Countercurrent exchange is a highly efficient, naturally occurring phenomenon of thermal or chemical transfer between fluid bodies. This process takes place through a conductive surface in the case of heat or a partially permeable membrane in the case of chemical exchange. In countercurrent exchanges, the donor and recipient fluid always flow in opposite directions, a characteristic that lends the process both its efficiency and its name. Countercurrent exchange is found in many biological systems such as mammalian kidneys, bird lungs, and fish gills and is a commonly used industrial chemical and thermal transfer system. A similar system is the concurrent exchange process which is less effective and features transfers between fluids flowing in the same direction.
The exchange of thermal energy or suspended substances between flowing fluids is a common phenomenon both in nature and industry. These current flow exchanges can be divided into two groups: concurrent and countercurrent. Both involve exchanging heat or suspended chemicals between fluids flowing in adjacent vessels either via conductive surfaces or semi-permeable membranes, respectively. As the fluids flow across their shared areas, heat and chemicals naturally flow from high to low concentration areas until equilibrium is reached. It is this characteristic of elemental transfer that makes the countercurrent exchange method the more effective of the two.
The process of oxygen transfer in the gills of a fish is a good example of the benefits of countercurrent exchange. As oxygen poor blood meets an opposed flow of oxygen rich water, the oxygen begins to diffuse out of the water and into the blood stream. This causes the concentration of oxygen in the water to drop and that in the blood to rise. Due to the fact that the flow directions are opposed, the blood will always be flowing over water with a higher concentration of oxygen and the exchange will continue until the flows diverge. In concurrent flows, however, the two fluids flow in the same direction and the relationship between concentrations quickly reaches equilibrium, thereby effectively stopping the exchange.
This means that unlike the concurrent variant, countercurrent exchange systems continue to transfer the relevant element over the full exchange area for greater efficiency. This efficiency usually allows transfer values of 100% with the recipient flow exiting the system with the same concentration of heat or chemicals as that of the donor flow. The same cannot be said of concurrent exchanges, however, with average transfer values running in the region of 50%. This makes the countercurrent exchange method appropriate for industrial processes such as regenerative heat exchange and biological transfer methods including renal and pulmonary functions.