Editing Countercurrent exchange (section)

=== Countercurrent exchange of heat in organisms ===

[[File:Arm counter-current flow.jpg|thumb|The arterial and deep vein blood supply to the human arm. The superficial (subcutaneous) veins are not shown. The deep veins are wrapped round the arteries, and the consequent counter-current flow allows the hand to be cooled down considerably without loss of body heat, which is short-circuited by the counter current flow.<ref name =knut>{{cite journal |last1=Schmidt-Nielsen |first1=Knut |title=Countercurrent systems in animals |journal=Scientific American | date=1981 |volume=244 |issue=May |pages=118–128 |doi=10.1038/scientificamerican0581-118 |pmid=7233149 |bibcode=1981SciAm.244e.118S }}</ref><ref name=grays>{{cite book |last1=Williams |first1=Peter L. |last2=Warwick |first2=Roger |last3=Dyson|first3=Mary |last4=Bannister |first4=Lawrence H. |title=Gray's Anatomy| pages=691–692, 791, 10011–10012 |location=Edinburgh |publisher=Churchill Livingstone | edition=Thirty-seventh |date=1989|isbn= 0443-041776 }}</ref>]]

In cold weather the blood flow to the limbs of birds and mammals is reduced on exposure to cold environmental conditions, and returned to the trunk via the deep veins which lie alongside the arteries (forming [[vena comitans|venae comitantes]]).<ref name=grays /><ref name=scholander>{{cite journal |last1=Scholander |first1=P. F. |title=The wonderful net |journal=Scientific American | date=1957 |volume=196 |issue=April |pages=96–110 |doi=10.1038/scientificamerican0457-96 |bibcode=1957SciAm.196d..96S }}</ref><ref>{{cite book |last1=Gilroy |first1=Anne M. |last2=MacPherson |first2=Brian R. |last3=Ross|first3=Lawrence M. |title=Atlas of Anatomy| pages=318, 349  |location=Stuttgart|publisher=Thieme Medical Publishers|date=2008|isbn= 978-1-60406-062-1 }}</ref> This acts as a counter-current exchange system which short-circuits the warmth from the arterial blood directly into the venous blood returning into the trunk, causing minimal heat loss from the extremities in cold weather.<ref name =knut /><ref name=grays /> The subcutaneous limb veins are tightly constricted, thereby reducing heat loss via this route, and forcing the blood returning from the extremities into the counter-current blood flow systems in the centers of the limbs. Birds and mammals that regularly immerse their limbs in cold or icy water have particularly well developed counter-current blood flow systems to their limbs, allowing prolonged exposure of the extremities to the cold without significant loss of body heat, even when the limbs are as thin as the [[Bird anatomy#Skeletal system|lower legs, or tarsi]], of a bird, for instance.<ref name=scholander />

When animals like the [[leatherback sea turtle|leatherback turtle]] and [[dolphins]] are in colder water to which they are not acclimatized, they use this CCHE mechanism to prevent heat loss from their [[Flipper (anatomy)|flippers]], tail flukes, and [[dorsal fin]]s. Such CCHE systems are made up of a complex network of peri-arterial venous [[plexuses]], or venae comitantes, that run through the blubber from their minimally insulated limbs and thin streamlined protuberances.<ref name=scholander /> Each plexus consists of a central artery containing warm blood from the heart surrounded by a bundle of veins containing cool blood from the body surface. As these fluids flow past each other, they create a heat gradient in which heat is transferred and retained inside the body. The warm arterial blood transfers most of its heat to the cool venous blood now coming in from the outside. This conserves heat by recirculating it back to the body core. Since the arteries give up a good deal of their heat in this exchange, there is less heat lost through [[convection]] at the periphery surface.<ref name=knut />

Another example is found in the legs of an [[Arctic fox#Adaptations|Arctic fox]] treading on snow. The paws are necessarily cold, but blood can circulate to bring nutrients to the paws without losing much heat from the body. Proximity of arteries and veins in the leg results in heat exchange, so that as the blood flows down it becomes cooler, and does not lose much heat to the snow. As the (cold) blood flows back up from the paws through the veins, it picks up heat from the blood flowing in the opposite direction, so that it returns to the torso in a warm state, allowing the fox to maintain a comfortable temperature, without losing it to the snow. This system is so efficient that the Arctic fox does not begin to shiver until the temperature drops to {{convert|-70|°C}}.