Why is it that when you go out on a bright summer day and spend an hour or so in the sun, you most likely get a sunburn? That is, unless you've taken the time before that to get a nice gradual tan. If you have a tan, you can go out in the sun and not get burned. Of course, none of this applies if you have "fair skin." The fair-skinned among us never get a tan, so they always get burned. If you take the time to look at skin and sunlight in some detail, all of this begins to make sense, and you can learn a lot about your body in the process.
Skin is one of the most amazing organs in the human body. It is hard for us to think about it as an organ, however. We tend to think of organs as boxy things. Your heart, liver, kidneys - those are obviously organs. But skin is an organ too. @Merriam-Webster Collegiate Dictionary defines an organ as a) a differentiated structure (as a heart, kidney, leaf, or stem) consisting of cells and tissues and performing some specific function in an organism; or b) bodily parts performing a function or cooperating in an activity."
By that definition, skin is definitely an organ. Skin is made up of very specific cells and tissues, and their collective purpose is to act as the boundary between "you" and "the world." One of the interesting things about skin that makes it different from the internal organs is that it has to interact extensively with the real world. Therefore, it is loaded with sensors, and it also has a very tough, layered design so it can handle diverse environmental factors, such as abrasion and sunlight.
If you take a look at the cross-section of typical skin (like the skin on your arm or leg) you find that it is made up of two main layers: the epidermis on the outside, and the dermis on the inside. The epidermis is the barrier between you and the world, while the dermis is the layer containing all the "equipment" -- such as nerve endings, sweat glands, hair follicles, and so on. Here's a description to help you understand what is going on:
In the skin's subcutaneous layer you can see the blood vessels. These vessels branch extensively into the dermis to supply the sweat glands, hair follicles, sebaceous glands, and erector muscles with blood. They also fan out into the dermis' capillary bed. It turns out that the dermis is loaded with capillaries, very small blood vessels. Capillaries satisfy the nutritional needs of the cells in the dermis, and they also help the skin perform an important cooling function in humans. The epidermis has no direct blood supply, but instead is supported and fed by the dermis. The dermis also contains nerve endings, which let you sense heat, cold, pressure, pain, etc. These help you protect yourself from burns, punctures, and so on by warning you when there is danger to your skin. Functionally speaking, therefore the dermis is where the action is.
The epidermis is your interface to the world. It has two main layers, the inner of which is living and the outer of which is dead. The dead skin cells of the outer layer are what we can actually see, and they are constantly flaking off and being replaced by new cells being pushed outward.
The living, inner layer is called the malpighian layer. It is in direct contact with the dermis, which feeds and supports it. It is here that the sun affects the skin during tanning. The malpighian layer is itself layered like this: The bottom layer, in direct contact with the dermis, is the basal layer.
