Skin Composition, from the Outside In
Our skin is a tough but supple membrane composed of three layers -- the epidermis, the dermis and the hypodermis -- that work in support of one another. Affixed to the skin are our fingernails and toenails, hair follicles, and glands that secrete sebum (we'll learn more about that term later) and sweat.
The epidermis is the thinnest of the skin layers and is also the outermost one. It has the important role of serving as our armor against infections and diseases. It also contains melanin, a pigment that gives our skin its color (we'll talk more about that in another section).
When our skin is healthy and glowing, it's a thing of true beauty, so you might be surprised to learn that the epidermis -- the layer that everybody gets to see -- is actually composed of dead skin cells that are always sloughing off and spreading bits of your DNA all over the place. The oldest skin cells are shed to make room for the younger ones that lie beneath them. But unlike snakes, we shed our skin gradually -- it takes about 35 days for us to replace all of the skin on our body [source: Arizona State University].
There aren't any blood vessels in the epidermis; they're located right below it in the dermis, the middle layer of our skin. If you cut yourself and bleed, it means you've torn through the epidermis and left the dermis exposed.
The dermis is full of collagen, which gives your skin its firmness, and that's where you'll find sweat glands and hair follicles. It also contains plenty of nerve endings that allow you to feel sensations such as heat, cold and pain. The dermis is like your body's own smoke detector. Pain felt in the dermis is a signal to the rest of the body to remove itself from whatever stimulus is causing so much hurt and discomfort.
The innermost layer of our skin is the hypodermis, which is what connects our skin to the bone and muscle beneath it. The hypodermis is made up of subcutaneous tissue, wonderful stuff that insulates our body and controls its temperature.
We mentioned sebum earlier, a substance that shields our epidermis from the elements. On the next page, you'll learn all about the oils and secretions that coat your skin.