Cortical vessels

Preglomerular vessels

INTERLOBULAR ARTERIES

The most obvious vessels of supply to the cortex are the interlobular arteries. These are mostly relatively short, straight vessels which take origin from the convex surface of the arcuate arteries or are formed from the terminations of the latter as they turn up towards the surface of the kidney. They may have an uncomplicated course or they may branch once or twice to give arteries of the second or third order.

In the final part of their course they run directly towards the surface except in the renal columns where their arrangement is somewhat complicated. The renal columns of Bertin consist of the fused cortices of adjacent lobes so that there is a cortico-medullary junctional zone on each side of the column.

Fig. 1.4: Diagram to show long and short-looped nephrones and
their relation to the zones of the kidney. From Moffat (1975), by
permission of Cambridge University Press.

Arcuate arteries lie in each of these zones so that there is at least one arcuate artery on each side of the column (Fig. 1.2/Read: Interlobar and arcuate arteries) and the interlobular arteries from these run in towards the central axis of the column where they intermingle to a considerable extent.

Some of the interlobular arteries reach as far as the surface of the kidney (Figs 1.2 and 1.5/Read: Cortical vessels) and emerge to form anastomoses with the capsular vessels. These perforating arteries may thus play an important part in the collateral circulation which develops when there is a slow onset of renal artery obstruction. The number of Such arteries in any one kidney is very variable and the total number probably decreases with age. The distribution of the perforating arteries has been described in detail by Eliska (1968).

AFFERENT ARTERIOLES

The afferent arterioles of the glomeruli are mostly branches of the interlobular arteries although some of the juxtamedullary afferents take origin directly from the arcuate arteries. They arise in a characteristic fashion. Those to the deepest glomeruli are directed towards the medulla so that they arise at an acute recurrent angle.

The most superficial arterioles are directed towards the surface of the kidney so that they almost continue the line of the parent trunk. The intermediate arterioles mostly arise at approximately right angles. This pattern is most marked in certain animals but it is usually very obvious in the human kidney. Many authors, studying injected specimens, have reported the presence of a constriction at the origin of each afferent arteriole, particularly in the juxtamedullary region.

This is almost certainly an artifact, the narrowing being produced in the lumen of the arteriole as it passes through the thickness of the wall of the parent vessel which is contracted after fixation. In certain species such as the rat and the dog, the origin of each of the juxtamedullary afferent arterioles is provided with a pair of valve-like intra-arterial ‘cushions’ which project into the lumen and are composed of modified smooth muscle cells (F ourman & Moffat 1971, Moffat & Creasey 1971).

They are not found in the human kidney, however, and cannot therefore be the cause of the constrictions. The wall of the afferent arteriole in the initial part of its course is composed of one or more layers of smooth muscle cells surrounding an internal elastic lamina and a smooth endothelium. As the vessel is traced towards the glomerulus, however, the elastic tissue diminishes in amount and finally disappears while the smooth muscle cells of the media become rounded and epithelioid in appearance and many, but not all, take on the characteristic form of the granular cells of the juxtaglomerular apparatus (p. 26).

Fig. 1.5: Surface view of the human kidney, infected with
Microfil and cleared, to show the perforating arteries.

Postglomerular vessels

The glomerular capillary knot, which will be described later, drains into an efferent arteriole which, throughout the greater part of the cortex, has only a short course before it breaks up into peritubular capillaries. In the juxtamedullary region, however, the efferent arterioles are long and play a major role in the blood supply of the medulla so they will be described later. 

The efferent arterioles of the most superficial glomeruli continue the line of the afferent arteriole until they reach the surface of the kidney where they break up into capillaries that radiate out from the end of the arteriole. These are usually called ‘star vessels’ (the Quellpunkte of German authors) and are important in animal experiments since they are easily recognised in vivo and offer a means of investigating the composition and pressure of postglomerular blood by micropuncture techniques.

The most superficial layer of the cortex is supplied with blood entirely by these ascending vessels and is therefore devoid of glomeruli (except in the very useful Munich strain of rats which have a few glomeruli on the surface). It is often called the cortex corticis.

The diameter of the cortical efferent arterioles is usually said to be slightly less than that of the afferent but these measurements are based on injected and/or fixed specimens and cannot be regarded as reliable. The glomerular filtration rate depends, at least partly, upon the relative diameters of the afferent and efferent arterioles which are, apparently, independently variable.

The wall of the efferent arteriole is certainly thinner than that of the afferent. In some species, parts of this vessel consist only of an endothelium surrounded by a few pericytes (Dieterich 1970) but in the human kidney it contains one or more layers of smooth muscle.

The cortical capillaries

This capillary plexus is derived entirely from the glomerular efferent arterioles. A few authors have described so-called Ludwig’s arterioles, which are aglomerular branches of the interlobular arteries that break up directly into peritubular capillaries, but modern investigators have been unable to find such arterioles.

As in all capillary plexuses, the meshes of the cortical plexus are adapted to the shape of the structures that it surrounds. Throughout most of the cortex, therefore, the plexus has the appearance of a small-meshed, three-dimensional network resembling crumpled chicken wire, since the capillaries Surround the proximal and distal convoluted tubules.

In the region of the medullary rays, however, the capillaries form an easily recognized longitudinal plexus since here they are arranged around the loops of Henle and the cortical collecting ducts. The pattern is therefore rather similar to that of the medulla.

The peritubular capillaries have a very close relationship with the proximal and distal tubules, being separated from the basal infoldings of the tubular cells only by the respective basal laminae, as befits their important function in removing the reabsorbed salts and wate. from the tubules. The peritubular blood can, in fact, affect the reabsorption of sodium by changes in its own composition and osmolality.

These peritubular capillaries have a very thin wall which is composed only of a thin endothelium supported by a basal lamina and a few scattered collagen fibres. The endothelium is of the fenestrated type, that is the cytoplasm is extremely attenuated, except in the region of the nucleus, and it contains a large number of rather regularly arranged circular apertures or fenestrations, each about 70 nm in diameter. These are not complete perforations such as those which are found in the glomerular capillaries, but each is closed by a very thin membrane (Fig. 1.6).

Fig. 1.6: The wall of a peritubular capillary (Human). The arrows indicate the endothelial fenestrations, each of which is closed by a membrane.

The cortical veins The peritubular capillaries of the cortex drain by means of very short wide trunks into the interlobular veins which have a similar course to that of the corresponding arteries. They, in turn, drain into the arcuate veins. In places, the superficial layers of the cortex drain peripherally towards the surface of the kidney where they enter rather larger veins lying on the surface.

These converge from all directions towards the ends of a number of long interlobular veins, forming the so-called stellate veins, which are rather a prominent feature on the surface of the kidney. These should not be confused with the ‘star vessels’ described above in spite of their name.

They ‘can be seen clearly with the naked eye whereas the‘ star vessels’ are capillary size. The stellate veins do not anastomose with the capsular vessels, although an occasional perforating vein does so.

Read the full article: The Anatomy of the Renal Circulation