Skin protects everything that lies beneath it. It acts as a cushion against insult to the body. It alerts the body of danger (through sensation). Its continuous, tightly connected surface, lightly coated with oily sebum maintains a distinct fluid environment within our bodies from that environment encountered outside of our bodies. Also, the skin, by being such an effective barrier, is our bodies' first line of defense against invading parasites/diseases.
Our bodies are supposed to maintain a specific temperature, 37 ºC (or 98.6 ºF) at all times. We need to maintain that temperature in order for our cells to perform properly. So, our bodies use energy to produce heat just to keep our temperature up.
Meanwhile, heat continually escapes from our bodies. It escapes in four ways: 1) radiation; 2) conduction; 3) convection; and 4) evaporation. These four ways require some further explanation. After the following explanation, we will return to the notion of insulation and how that helps to prevent heat loss.
- Radiation: This is the main way we lose heat!
You know how if you light a match, the heat spreads out in all directions from the flame? We talk about the heat as radiating out from the heat source, the flame. Well, our bodies radiate heat, too. They contain heat, much like a flame contains heat (but just less of it). Heat radiates out from our bodies in all directions. Heat can be described as traveling in rays that are called infrared heat rays.
Some organisms can actually see infrared rays. For example, killer snakes have organs that detect these rays and use them to find warm-blooded animals--even in the dark. For these snakes, it is as if they see the heat rays radiating out of the warm-blooded animal. Humans do not have any ability to see infrared heat rays, that's why the notion of their existence seems weird to us. But humans have manufactured machines that can sense these infrared heat rays-- so that through this machine people seem colored from red (very hot) to blue (very cold).
All that this means is that if your body is up against something that is colder than it, heat gets transferred from your body to the colder item. For example, after you have been sitting in a chair for a while, you may notice that you have warmed it up. Another more important example is that the air that is around your body, assuming it is cooler than 37 ºC also warms up just by being next to your body.
You know that heat rises and cold air falls. We all know that. Well, the warmer air that is next to your body, as mentioned above in the "conduction" section, also rises. When it does so, cooler air that has fallen replaces it. This means that we continually lose the insulating, warmer air layer from our bodies, and so we end up losing more heat.
In order for water to evaporate, it needs more kinetic energy (energy of molecular motion)... this is proportional to temperature. So, another way to say it is that water needs higher temperatures or more heat to evaporate. Where does it get this heat from? The water comes from within our bodies and is released to the surface of our bodies by the sweat glands. If it is going to evaporate, it needs more heat-- so it gets this heat from our bodies and takes it with it when it evaporates. That leaves less heat behind, and we feel cooler.
Back to insulation...
The underlying layer of hypodermis with its adipose tissue (as well as the dense nature of the dermis) creates an insulating layer against heat loss. For example, infrared rays may not make it through the adipose, but instead may be absorbed by the adipose tissue.
Also, when we begin to get cold, we excite our arrector pili muscles and then our skin hairs stand up on end. This increases the thickness of the insulating layer of air around the outside of our bodies, and helps to slow down conduction and convection heat loss.
Finally, we can slow down sweat gland secretion, to decrease the loss of heat through evaporation. But since we can also speed it up, this fits even better in the "temperature regulation" section.
This is a bit different from mere insulation. In order to regulate temperature you have to be able to both increase it and decrease it. Imagine walking outside on a hot summer's day... it is hotter outside than your body temperature. How come you don't overheat right away? How come you can stay outside for hours (if it is not really, really hot) without passing out? Then you go inside. You enter an air conditioned building, which is set really low. It seems cold at first, but you "get used to it." How come? What did your body do?
Your body handles these situations by having your skin respond to them. Your skin can help you to lose or retain heat, depending on the situation. Kind of like having a thermostat in your home that always keeps it at 72 ºF, whether it has to run the heat or the air-conditioning to do it. Such a fancy thermostat in our homes would be rather expensive. Our skin does it automatically, using only cellular energy.
The main way that our body handles temperature changes is by altering blood vessel diameter in your skin. If you are too hot, then your blood is also too hot. You cancool down the entire body by cooling down your blood. How? Dilate your dermal blood vessels. Dilation increases their diameter, and, since we have a constant volume of blood that is running through our blood vessels under pressure, if this blood has a lower pressure way to go, like through large vessels, it will (*click here for another explanation of this pressure notion). So blood will flow into your dermal blood vessels. This causes it to run very near the surface of your body, and allows for heat to escape from your blood quite easily into the air.
Note that in the above situation, more blood enters the skin-- this causes your skin to look flushed. That is a common symptom of being hot.
If you are too cold, you can warm up your body by constricting your dermal blood vessels. This prevents too much blood from entering your skin, keeping it well inside your body and away from any possible heat loss to the environment. Your book describes this mechanism in Figure 6.12.
The other means by which we can regulate our temperature are: regulating sweat gland secretion, contracting our arrector pili muscles, and shivering. Shivering, however, occurs in muscles deeper than the skin... it's just that the heat that makes is trapped inside our bodies by our insulation. Shivering works because active muscles release heat, so it forces muscles to become active.
Keep in mind that this type of temperature regulation shows very fine tuning characteristic of all homeostatic mechanisms.
*note: Think about it this way, if you want to water your huge garden with a hose, and you have two different hoses available (one with a wide diameter and one with a small diameter), which hose would you use? You would probably use the wide diameter hose, because with the same amount of water pressure from your faucet, you could water your garden much faster. Water runs through a wider hose faster than it does through a narrow hose. Same with blood. Blood will go faster and more easily into larger diameter blood vessels. Since blood is constantly moving through our bodies' blood vessels, more will go through the dilated vessels over the course of minutes or hours than will go through constricted vessels