 Dear students, in this topic we shall discuss the counter-current exchangers in the circulatory system. The circulatory system has special arrays of blood vessels at certain locations of the body. These form the systems in which the arterial and venous blood flows in close proximity in opposite directions. The counter-current exchangers have adaptive values which play a significant role in thermoregulation. They help to exchange heat between the bloods flowing in the arteries and veins. They regulate the rate of heat loss through the limbs and other body extremities. They also serve to exchange gases and ions between the arterial and venous bloods. Dear students, the counter-current exchangers work by regulating the extent of blood flow to the body periphery and extremities. They also work by trapping the heat within the body core. The counter-current exchangers are present commonly in the limbs of birds and mammals. They are found extensively in the body of fishes while in humans they are found only in the kidneys. Dear students, the counter-current exchangers have an advanced form of a specialized structure which is called retimerabilia. In this, the arterial and venous capillaries form a counter-current system. A counter-current network of arterial and venous capillaries is known as retimerabilia. Retimerabilia is formed when an artery, before entering into a tissue, divides into large numbers of small arterial capillaries. These arterial capillaries are then surrounded by a series of venous capillaries which leave the tissue. In retimerabilia, it performs many types of functions in the body. They form an extensive heat exchange surface between the in-flowing and out-flowing blood. Tuna fish are found in large numbers of retimerabilia which regulate the temperature of the brain, muscles and eyes. Other than this, some fish's swim bladders are also retimerably associated like eels. They function here as carbon dioxide exchangers.