Counter-current multiplication

If specific ATPases are present in a loop system that actively transports the solute from the solution in T' to the solution in T, counter-current multiplication occurs (Figure 4.17). This system is used to create a hypertonic environment by increasing the concentration of a solute with low ATP consumption, while active transport works against small concentration gradients (20 mM in the example of Figure 4.17) within each sector of the loop system.

This is possible because the single transport effect within each sector is multiplied by the fact that the solution in T, which has been enriched in solute by flowing from sector 1 to sector 5, in turn becomes a supplier of solute for the solution that eventually arrives in sector 1 of T (by flowing from sector 5 to sector 1 of T').

Figure 4.17. A solute can be concentrated in a looped tube if, at minimum in the innermost section, there is active transport. The numbers, representing the solute concentrations in mol/L in the tubes and the quantities exchanged in moles at the arrows, are purely illustrative

A simple system that uses a counter-current multiplier is the one allows teleosts to increase the volume of the swim bladder, to decrease the specific weight of the animal and allow it to move towards the surface of the sea (Figure 4.18).

Figure 4.18. Increased metabolism in the afferent capillaries causes the swim bladder to fill with O₂, which increases its volume and lowers the specific gravity of the animal. The degree of sphincter openness regulates the flow of O₂ to the oval body from where it passes, by partial pressure gradient, to the circulatory system

Each afferent capillary of the very dense system (about 100,000 vessels) that impacts the swim bladder bends into a loop and becomes an efferent capillary arranged in parallel to the first. Within the gas gland, located near the swim bladder and irrigated by the dense network of capillaries, the increase in metabolism and consequent ATP consumption produce a large amount of carbon dioxide and lactic acid that lower the pH of the blood: the consequence, similar to the increase in solute concentration in Figure 4.18, is a progressive increase of the pO2 due to the release of oxygen by hemoglobin (a phenomenon called the Bohr effect).

Oxygen passes, by partial pressure gradient, into the swim bladder with an increase in volume and decrease in specific gravity.

Another use of the mechanism described is Henle's loop of the mammalian kidney nephron (Data sheet 4.4). The counter-current multiplier system, associated with a counter-current exchanger system used by the vasa recta running parallel to Henle's loop, is used to produce and maintain a high osmolarity in the renal interstitium, which can be as high as 2000 mOsm in the deep medullary zone, compared with 300 mOsm in the cortical zone. This high osmolarity is used to produce hypertonic urine by osmotic reabsorption of water, which passes from the collecting duct to the renal interstitium and vasa recta.