Counter-current exchange

A system exists that makes it possible not to disperse an excessive quantity of solute into an environment in which it is present at a low concentration and not to absorb an excessive quantity from an environment in which it is present at a high concentration. Such a system occurs when the mechanism of two solutions that flow in counter-current takes place in a single tube folded like a loop.

Consider if a solution at high concentration, for example 200 mM, enters a straight tube and arrives unaltered at the end of the tube where, in the external environment, the same solute is present at low concentration, for example 50 mM. The solution would lose solute, which would diffuse in great quantity towards the outside because of the high concentration gradient (in this example, 150 mM).

Consider if instead we have the situation of Figure 4.16, in which the tube T is bent as a loop and forms the tube T', which is placed parallel to the first. As the solution in T flows from sector 1 to sector 7, it yields a part of the solute, which is taken up by the solution in T', with a mechanism analogous that in Figure 4.14, and arrives in sector 7 at low concentration, therefore with a small concentration gradient towards the outside.

Figure 4.16. In a looped tube, it is possible to limit the loss of solute to the environment by recovering it from the inlet to the outlet tube. The numbers, representing the solute concentrations in mol/L in the tubes and the quantities exchanged in moles at the arrows, are to be considered examples only

This gradient generates a small flux and a reduced loss of solute. Also, in this case a stable disequilibrium is established, because while the solutions flow continuously, a small concentration gradient is generated within each sector, which in turn generates the flow of solute from T to T'.

Of course, if the aim of the counter-current exchange system is not to absorb an excessive quantity of solute from an environment where it is present at high concentration, in the tube T (Figure 4.16) the concentrations will increase from sector 1 to sector 7; in the tube T' they will decrease from sector 7 to sector 1; the flows will be from T' to T.

The counter-current exchange can involve solutes but also heat. For example, for feeding, river birds remain immersed for long periods with their long legs in water at low temperature. They have the arteries and veins of the legs arranged like the tubes T and T' in Figure 4.16: the arterial blood arrives at sector 1 of T at body temperature but it drops at the end of the leg at a lower temperature, reducing the loss of heat to the external environment due to a reduction in the gradient.