Introduction

The renal medulla is the innermost part of the kidney itself. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the arcuate arterioles. The arcuate arterioles each in turn brach into interlobar arterioles, which finally reach the glomeruli. At the glomerulus the blood reaches a highly disfavourable pressure gradient and a large exchange surface area, which forces a lot of the blood out of the vessel into the renal tubules. Flow continues through the renal tubules, including the proximal tubule, the Loop of Henle, and finally leaves the kidney by means of the collecting duct, leading to the renal urethra.

By this point it is hoped that the fluid within the renal tubules has obtained significant concentration, similar to that of urine. This is important because undoubtedly one of the major functions of the kidney is that of water conservation. If one compares the disparity between the 8 glasses of water a day recommended by medical science, and the fact the body can go three days without water you can see how water conservation is a crucial physiological ability. One of the major ways in which this is effected is as the fluid in the renal tubules descends deep into the medulla it encounters an increasing interstitial small solute concentration. This exerts an osmotic pressure across the tubular membrane, pulling fluid back out of the tubules and resorbing it into the body, hence the hypertonicity of small solutes in the renal medulla is crucial for water conservation within the body.

The renal medulla is however not inhabited soley by the renal tubules, there is also notably the renal microcirculation. This starts in the form of the descending vasa recta (DVR), which take a strikingly stright path deep into the medulla. At various points throughout its depth, the DVR branches into fine capillaries. These capillaries spread through the medulla and anastomise to form the efferent vessels, the ascending vasa recta.

One of the primary functions of the renal microcirculation is that of nutrient provision and metabolite exchange for purposes of maintaining cellular function of those tissues making up the renal medulla.

The permeability dillema

For the microcirculation to fulfil its function of maintaining cellular requirements it requires a reasonably high permeability. A flow through a high permeability vessel passing through the depths of the renal medulla has the potential to cause the wash out of small solutes from the kidney, rendering it unable to produce a concentrated urine.

It is this dillema between the two conflicting requirements of the microcirculation, and the extraordinary and beautiful structure with which these are achieved which makes the renal medullary microcirculation such an interesting topic to study.It is this dillema between the two conflicting requirements of the microcirculation, and the extraordinary and beautiful structure with which these are achieved which makes the renal medullary microcirculation such an interesting topic to study.:V F Murphy

A question of lymphatics

It has been noted by many researchers that there is a striking absence of lymphatics particularly in the inner reaches of the medulla. Knowing that the DVR is particularly permeable to plasma proteins, the question arises of how plasma proteins which must enter the medulla are removed, and oedma is prevented. Wang and Michel (2000) presented a striking model illustrating that plasma proteins are in fact removed via the AVR, by means of convection.

The countercurrent mechanism

It has been proposed that the countercurrent mechanism is key in the unique function of the renal microcirculation. The countercurrent mechanism can be demonstrated to be the dominant, but not only factor, in the enhancement of hypertonicity deep within the renal medulla. This was demonstrated by numerical modelling of the renal medulla which is described elsewhere on this site.

Resorbtion of fluid from the renal medulla

It is important that fluid is removed from the medulla once it has been resorbed from the renal tubules. This is generally held to be the function of the AVR.