Sweat glands draw almost all the water and sweat of blood plasma are surrounded by a dense capillary network in the deep layers of the dermis. As explained in Chapter 29, regulating the nervous mechanism of sweating not only causes the formation of sweat, but also greatly increases skin blood flow. All hair from the upper layers of skin are drained by venules, which form a venous plexus in the superficial dermis and eventually flow into many large venules and small veins in the dermis. The vascular network described above is modified in the tissues of the hands, feet, ears, nose, and in some areas of the face in that direct connections between vascular arterioles and venules, known as arteriovenous anastomoses , occur mainly in the superficial tissues of the dermis. However, relatively few arteriovenous anastomoses exist in the majority of human skin on the limbs and torso. If a large amount of heat must be dissipated, dilated arteriovenous anastomoses allows significantly increased skin blood flow to warm the skin, increasing heat loss to the environment. This allows the vasculatures BLOOD AND 286 PART IV cardiovascular physiology passively expand to varying degrees. The reddish color of the hands, face and ears on a cold day demonstrates increased blood flow and vasodilation due to low temperatures. To some extent, this cold-mediated vasodilation is useful because it reduces the risk of cold injury on exposed skin. However, if this process included most of the body surface, as occurs when the body is immersed in cold water or inadequate clothing is worn, the heat loss would be rapid and result in hypothermia.