The vascular bundles of the outer medulla

These are composed basically of the intermingled descending and ascending vasa recta but they are also, in animal kidneys, closely related to the descending limbs of the loops of Henle. The bundles are not very noticeable in normal sections of the kidney since they run parallel to the usual plane of sectioning, but in sections cut at right angles to the long axis of the bundles they are a very prominent feature of the outer medulla (Figs 1.15 and 1.16).

The structure of the vascular bundles has been studied in a number of species but so far there have unfortunately been no detailed studies of the human kidney. The most comprehensive descriptions have been given by Kriz and his colleagues, who studied rodents (Kriz 1967, Kriz et a]. 1972, Kriz & Koepsell 1974).

In these species, at least, each bundle consists of a central core composed of closely intermingled ascending and descending vasa recta. These vessels are in very close contact, being separated one from another only by their basement membranes or by thin perivascular cells.

Around the periphery of each bundle there is a ‘shell’ of intermingled ascending vasa recta and descending loops of Henle (in the outer medullary zone all descending loops are thin-see Fig. 1.4/Read: Cortical vessels).

These descending loops all belong to short-looped nephrons; the long 100ps, Whose glomeruli are in the juxtamedullary region, are situated outside the vascular bundles along with the thick ascending limbs and the collecting ducts.

Fig. 1.15: Two vascular bundles from a human kidney,
cut transversely. The descending vasa recta contain injection
medium, the ascending vasa are empty. Haematoxylin and eosin.

In the human kidney, the bundles are an equally prominent feature (Figs 1.15 and 1.16) but it is doubtful if the loops of Henle are involved in their make-up, as they are in rodents. Otherwise, the intermingling of descending and ascending vasa recta is similar to that in rodents although, on the whole, rather more vasa recta are involved in each bundle and preliminary studies of the human kidney have suggested that the numbers of ascending and descending vasa recta are approximately equal.

Fig. 1.16: Transverse section through a vascular bundle of human kidney. Some of the descending (D) and ascending (A) vasa recta are labelled. Thick Spurr section, methylene blue.

The anatomical arrangement thus strongly suggests the possibility that countercurrent exchanges (but probably not multiplication) can take place, not only between ascending and descending vasa recta but also between ascending vasa recta and descending loops of Henle (Kriz & Lever 1969) and there is a certain amount of evidence for this derived from animal experiments.

However, a recent study of the development of the vascular bundles (Speller & Moffat, 1977) suggests that the complex arrangement of the components of the vascular bundles in animal kidneys is an inevitable result of developmental processes rather than a teleologically significant relationship.

Since the collecting ducts are the first components of the medulla to differentiate, followed closely by the juxtamedullary nephrons with their long loops, these components form a series of units, to the periphery of which are added increasing numbers of short loops whose ascending limbs lie nearer to the collecting ducts than the descending limbs.

When the vasa recta appear, they have no choice but to develop in the loose connective tissue between the units so that the most peripheral components of the units, i.e. the descending limbs of the short loops, lie at the periphery of the vascular bundles, where most of the ascending vasa recta are found.

In order to produce and maintain the high osmolality of the medullary structures, water which has entered the interstitium from the collecting ducts, from the descending limbs of the loops of Henle, and presumably from the descending vasa recta, must be removed from the medulla. It is almost certain that there are no lymphatics in the medulla (see Moffat 1975 for discussion) so that the most obvious route for water removal is via the ascending vasa recta.

In other words, there must at times be a greater volume of blood leaving the medulla than is entering it and it is thus of great interest to compare the relative size and numbers of descending and ascending vasa recta. It is difficult to do this in fixed specimens, owing to the likelihood of fixation artifacts, although in animal specimens which have been fixed during a mannitol diuresis it is obvious that descending vasa recta are both fewer and smaller than ascending vasa recta.

Marsh and Segel (1971) have examined the relative size and number of the vasa recta in the hamster papilla in vivo and have found that the mean ratio of the numbers of ascending : descending vasa recta Was 1-71: 1 while the ratio of their respective radii was 1-35: 1. It seems likely, therefore, that the vessels play an important part in medullary function.

Read the full article: The Anatomy of the Renal Circulation