Skin function

The skin looks like a simple covering, but it is really an organ doing several jobs at once. Together with its accessory structures, such as the sweat glands, it protects the body from microbes and from chemicals, keeps it from drying out, acts as a sensory organ that reports what you touch, helps hold temperature steady, and makes vitamin D.

Two of those jobs come down to how the skin is built. It slows water loss thanks to its outermost layer, the stratum corneum, loaded with keratin, the tough protein that hardens it, and with glycolipids, fats that seal it; between the two, that layer works like the waterproofing on a rooftop. And the skin is also the first thing to take the hit when something scrapes it, or when it meets dirt, microbes or harmful chemicals.

The skin handles the third job, temperature, together with the sympathetic nervous system, the part of the nervous system that triggers the fight-or-flight response: when sweat evaporates from the surface, the body cools down, and the arterioles of the dermis, the small vessels in the deep layer of the skin, widen so the heat in the blood can leave through them. It is like opening the windows of a house.