Countercurrent exchange is evident in the flow of countercurrent exchange is evident in the flow of blood in the dorsal vessel of an insect and that of air within its tracheae water across the gills of a fish and the blood within those gills air within the primary bronchi of a human and the blood within the pulmonary veins water across the skin of a frog and the blood flow within the ventricle of its heart

Countercurrent exchange is evident in the flow of water across the gills of a fish and the blood within those gills. Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or component, between two flowing bodies flowing in opposite directions to each other. Biological countercurrent systems may be downhill exchanges and hairpin multipliers.