Ouch! Nerves give you your sense of touch. So that means they let you feel pain, too. It may sound strange, but feeling pain helps keep you healthy. T ink about a paper cut. T e edge of a sheet
of paper hits your skin at just the wrong angle. It slices through the epidermis. It digs into the dermis. T e paper touches nerve endings. Ouch! You feel pain. You jerk your finger
away. T at’s good. Now the cut won’t get any deeper. Plus, the pain lets you know you’re hurt. You can clean up the cut. T e cut will bleed a bit. T at’s because the
paper breaks through tiny blood vessels in the dermis. T is may sound strange, too, but bleeding can help you stay healthy. Here’s how. T e cut is not just a pain. It’s now a way
for harmful germs to get into your body. Your body sees the germs as invaders. Your blood comes to the rescue. It has special fighter cells in it. T ey attack and destroy the germs. T e skin around the injury swells and gets red. T e red, swollen skin is a sign that your body is fighting the germs.
Keeping Cool T ese blood vessels also help keep you the right temperature. If you get too hot, these vessels expand. More blood flows near the surface of your body. T at lets heat escape. If you get too cold, the blood vessels shrink. Blood flow slows and more heat stays in your body. Sweat glands in your dermis help cool
you, too. When you get too hot, sweat oozes through pores, or holes, in your skin. It coats your epidermis. As it evaporates, it cools your skin and you. Other glands in your dermis make an oil. It
waterproofs your skin. T at way, you don’t get too soggy. T is is just one more way your skin keeps you healthy.
22 NATIONAL GEOGRAPHIC EXTREME EXPLORER
Extra Padding Beneath the dermis is the last layer. It’s called the hypodermis. T is layer protects the organs inside your body. It’s mostly made of fat cells. T is layer has two important jobs. First, it
keeps your skin in place. It sticks to the tissues under your skin. Second, it acts like a cushion. T is fatty layer absorbs the shock of all the little hurts your body takes every day. Some parts of your body need more cushion
than others. So the thickness of the hypodermis varies. You have a good amount of fat on your upper legs, for example. You have very little fat on your eyelids. Some animals have super thick layers of fat
beneath their dermis. T en it’s not called fat. It’s called blubber. For a seal or walrus, blubber makes up half
of the animal’s weight. T is fat doesn’t just act as a cushion. It also keeps them warm in the cold places where they live. Animals also depend on their blubber when when food is scarce. If they need energy, their bodies can slowly absorb the fat.
Teamwork From the outside in, your skin works in many wonderful ways. Each layer protects you, keeping out invaders and cushioning blows. Together, these layers form an organ system.
It’s called your skin system. Your skin doesn’t work alone, though. It teams up with other body systems. It works with your blood, part of your circulatory system. It works with your nerves, part of your nervous system. All these body systems work together to
keep you healthy. Yet your skin is your the first line of defense. It’s what comes between you and the outside world. So return the favor. Take good care of your skin. Remember, you owe a lot to the skin you’re in.
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