The countercurrent multiplier system is a sophisticated apparatus that evolved in mammals and birds to con-serve water. It forms a longitudinal concentration gradient in the medullary interstitium that increases in strength toward the papilla. This gradient is crucial for water reabsorption from the renal tubules, which is a passive process that depends on osmotic pressure from the interstitium.
|MODEL OF THE COUNTERCURRENT MULTIPLIER: PART I|
The creation and maintenance of this gradient is best understood by ﬁrst considering a simpliﬁed model of the loop of Henle. In this model, a tube of ﬂuid is divided by a membrane in all but its most inferior aspect. The left side represents the entire descending limb, whereas the right side represents the entire ascending limb. Fluid enters at the top of the left-sided column, travels beneath the membrane, and then exits at the top of the right-sided column. The dividing membrane is impermeable to water but contains active transporters, which pump solute from the ascending limb to the descending limb. These transporters are powerful enough to establish a transmembrane gradient of about 200 milliosmoles (mOsm).
In Panel 1, the entire tube is ﬁlled with ﬂuid concentrated at 285 mOsm, which is roughly equal to the osmolality of ﬁltrate as it enters the descending limb. A transmembrane gradient is established as the transporters pump solute across the membrane.
In Panel 2, ﬂuid begins to move through the circuit. Thus, at the hairpin turn, concentrated ﬂuid from the descending limb mixes with less concentrated ﬂuid from the ascending limb. As a result, a ﬂuid of average concentration is formed. Because the active transporters can establish a 200 mOsm gradient, the last part of the descending limb becomes correspondingly more concentrated.
In Panel 3, the ﬂow process continues, and as con-centrated ﬂuid continues to rise in the ascending limb, reestablishment of the 200 mOsm transmembrane gradient causes a corresponding rise in the concentration of ﬂuid in the descending limb. At this stage, solute is still being retained within the system, and thus the outgoing ﬂuid is less concentrated than the incoming ﬂuid.
In Panel 4, steady state has been reached, meaning that no additional solute is being added to the system. Thus the incoming and outgoing ﬂuid are iso-osmotic. The overall effect of this process has been to establish high longitudinal gradients, whereas the transmembrane gradient is comparatively small.
In Panel 5, which represents the actual loop of Henle, these same events occur but with important differences. First, the limbs are separated by an interstitium, rather than a single membrane. The ascending limb is impermeable to water but reabsorbs solutes into the interstitium. The descending limb, in contrast, is permeable to water but not to solutes. As a result, the concentration of ﬂuid in the descending limb rapidly equilibrates with the concentration in the interstitium. Another difference is that the ﬂuid leaving the loop of Henle is hypo-osmotic to the ﬂuid coming in, reﬂecting the fact that a small amount of solute is continuously lost from the interstitium, preventing a steady state from being reached.
|MODEL OF THE COUNTERCURRENT MULTIPLIER: PART II|
In Panel 6, the collecting duct is added to the model and runs parallel to the loop of Henle. In the presence of ADH (see Plate 3-17), the collecting duct becomes permeable to water, which is reabsorbed from the col-lecting duct lumen into the interstitium. This process is entirely passive, depending on the osmotic pressure of the interstitium. Thus the maximum concentration in the medullary interstitium determines the maximum concentration of the ﬁnal urine.
The addition of the collecting duct also illustrates how urea contributes to formation of the interstitial concentration gradient, especially in the inner medulla. In the presence of ADH, the inner medullary collecting duct becomes permeable to urea. As water is reabsorbed from the cortical and outer medullary collecting ducts, urea becomes highly concentrated within the tubular ﬂuid. Once the inner medulla is reached, urea ﬂows out of the collecting duct and accumulates in the interstitium, contributing to the concentration gradient. Thus, as further described on Plate 3-17, ADH not only promotes water reabsorption from the collecting duct, but it also activates mechanisms that strengthen the concentration gradient, thereby ensuring water reabsorption is maximal.
Once deposited in the interstitium, some urea drifts from the inner medulla and is secreted back into the proximal tubule and loop of Henle. By reentering the tubular ﬂuid in this manner, urea is returned to the inner medullary collecting duct to once again be reabsorbed. This process, known as urea recycling, tends to minimize urea depletion from the inner medulla.
|MODELS TO DEMONSTRATE PRINCIPLE OF COUNTERCURRENT EXCHANGE SYSTEM OF VASA RECTA IN MINIMIZING DISSIPATION OF MEDULLARY OSMOTIC GRADIENT|
The ﬁnal elements that need to be added to this model are the capillaries of the vasa recta, which are permeable to water. If these vessels passed straight through the interstitium (Panel 7), osmotic pressure would draw out plasma and dilute the concentration gradient. Instead, the capillaries turn back upon them-selves (Panel 8), and thus water that efﬂuxes from the descending capillaries is reabsorbed in the ascending capillaries. This process is known as countercurrent exchange.
The blood leaving the medulla, however, does not completely reabsorb all of the efﬂuxed plasma. Thus outgoing blood is slightly hyperosmotic compared with incoming blood. As a result, the anatomic conﬁguration of the vasa recta minimizes, but does not completely prevent, solute loss from the medulla. These losses are also small, because the blood ﬂow to the medulla is very low. Release of ADH further constricts the vasa recta capillaries, ensuring maintenance of the high interstitial concentrations required for maximal urine concentration.