Medullary vessels

By Supriyadi Pro -
Juxtamedullary efferent arterioles - The efferent arterioles of the juxtamedullary glomeruli differ in many respects from those of the cortical glomeruli, being longer, wider, and having a thicker wall containing a well-developed smooth muscle coat. It should, perhaps, be mentioned here that this terminology for glomeruli (cortical and juxtamedullary) is inaccurate since all glomeruli are in the cortex (or subcortex) but it has been used by so many authors that it has now become a convention.

The juxtamedullary glomeruli are those whose efferent arterioles supply the medulla rather than the cortex. Even these, however, are not necessarily juxtamedullary in the geographical sense since some of them lie almost as far out as the middle zone of the cortex.

It seems likely that when more is known about changes in the distribution of blood flow through the different layers of the cortex as a result of changes in salt and water balance and in response to various pathological conditions it will be possible to produce a more satisfactory classification of glomeruli based on functional rather than anatomical attributes.

The efferent arterioles give small branches to the subcortical capillary plexus and then break up to form descending vasa recta.

Picture: Juxtamedullary glomeruli and their efferent arterioles
Fig. 1.7: Juxtamedullary glomeruli and their efferent arterioles. To reach the medulla,
the efferents or their branches have to sweep around the arcuate vessels,
two of which are visible in transverse section. A number of efferent arterioles
contribute to each bundle of descending vasa recta. Human Microfil injection.

DOUBLE EFFERENTS
Juxtamedullary glomeruli possessing two efferent arterioles are commonly seen, Smith (1956) estimating their incidence to be about 5 per cent of all glomeruli in this region. One of these efferents breaks up after a short course to supply the peritubular capillary plexus of the subcortical region while the other descends into the medulla and breaks up into a normal bundle of vasa recta.

It is doubtful if such double efferents have any functional significance as it seems probable that the shorter of the two simply represents a precocious capillary branch of the normal efferent.

The vasa recta
The juxtamedullary efferent arterioles mostly begin on the cortical side of the arcuate vessels so that in order to reach the medulla they or their branches have to sweep around these large vessels on either side, giving a very characteristic appearance (Fig. 1.7).

In the region of the arcuate vessels, the efferent arteriole breaks up abruptly into a bundle of descending vasa recta, each of which is not much smaller than the parent vessel (Figs 1.7, 1.8 and 1.9/Read: The medullary capillaries and Venous drainage of the medulla). There is thus a sudden enormous increase in the total cross-sectional'area of the vessels entering the medulla with, presumably, a corresponding decrease in pressure and flow rate.

Picture: The formation of vascular bundles
Fig. 1.8: The formation of vascular bundles. Some efferent arterioles
supply more than one bundle and each bundle receives contributions from
a number of efferents. The arrow indicates an aglomerular vessel arising
from an arcuate artery Human, Microfil injection. 

This has not yet been confirmed since pressures in the juxtamedullary zone can only be estimated by extrapolation from pressures measured in the vasa recta near the tip of the papilla. The number of descending vasa recta derived from each afferent arteriole varies from one Vessel to another but a rough estimate puts their mean number as between twelve and twenty-hve.

As will be described later, these descending vasa recta are intermingled with even more ascending vasa recta to form a vascular bundle. In animal kidneys (but probably not in man) the descending limbs of the short loops of Henle are also involved in the bundles.

Each efferent arteriole may supply vasa recta to a number of bundles and each bundle receives vasa recta from a number of efferent arterioles which sometimes travel a considerable distance to reach the bundle of their choice (Fig. 1.8). In this way, the number of descending vasa recta in the average vascular bundle may be as many as thirty to thirty-dive, but bundles containing fifty or more descending vessels are quite common.

Read the full article: The Anatomy of the Renal Circulation

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