Engineers have known for decades that efficient, almost complete heat or other exchange could be achieved between two fluids flowing in opposite directions in separate tubes. Such countercurrent systems have evolved numerous times in living organisms for all types of exchange function. They are most commonly found in the circulatory, respiratory, and excretory (kidney) systems, serving in heat, oxygen, and ion exchange. Biological countercurrent systems can be classified into two main types: downhill exchanges and hairpin multipliers. In both cases, the basic mechanism is the same-exchange of substance between fluids flowing in opposite directions—but the consequences are very different.
Downhill exchange systems are commonest in the circulatory system where their morphological structure is a rete (network) of closely oppressed sets of small arteries and veins. They are also found in gills of fish and in the minute air tubules of the avian lung. In downhill exchanges, fluids flow in opposite directions in separate tubes with the possibility of exchange, for example, heat flow or diffusion of oxygen, between them. The fluid entering one tube is warmest at that end, while that entering the second tube is coolest at the other end. Heat flows from higher to lower temperature. Although the temperature differential between the two fluids is small at any point along the length of the countercurrent system, almost all the heat contained in the warmer tube is transferred to the cooler tube. Exchange of heat or oxygen occurs by passive diffusion. Most of the heat that entered the countercurrent system at one end leaves the system at the same end.
Retia of blood vessels thus serve as thermal isolating mechanisms within the body. Downhill exchange systems in the gills of fish and in the air tubules of birds permit maximum exchange of oxygen from the environment into the blood. Blood in respiratory capillaries flows against the water or air current and thus can pick up most of the oxygen contained in the external fluid. The advantage of downhill exchangers is that they achieve greater efficiency without extra energy cost simply by arranging flow in a countercurrent rather than in a concurrent fashion.
Hairpin multiplier systems take their name from the structure of the tubes, which have a hairpin turn between the afferent (descending) and the efferent (ascending) limbs. Hairpin countercurrent systems are found in the nephron (the loop of Henle) of the kidney and in the capillary system of the gas gland in the swim bladder of many fish. In contrast to downhill systems, which operate by passive transport, hairpin multipliers must employ active transport of materials. These are always materials pumped out of the efferent limb of the system. See also Kidney; Respiratory system; Swim bladder.
