from the cavity at the end of the renal tubule only by two exceedingly thin cell layers, that is the wall of the inner cup of Bowman’s capsule and the wall of the glomerular capillary itself. Thus filtration of the liquids to be excreted from the blood into the nephridial tube is made easily possible. Once through the inner wall of Bowman’s capsule, the excretory filtrate passes down the neck into a thick-walled, kinked-up glandular portion of the tube, known as the proximal convoluted tubule, whence it continues around a non-glandular hairpin curve, Henle's loop, into a second thick-walled, kinked-up glandular part, the distal convoluted tubule, which opens in turn into a collecting tubule (Fig. 364). Eventually the collecting tubules of neighboring units join into larger common channels, called the ducts of Bellini, that finally open into the renal pelvis at the tips of the papillae. Thus the entire urinary unit from glomerulus to pelvis is a continuous canal, the walls of wrhich vary much in character and function. As many as ten to twenty-four ducts of Bellini, or papillary ducts, may empty into the calyx of the pelvis from a single papilla of the human kidney. It is the pelvis, or enlarged end of the ureter, which, by way of the bladder and urethra, establishes a highway of communication with the outside world.
The blood stream passes through the kidney in the following manner (Fig. 366). It enters at the hilus by way of the renal artery that subdivides in the columns of Bertini into arterioles and finally into arterial capillaries. The latter knot up into the glomeruli, which are entirely arterial, forming a rete mirabile, rather than an arteriovenous capillary transition. The afferent twig entering the glomerulus is larger than the efferent twig which makes its exit near by, on the same side that the afferent twig enters. The inequality in the size of these twigs tends to build a pressure within the glomerulus that is probably higher than in any other capillaries. The emerging efferent twig soon breaks up into a capillary network which entangles the convoluted tubules. Here in this capillary network the arterio-venous transition occurs, since at this point intimate contact with the glandular walls of the convoluted tubules occurs, which makes possible the change in the blood from arterial to venous character.
The venules emerging from the capillary network anastomose with neighboring venules from other units in the columns of Bertini between the Malpighian pyramids, finally joining to form the renal vein that emerges from the kidney at the hilus.
The cortical region presents to the naked eye a granular appearance because in it are embedded the glomeruli and the convoluted tubules of the urinary units, while the striated appearance of the Malpighian pyramids is due to the presence of the parallel Henle’s loops and the collecting tubules (Fig. 364).
It is estimated that the human kidneys may include as many as 2,000,000 urinary units that establish continuous open channels from the glomeruli to the pelvis, through which some of the components of the onward-rushing blood stream are diverted and eventually discarded.
The secretion of the urine probably involves two processes: (1) filtration at the glomerulus and (2) reabsorption at the convoluted tubules. Water and all other non-protein constituents of the blood plasma are filtered into the tubule at the glomerulus. This very dilute fluid is then concentrated at the convoluted tubules by active secretion back into the blood stream of: 99 per cent of the water; all of such useful substances as glucose and amino acids; most of the inorganic ions such as sodium, chlorine, and potassium; little or none of the urea, uric acid, creatinin, and sulphate. It is possible that under certain conditions substances may be added to the urine in the tubules.
The composition of urine varies enormously in different animals and at different times in the same animal. This is because the blood from which the urinary excretion is obtained by the kidneys is such a kaleidoscopic modifiable fluid tissue that it reflects constantly different states of metabolism within the body. Furthermore, under pathological conditions still other variations from the normal in the composition of the urine appear. Consequently urine analysis is an important aid to the diagnostician in finding out what is going on within the body. This fact was realized even by the medical practitioners of former centuries to whom the refined chemical technic of modern urine analysis was not available. At least certain abnormalities of urine could be discovered by simple visual inspection and this was very generally, and no doubt more or less oracularly, done. In the crude woodcuts of old medical books of the sixteenth and seventeenth centuries there recurs over and over again, like a motif in architecture or music, the urine motif in the form of a urine flask, which plainly tells the story of the importance given to the examination of samples of urine in the medical diagnosis of those days.
Carnivores generally have an acid urine, while that of herbivores tends to be alkaline, except when they are feeding largely upon milk. It is usually more concentrated in animals, such as turtles and birds, which drink sparingly. In man its specific gravity varies from 1.016 to 1.020, and normally about a liter and a half is produced every twenty-four hours.
Urine is characterized by nitrogenous waste products, such as urea, creatinin, hippuric acid, ammonia, and uric acid, although nitrogen-free constituents and inorganic substances, such as sodium chloride, and sulphates and phosphates of sodium, potassium, calcium, and magnesium are also present.
In mammals, amphibians, and fishes, urel, formed from the blood in the liver, is the prominent nitrogenous compound present, while in reptiles and birds, with a minimum of water as a component, it is uric acid.
Mitchell gives a table that accounts for about 99 per cent of the constituents in most human urines, in which 95.1 per cent is water; 2.55 per cent nitrogen-containing constituents; 1.26 per cent inorganic materials; and .052 per cent nitrogen-free substances.