The medullary capillaries and Venous drainage of the medulla

The medullary capillaries - As the efferent arteriole passes through the subcortical region it gives small branches to the coarse meshed capillary plexus which is found in this region (Fig. 1.9). This plexus is continuous both with the cortical capillary plexus and with a much more dense capillary plexus which occupies the outer medulla. The outer medullary plexus is less conspicuous in the human kidney than in most animals, where it has a closely packed ‘frizzled’ appearance (Fourman & Moffat 1971).

The vascular bundles traverse. this plexus (Fig. 1.10) and individual descending vasa recta supply it by peeling off the periphery of the bundles and breaking up into capillaries. These vessels often loop sharply backwards before forming their capillaries, so that in sections where the termination of the vessel may not be visible the appearance may suggest that the vessel is forming a hairpin loop before climbing out of the medulla again. The question of the so-called ‘U-loops will be discussed in more detail later.

Not all the vasa recta continue the line of their parent vessel directly. Those which are coming from glomeruli in the renal columns of Bertin are at first directed towards the surface of the kidney and then have to bend sharply back over the fat and connective tissue which lies deep to the calyceal fornix to reach the papilla.

In the immature kidney, in about 25 per cent of papillae, these vessels are closely related to the calyceal cavity itself and appear to be vulnerable to any increase in pressure within the calyx such as might be produced by vesico-ureteric reflux (Moffat & Laurence 1976).

The inner medulla, including the papilla, contains an elongated plexus of capillary loops which form a ‘trombone pattern’. The shape of the meshes of the plexus is the result of the general longitudinal arrangement of all the tubules and vessels of the inner medulla.

This plexus appears to be much more sparse than the outer medullary or the cortical plexuses and, in animals, it has been estimated that only about 1 percent of the total renal blood flow passes through this region.

In spite of this, the blood flow is still about fifteen times that of resting muscle (Thurau 1964). This extensive inner medullary plexus is supplied by those vasa recta Which have survived the passage through the outer medulla. These vessels leave what is left of the vascular bundles in turn, and break up to join the capillary plexus, so that the bundles rapidly lose their identity after leaving the outer medulla.

The vasa recta break up into capillaries at different levels, the longest of them having an unbranching course almost to the tip of the papilla (Fig. 1.3/Read: The renal veins and their tributaries). There is thus a rapid transit system by which blood is delivered almost simultaneously to all levels of the medulla so that an overall increase in medullary blood flow can take place very rapidly.

Here again, as in the outer medulla, the appearance of the elongated capillary plexus in sections gives the impression that the vasa recta are forming hairpin loops and this appearance is presumably the reason for the description by Trueta et a]. (1947) who believed that the vasa recta looped back in the medulla to join the venous system directly. This is certainly not the case (F ourman & Moffat 1964) although diagrams illustrating this arrangement are still found in many textbooks of physiology.

Fig. 1.9: Branches from efferent arterioles supply the subcortical capillary plexus. The elongated capillary plexus of a medullary ray is seen in in the top left-hand corner. Human, Microfil injection.

Fig.1.10: Section of injected human kidney to show two vascular bundles
and the outer medullary capillary plexus.

Venous drainage of the medulla

Ascending vasa recta arise from the capillary plexus of the inner medulla and these become crowded together into bundles, along with the corresponding descending vessels, as they approach the outer medulla. In the latter zone, part of the dense capillary plexus drains into new ascending vasa recta which are added to the periphery of the bundles so that the latter become larger and larger as they ascend through the outer medulla.

In the subcortical region, the ascending vasa recta splay out from the vascular bundles and open into the arcuate veins either directly or by means of small collecting veins. Many of them, however, sweep around the arcuate vessels on either side and end up by opening into the interlobular veins.

In most animal kidneys, an additional series of large and very irregular veins drain from the superficial surface of the outer medullary plexus and open into the arcuate veins. These are a very prominent feature of the subcortical zone in animal kidneys but they are not large in the human kidney.

Read the full article: The Anatomy of the Renal Circulation