Biological Occurrences of Counter-Current Flow
Counter-current flow is widely found in fish and animals. Counter-flowing blood conserves body heat for whales, seals, cranes, herons, manatees, sloths, anteaters and armadillos. Other animals don't have counter-current arrangements even though they live in cold climates. These include ducks, geese, sea gulls, foxes and huskies.
Camels have scroll-like passages called turbinates in their nose which turn back upon each other creating counter-current airflow to conserve both energy and water.
The bumblebee produces lots of heat with its flight muscle. Counter-current blood flow is used to regulate heat transfer during warm weather and during vigorous wing activity. Kidneys concentrate urine by counter-flowing body fluids in the loop of Henle.
Fish gills are another fine example of counter-current efficiency. Animals which breathe air inhale 200 cubic centimeters of oxygen with each quart, but a quart of sea water has only 5 cubic centimeters of oxygen. A fish can take up 80% of that oxygen by arranging capillaries in the gill plate to allow blood to flow counter to the flow of water across them. Go to a fish monger and obtain gillrakes from fish being cleaned. You'll be able to see the folded double loop.