The skin is composed of three main layers. The deepest is the fatty subcutaneous layer that acts as a cushion against bumps and lumps and provides insulation.
Directly above it is the dermis, made of strong elastic fibers and collagen. This is where miles of blood vessels flow that expand and contract when environmental temperatures rise or fall. This also is where you find hair follicles, a network of nerves and substance-secreting glands. Sweat glands work overtime to cool us down when we overheat and can produce up to a liter of sweat a day. Sebaceous glands produce an oily, waxy substance called sebum that provides lubrication and waterproofing for the epidermis. It is what clogs pores in people with acne. It protects the epidermis, the uppermost layer exposed to the outside world.
The epidermis contains the tough horny layer of cells called the stratum corneum. Every five weeks or so, these flattened dead cells slough off and are replaced by new ones. Skin cells constantly fall off when we squirm around in our beds and rub against our clothes. Humans can shed up to nine pounds of skin every year. This natural turnover allows the fatty substance produced in the dermis to migrate freely to protect the surface layer. The epidermis is thicker on the soles of the feet and palms, and thinner on forearms and around the eyes.
The epidermis contains melanocytes responsible for producing melanin, the pigment that gives skin its color and absorbs energy from the sun. This is the layer that gets the most exposure to the elements and suffers the most abuse.