 Dear students, in this topic we shall discuss the counter current heat exchange systems. Dear students, the limbs and extremities of the endothermic animals, for example the gills, ears and nose are thin, have large surface area and are well vascularized. But still they are not massively insulated, they are the major avenues of heat loss. The counter current heat exchange systems reduce the heat loss from these peripheral extremities and tissues. These counter current heat exchange systems are found in homeothermic animals of cold climates like cetaceans that is whales and dolphins, seals, wading birds, arctic land animals and they are also contained by many heterothermic fishes. Dear students, the basis of counter current heat exchange system is that the arterial blood which is coming from the core of the body is always warm. However, the venous blood which returns from the peripheral parts may be very cold. This cold blood which comes from the peripheral tissues can make the core of the body cold. Similarly, the warm blood coming from the core of the body is going to the periphery that may lose heat. Up this heat loss with the help of this counter current heat exchange system. In this system the arteries which are bringing blood in the extremities are aligned in such a way that they pass just parallel to the outgoing veins. Due to this arrangement the warm blood in the arteries which is coming from the core transfers its heat to the returning venous blood. The warm blood which is coming from the core of the body is going to transfer its heat to the cold blood. The effect of this is that the arterial blood is flowing slowly and it is getting cooler because it has transferred its heat to the venous cold blood. Similarly, the venous blood is getting warmer and not as cold as it was coming from the periphery. Dear students, When this arterial blood reaches extremely extreme, it is already cooled. It is just a few degrees above the ambient temperature. So it has minimized its capacity to lose heat to the environment. Now it has a little bit of heat which if it loses then the loss of the body will not happen. Similarly, the returning blood which is coming from the venous blood that has become warmed by the arterial heat transferred and it is just near to the core body temperature when it will reach core. Overall, its result there is little heat loss to the environment. Dear students, A highly evolved system of counter current heat exchange is found in many animals and this system is known as Riti Mirabili. This system is found in many homeothermic and heterothermic vertebrates. In this system, the artery which is carrying the warm blood towards the extremity or towards the periphery, the artery is encircled on the venous blood coming from the periphery. And this encirclement of veins becomes the game of the artery, it absorbs the heat on the periphery. And in this way, the principle of counter current exchange is that the heat is transferred to the outside, it is successfully gained from Riti Mirabili. Dear students, counter current heat exchange system has many advantages. It reduces the heat loss into the environment, it provides oxygen and nutrient supply without reducing the blood flow to the peripheral parts or extremities. It is better arrangement as compared to vasoconstriction which reduces blood flow towards peripheral parts. This system is not reducing blood flow, therefore nutrients and oxygen supply reaches the extremities. The second advantage of this system is that due to this, the extremities or the peripheral parts can be maintained at a high or low temperature, without causing any type of damage.