CHAPTER XXII.

URINARY EXCRETION.

The urine is the most important fluid excretion, for by it, in mammalia, nearly all the nitrogen of the used-up proteid leaves the body in the form of

urea. The construction of

the urinary glands is peculiar, and requires special notice.

STRUCTURE OF THE

KIDNEYS.

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The kidneys may be called complex tubular glands, because the tubes of which they are posed are made up of a number of parts essentially differing from one another both in their structure and in their relation to the blood vessels.

The tubes begin by a small rounded dilatation (Malpighian capsule), which is lined by thin flattened epithelium. Opening from this capsule, Fig. 171 (g), is found a tortuous tubule (f), lined by peculiar large, rod-beset, epithelial cells, which occupy the

FIG. 170.

d. Commencement of ureter leading from central sac or pelvis.

c. Papillæ, where the tubes open into pelvis. (Cadiat.)

greater portion of its diameter. This convoluted tubule (f)

leads into a tube (e) of much less external diameter, but about equal lumen, owing to the thinness of its lining epithelium, the cells of which are more flattened and much thinner than those in

the tortuous tubes. This thin tube forms a loop extending down to the medullary pyramid and returning to the cortex, where it can be seen to become again convoluted (d) and then to open

into a straight collecting tube. The collecting tubes (c, b) receive many similar tributary tubes on their way toward the apex of the medullary pyramid, where they pour their contents into the pelvis of the kidney. The epithelial lining of these collecting tubes is of the ordinary cylindrical type.

We thus find four kinds of epithelial cells in the various parts of the urinary tubules, viz., scaly cells in the capsule; peculiar rod-beset glandular cells in the convoluted tubes; flattened cells in a great part of the loop; and ordinary cylindrical cells in the large straight tubes. (Figs. 172 and 173.)

BLOOD VESSELS.

The renal artery, on its way from the hilus to the boundary between the cortical and medullary portions of the kidney, breaks

up suddenly into numerous small branches; these vessels then form arches, which run along the base of the pyramids. From the latter, straight branches, called interlobular arteries, pass toward the surface, and give off lateral branchlets, which form the afferent vessels to the neighboring Malpighian capsules. Within the capsules the afferent arteries at once break up into a series of capillary loops, forming a kind of tuft of fine vessels -the glomerulus, which fills the cavity at the beginning of the tubules, and is only covered by thin, scaly epithelial cells, and thus separated from the urine. It is a singular fact that in the renal circulation the efferent vessel, on leaving the glomerulus, does not, like most veinlets, unite with others to form a large vein, but again breaks up into capillaries, which form a dense

meshwork around the convoluted tubules. The blood is thence conveyed to small straight veins corresponding to the intralobular arteries.

Another striking peculiarity of the renal vessels is that a distinct set of arteries, starting from the same point as the interlobular (between the cortex and medulla), pass toward the centre of the gland into the pyramids. They consist of bunches of straight arterioles, which lie between the straight and the looped tubules. Corresponding with these straight arteries are minute

Glomerulus, treated with silver nitrate, showing the endothelium.

straight veins, which carry the blood back to the vessels at the base of the pyramids.

In the kidney, then, we have three sets of capillary vessels, which differ in their position, the form of their meshes, and their relation to their parent artery. Probably the pressure exerted by the blood in them, and the rapidity of its flow through them, differ also:

1. The capillaries in the glomeruli are loops collected into a

tuft by their covering of delicate epithelium. On account of their relation to the afferent artery which ends abruptly in these capillaries, and to the smaller efferent vessel that leads to a secondary plexus of capillaries, the pressure within the glomerulus must be very great compared with that of the general capillaries of the body, and must vary much with changes in local blood pressure.

2. The secondary capillary plexus, with its narrow meshwork closely investing the tubules, can only be under comparatively trifling pressure which varies but little, on account of the blood having first to pass through the capillaries of the glomerulus. Their current of blood

must also move slowly, since the bed of the stream is here very great.

3. The straight vessels, with long meshed capillaries, in the pyramids between the looped and straight tubules, are unlike the two preceding. In these straight vessels the blood probably flows with greater velocity than in those around the convoluted tubes; and their blood pressure is less than that in the glomeruli, but greater than that in the intertubular capillaries.

THE URINE.

When freshly voided, the urine of man in health is a clear straw-colored fluid, with a peculiar aromatic odor. The intensity