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 mesh work around the convoluted tubules. The blood is thence conveyed to small straight veins corresponding to the intralobular arteries.
Fig. 173. Portion of the Convoluted Tubule, showing peculiar fibrillated epithelial cells.
Another striking peculiarity of the renal vessels is that a distinct series 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 straight veins, which carry the blood back to the vessels at the base of the pyramids.
Fig. 174. Glomerulus, treated with silver nitrate, showing the endothelium.
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.