The largest organ in the human body is also one that most people don’t even think about. Little kids complain when parents try to clean it. School-age kids and teenagers grumble when parents pester them to put sunscreen on it.

The skin has three layers. The outside layer (epidermis) is actually made up of about 20 layers of tightly packed cells. Your body sheds skin cells so quickly that you have an entire new layer of epidermis every month.

The middle layer of skin (dermis) is thicker and more complex than the epidermis. It contains nerves, glands, hair follicles and blood vessels that help skin function. Sebaceous (oil) glands make a gooey substance called sebum that moves along hair shafts to the surface. Sebum keeps the epidermis soft and smooth. It also helps keep you waterproof.

The bottom layer of skin (subcutaneous layer) is made up of fat cells and connective tissue that binds skin to muscles and bone.

Every square inch of skin contains 20 feet of blood vessels, 100 oil glands and 650 sweat glands. It also contains more than 1,000 nerve endings that sense touch, pain, temperature and pressure. When one of those nerve endings is stimulated, a signal is sent to the brain, and you “feel” the corresponding sensation.

Fighting infection

Skin is the first line of defense against germs that could enter the body and make you sick. That’s why breaks in the skin — cuts, scrapes and burns — are more likely to get infected than normal skin. In addition to providing a barrier that keeps germs out, the epidermis contains cells that help your immune system fight infection.

Temperature control

Skin plays a big role in maintaining body temperature. On cold days, the brain reduces the amount of blood that travels to your skin. That helps retain heat within your body. On warm days, the brain increases blood flow to the skin, which has the opposite effect. The subcutaneous fat layer acts as insulation that helps maintain body temperature. (The blubber of sea mammals, which helps them stay warm in frigid water, is made from fat.)

The dermis contains sweat glands that help you to stay cool. Nerves that are temperature sensitive signal the brain if you are in a warm environment. The brain then signals the sweat glands to become active. (The same thing can happen when you get nervous, which is why your hands, feet and armpits become sweaty in certain situations.)

Sweat travels up tiny tubes to the epidermis. When the sweat evaporates, it cools the surrounding skin. (You can see how sweat keeps you cool by licking the back of your hand and gently blowing on the wet skin.)

Skin color

The epidermis contains cells that produce a dark pigment called melanin. The amount of melanin in skin determines its color: the more melanin, the darker the skin. A person tans because ultraviolet light simulates the release of melanin into the epidermis. However, tanned skin is damaged skin. So do your epidermis a favor and put on lots of sunscreen!

— Howard J. Bennett

Bennett is a Washington pediatrician. His Web site, www.howardjbennett.com, includes past KidsPost articles and other cool stuff.