Understanding what your skin does for you

Controlling your internal temperature

Your skin plays an important role in homeostasis, specifically thermo-regulation (see Chapter 2). It has mechanisms that increase body temperature when you’re cold and decrease it when you’re hot.

Your body is continuously converting energy from food into energy in the form of ATP (see Chapter 3). About 60 percent of food energy is converted to heat in the metabolic reactions that produce ATP, and more heat is given off in the reactions that use ATP, such as muscle contraction. This heat replaces the heat that your body loses continuously to the environment. (See Chapter 16 for an overview of ten basic chemistry and physics concepts.)

Human physiology employs specially adapted integumentary structures for thermo-regulation. When your internal temperature rises above its set point, the blood vessels in the dermis dilate, dissipating heat from the blood to the environment through the epidermis. The sweat glands are activated, and heat escapes the body as the sweat evaporates.

When your skin is cold, the sweat glands aren’t activated, thus slowing down evaporative cooling and raising your internal temperature by retaining the heat produced in metabolism. Blood vessels in the dermis constrict, limiting heat escape from the blood. If body temperature falls to about 97 degrees, shivering begins automatically, and these muscle contractions generate heat.

Normal body temperature for a human is 97 to 100 degrees Fahrenheit (F). This is the optimal range for the very temperature-sensitive reactions of metabolism. A few degrees higher, and the body goes into convulsions. A few degrees lower, and metabolism gradually shuts down, sending the body into what may be its final sleep.

Other mammals, including other primates, rely on a more or less dense covering of hair on the epidermis to conserve body heat. During the evolution of the human species, natural selection favored the development of a very light coat of hair over most of the body. Turn to Chapter 17 for some information related to that evolution.

 

Your skin is sensational

So how does your body know when it’s cold or hot? How do you know when you get a cut or splinter? How can you tell the difference between being tickled with a feather and punched with a fist? Because the dermis contains nerve endings that serve as specialized receptors for hot and cold, touch, pressure, and pain. The receptors, such as lamellated (Pacinian) corpuscles, tactile (Meissner’s) corpuscles, and Ruffini’s corpuscles, are sprinkled throughout the dermis and are connected to the nerves that run through the dermis and subcutaneous layer (hypodermis). See Chapter 7 for more about these receptors.

Not every square inch of skin contains all types of nerve endings. At one spot on your skin, you may sense light touch, while a few centimeters away, you may sense pressure; some spots sense cold, and some spots sense heat. These mixed messages connect with the network of nerves up to your brain, which does a pretty good job of making sense of the different kinds of information.

 

Your skin is self-healing

Damage to the skin, such as a cut, triggers epidermal cells to divide more rapidly, trying to fill in the space. If the cut extends into the dermis or deeper, and a blood vessel is broken, the clotting blood will first cover the area allowing time for the epidermal cells to divide and fill in the space. A covering, natural (like a scab) or otherwise (like a Band-Aid) keeps nutrient-rich fluids from seeping out, as well as germs from getting in. Special cells called phagocytes will come in and clean up the debris. Inflammation is also an essential part of this process. The dilation of the blood vessels brings more resources to the area to speed the process.

If the cut is particularly deep, the skin cells will not be able to divide fast enough to fill it in. As a result, special cells in the dermis, called fibroblasts, will be triggered to produce collagenous fibers. These bulky fibers are what can lead to a scar.