Tubulo-vascular relationships

By Supriyadi Pro -
In the past, the vascular system of the kidney has been described in great detail while the arrangement of the tubules, and their fme structure, have been equally well studied. It is only relatively recently, however, that the two major components of the kidney have been studied in relation to each other. One theory of glomerulo-tubular balance suggests that tubular reabsorption is mediated by the composition and osmolality of blood in the peritubular capillaries.

If each individual nephron is to regulate its own function in this way it is essential that the tubule should be surrounded by capillaries derived from the efferent arteriole of its own glomerulus. This relationship has been studied in the kidney of the dog (Beeuwkes 1971, Beeuwkes & Bonventre 1975) and in the human kidney (Beeuwkes, personal communication).

It was found that, while the relationship holds good for the subcapsular glomeruli, there is complete dissociation between the tubules derived from a juxtamedullary glomerulus and the capillaries supplied by its own efferent arteriole. These authors, having fixed the kidney by intravascular perfusion, partially injected the vascular system with orange ‘Microfil’ silicone rubber. Slices of the kidney were then cleared, and the Bowman’s space of selected glomeruli injected with white ‘Microfll’ by means of a micropipette. In this way it was often possible to study the vascular relationships along the whole length of a nephron.

In the dog kidney it was found that typical subcapsular glomeruli have long efferents which run upwards to form a capillary network near the surface of the kidney. This supplies all but the first millimetre or so of the corresponding proximal tubule. The initial part of the tubule is often supplied by capillaries derived from other efferents.

When the distal tubule could be visualized, it was found to be closely intertwined with its own proximal tubule and to share the same capillary network. The straight descending part of the proximal tubule is supplied by efferents derived from a number of midcortical glomeruli.

In the midcortical zone, the efferent either supplies the long-meshed plexus of the medullary rays or breaks up immediately into a complex local plexus. The majority of the proximal tubules belonging to the glomeruli of this zone are supplied over less than half their length by capillaries derived from their own glomerular efferent. The straight parts of the proximal tubules are supplied by efferents from the deeper glomeruli. The distal tubules are closely associated with the proximal tubules and share a common blood supply.

In the inner cortical zone the efferents break up to form vasa recta and some also give branches to the local capillary plexus. The proximal tubules of these glomeruli are completely dissociated from their own efferents except in a very few cases in which a small part of the tubule is perfused by some of the early capillary branches of an efferent arteriole. The straight segment of the proximal tubules descends between the vascular bundles. The distal tubules share the capillary plexus which surrounds the proximal tubules.

In the human kidney (Beeuwkes, personal communication) the relationships are almost exactly similar to those in the dog although the efferent patterns are less well defined. Many midcortical efferents pass directly into the medullary rays and there perfuse relatively short segments of the loops of Henle and the collecting ducts, which might explain the patchy tubular lesions, particularly in the straight parts of the proximal tubules, which occur in acute renal failure.

In the human kidney only about 14 per cent of all nephrons have long loops and there are a relatively large number of purely cortical nephrons whose loops of Henle are entirely perfused by the capillary plexus in the medullary rays. Thus these nephrons cannot be affected by any exchanges which may take place in the medulla.

Tubulo-vascular relationships at the surface of the kidney were investigated by Steinhausen et al. (1970) in rats. Using intravenous Lissamine green as an indicator, they found that the efferent arteriole breaks up into capillaries in the region of the most distal part of the proximal tubule.

The capillaries then run proxirnally so that, in general, the flow of blood is in the Opposite direction to the flow of tubular fluid. This observation has not been confirmed, however, and both Solomon (1971) and Beeuwkes and Bonventre (1975) found that tubules often pass along one side of a capillary and then turn and run in the opposite direction.

Read the full article: The Anatomy of the Renal Circulation

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