SWEATING & THE BODY'S EVAPORATIVE COOLING PROCESS

98.6° Fahrenheit is considered a normal human body temperature.

97.5° to 99° Fahrenheit is considered a healthy body temperature range.

The average adult loses .07 L of water per day through sweat but depending on heat or exercise intensity, can lose as much as 2.5L each day.

Sweat must convert into vapor in order for evaporative cooling to take place.

How, then, does this process actually take place in our bodies each and every day, and how can a sock promote evaporative cooling?  The answer can only be properly illustrated with a brief look at basic science.

The body’s temperature is regulated first by the rate at which skin radiates heat and, second, by the evaporation of sweat vapor off the skin.  Perspiration, the movement of sweat through pores in the skin and panting (evaporation, this time, through pores in the mouth) are common temperature regulators in warm-blooded animals.  Both processes are controlled involuntarily by the brain.

What, exactly, is the nature of sweat?  Sweat is composed of about 98 percent water and about two percent dissolved salts and nitrogenous wastes, such as urea and uric acid.  Ostensibly, we are dealing with water.  Water (H₂O) is comprised of molecules that are in constant motion, traveling at different rates. The average speed of these particles depends on the liquid’s temperature.  A rise in temperature increases molecular velocity as well as aggregate kinetic energy.  If molecules gain enough energy, their fast-moving particles will begin to bump against their neighbors.  Eventually, particles near the liquid’s surface will impart sufficient speed, and therefore sufficient kinetic energy, to cause the surface particles to propel away from the liquid in the form of gaseous molecules or, more simply, as water vapor.

As the particles with the highest kinetic energy levels evaporate, the average kinetic energy of the remaining liquid (sweat) decreases.  Because a liquid’s temperature is directly related to the average kinetic energy of its molecules, the liquid cools as it evaporates.

It is a certainty that molecules evaporate from a liquid’s surface.  Intuitively, a larger surface area will allow more molecules to leave the liquid than will a smaller one, and therefore evaporation will occur more rapidly.

The evaporative process seems relatively simple but the entire process seems to depend on heat and temperature.  How, exactly, do these participate in the process?

A calorie is the amount of heat energy needed to elevate the temperature of one gram of water by 1° Celsius.  It requires about 580 heat calories to evaporate each gram of water at room temperature.

Because of a process known as diffusion, heat will always pass from a warmer object to a cooler one.  In cold weather your skin cools not because the wintry air itself cools the skin but rather the skin looses its heat as it diffuses out into the chilled surroundings.

It is pivotal to note that heat and temperature are not one and the same.  Heat is a measure of the total kinetic energy due to molecular motion in any body of matter.  Temperature, measures the intensity of heat due to the average kinetic energy of the molecules.  When the average speed of molecules increases, a thermometer registers a rise in temperature. 

Now—finally—we can tie temperature to the body’s evaporative process.  We know that as sweat evaporates, the average temperature of the liquid left behind drops.  This evaporative cooling occurs because the hottest molecules—that is, those with the greatest amount of kinetic energy—are the most likely to escape in the form of water vapor.  Simply stated, when sweat evaporates from the skin, it carts heat away with it and naturally cools the skin.

Similarly, high humidity on a hot day causes discomfort because the elevated concentration of water vapor in the outside air inhibits the evaporative process from the body and clothes.  This happens because the amount of moisture air holds depends on its temperature—the hotter the air, the more water it can potentially hold.

Now that we understand the physics of sweat, as it were, we ought to take a look at the biology.

Sweat glands are embedded in the middle of the dermis (the layer of connective tissue that lies beneath the outside epidermis).  Their ducts penetrate the epidermis to the outside and excrete sweat.  Capillary blood vessels and nerve endings pervade the entire dermis.  Respectively, they work to feed the skin's muscles, sweat glands, and blood vessels and are responsible for the body’s sensations of touch and heat.  Located under the dermis is the layer of very loose connective tissue that fixes the skin to the body.  This layer, though not technically part of the skin, houses many fat cells.

Sweat glands and the skin’s capillary blood vessels work in tandem to allow the body to thermo-regulate itself.  This is formally known as Homeostasis.  When the body is too warm, the flow of perspiration from the sweat glands to the ducts on the surface of the skin increases.

As we learned before, as perspiration evaporates, heat too is removed from the surrounding tissues as liquid particles transform into a gas.  The desired effect, of course, is a general cooling of the body’s surface.  At the same time, the capillaries in the skin dilate, which catalyzes an intensified flow of blood to the skin’s surface. Because blood rapidly loses heat by radiation to the outside, the greater the blood flow to the skin, the greater the heat loss.  Conversely, when the body is too cool, the capillaries constrict to reduce blood flow to—and heat loss from—the skin.

How does evaporation work, specifically, with the feet?  Sweat glands on the feet are known as Eccrine.  They are most numerous on the palms and soles of the feet.  Eccrine sweat glands open at the skin’s surface while the other type of sweat glands, Apocrine, open to and empty into hair follicles. 

What does this all mean?  Let’s start with a real-life example.  Exercising causes a person’s muscles to contract, producing heat.  If this heat is not transferred from the body to the surrounding environment, the individual’s internal temperature can rise to life-threatening levels.  During vigorous activity, humans excrete perspiration through pores in their skin.  These water secretions absorb body heat and use this energy to evaporate into their environment, carrying the heat energy away with them.  This explains, then, why our athletic shirts, shorts, and socks can become so leaden with sweat.  While sweat on the chest or back may easily evaporate into the outside air, what happens to a foot trapped beneath the garb of the athletic shoe and sock?  We can begin to see, finally, the vital importance of an evaporative cooling system imbedded within the sock itself.