Transport processes often involve streams of material transport. For example, O2 transport across gill epithelium involves movement of material from a water current over the gills into a circulatory system that is also moving. Material exchange in these situations tends to be favoured when the streams are flowing in opposing directions (counter-current exchange). To see why this is so, consider first the co-current process.
Here, the maximum gradient occurs where the two streams first come into contact. Exchange between them lessens the gradient and the maximum exchange possible is an averaging of the concentrations in the two streams.
With a counter-current system, the gradient is lower than the initial co-current gradient, but the gradient is maintained throughout the length of association of the streams. In this way all but some fraction () of the transported material is exchanged. The unexchanged fraction can be made smaller by increasing the length of contact between the two streams. This can be done by increasing the physical length of the zone of association of the two streams, or by decreasing flow rate in the streams. Counter-current mechanisms are almost always more efficient than co-current systems but the efficiency depends upon the time of association of the opposing streams. The long legs of wading birds contain very little metabolically active tissue, and represent a potentially extreme source of heat loss. Efficient counter-current blood flow in the legs allow much ( %) of this heat to be recuperated. In other systems, however, counter-current mechanisms may be only marginally more efficient than co-current mechanisms.
Glossary: Bulk flow, counter-current exchange, counter-current multiplication, gradient, conduction, convection.
Heat exchange: legs of birds, body heating of fish such as Tuna and Mako shark; Salt and waste excretion in kidneys; O2 and CO2 exchange in gills of many invertebrates and fish, in lungs of birds (but not those of mammals).