The Skin

The skin is the main organ through which a mammal exchanges heat with its surroundings, so many thermoregulatory effectors are found there.The skin has two layers above a layer of fat:
• The epidermis is the thin outer layer. Its surface is made of dead, keratinised cells that are waterproof and protect against infection.
• The dermis is the thicker inner layer. It contains blood vessels, sweat glands, hair follicles, nerve endings and thermoreceptors.
• Below this is the subcutaneous fat (adipose tissue), which insulates the body and stores energy.Structures in the dermis involved in thermoregulation:
• Sweat glands secrete sweat through a duct onto the skin surface, where it evaporates.
• Hair follicles, each with a hair erector muscle attached. When this muscle contracts, the hair is pulled upright.
• Arterioles supply a network of capillaries near the surface. They have smooth muscle in their walls, so their diameter can change, controlling how much blood reaches the surface.
• Shunt vessels (arteriovenous shunts) link arterioles directly to venules, so blood can bypass the surface capillaries.
• Thermoreceptors (nerve endings) detect skin temperature and send impulses along sensory neurones to the hypothalamus.How each of these structures is used to lose heat or to conserve it is covered in the next lessons.