The arterioles near the surface of the skin dilate and the arteriovenous shunt, which connects the arterioles and venules, constricts. Blood is forced closer to the surface of the skin through the capillary network. The skin is then cooled by radiation. This is known as vasodilation. Sweat glands in the skin produce 1 - 12 litres per day of sweat which evaporates from the surface of the skin. This causes cooling. Erector pili muscles in the skin relax and hair or feathers lie flat. This avoids trapping an insulating layer of air.