Sweating Part 2

Author: Sarah Marshall H.BSc CPT
How Sweat is Made

We are constantly sweating, even though we may not notice it. Sweating is your body's major way of getting rid of excess body heat, which is produced by metabolism or working muscles. The amount of sweat produced depends upon our states of emotion and physical activity. Sweat can be made in response to nerve stimulation, hot air temperature, and/or exercise. First, let's concentrate on how sweat is made in an eccrine sweat gland. When the sweat gland is stimulated, the cells secrete a fluid (primary secretion) that is similar to plasma -- that is, it is mostly water and it has high concentrations of sodium and chloride and a low concentration of potassium -- but without the proteins and fatty acids that are normally found in plasma. The source of this fluid is the spaces between the cells (interstitial spaces), which get the fluid from the blood vessels (capillaries) in the dermis. This fluid travels from the coiled portion up through the straight duct (Figure 2). What happens in the straight duct depends upon the rate of sweat production or flow:

Low sweat production (rest, cool temperature) - Cells in the straight duct reabsorb most of the sodium and chlorine from the fluid. This happens because there is enough time for reabsorption. In addition, water is reabsorbed osmotically. So not much sweat reaches the outside. Also, the composition of this sweat is significantly different from the primary secretion. There is not as much sodium and chloride, and there is more potassium.

High sweat production (exercise, hot temperature) - Cells in the straight portion do not have enough time to reabsorb all of sodium and chloride from the primary secretion. So, a lot of sweat makes it to the surface of the skin and the composition is close to, but not exactly like the primary secretion. The sodium and chloride concentrations are about half as much, and potassium is about 20 percent higher.

Sweat is produced in apocrine sweat glands in the same way. However, the sweat from apocrine glands also contains proteins and fatty acids, which make it thicker and give it a milkier or yellowish color. This is why underarm stains in clothing appear yellowish. Sweat itself has no odor, but when bacteria on the skin and hair metabolize the proteins and fatty acids, they produce an unpleasant odor. This is why deodorants and anti-perspirants are applied to the underarms instead of the whole body.

The maximum volume of sweat that a person who is not adapted to a hot climate can produce is about one liter per hour. Amazingly, if you move to a hot climate such as the American desert southwest or the tropics, your ability to produce sweat will increase to about two to three liters per hour within about six weeks! This appears to be the maximum amount that you can produce.

Cooling Down

When sweat evaporates from the surface of your skin, it removes excess heat and cools you. This is actually due to a neat principle in physics, which goes like this. To convert water from a liquid to a vapor, it takes a certain amount of heat called the heat of vaporization. This heat energy increases the speed of the water molecules so that they can escape into the air. Typically, all of the sweat does not evaporate, but rather runs off your skin. In addition, not all heat energy produced by the body is lost through sweat. Some is directly radiated from the skin to the air and some is lost through respiratory surfaces of the lungs. A major factor that influences the rate of evaporation is the relative humidity of the air around you. If the air is humid, then it already has water vapor in it, probably near saturation, and cannot take any more. Therefore, sweat does not evaporate and cool your body as efficiently as when the air is dry.
Finally, when the water in the sweat evaporates, it leaves the salts (sodium, chloride and potassium) behind on your skin, which is why your skin tastes salty. The loss of excessive amounts of salt and water from your body can quickly dehydrate you, which can lead to circulatory problems, kidney failure and heat stroke. So, it is important to drink plenty of fluids when you exercise or are outside in high temperatures. Sports drinks contain some salts to replace those lost in the sweat. Nervous or Scared?

As we mentioned, sweating responds to your emotional state. So when you are nervous, anxious or afraid, there is an increase in sympathetic nerve activity in your body as well as an increase in epinephrine secretion from your adrenal gland. These substances act on your sweat glands, particularly those on the palms of your hand and your armpits, to make sweat. Thus, you feel a "cold" sweat. Also, the increased sympathetic nerve activity in the skin changes it electrical resistance, which is the basis of the galvanic skin response used in lie detector tests.

Excessive Sweating

Excessive sweating -- usually on the palms of the hand or the armpits -- that is not caused by emotional or physical activity is called diaphoresis or hyperhidrosis. It is often an embarrassing condition. The cause or causes are unknown, but the condition may be due to the following:

hormonal imbalances (e.g., menopause in women)
overactive thyroid gland (The thyroid hormone increases body metabolism and heat production.)
certain foods and medications (e.g., coffee with its high amounts of caffeine)
overactivity of the sympathetic nervous system

This condition can be treated by medications and surgical procedures.

Question

Why do feet stink? Why do some people's feet smell worse than other people's? How can I get my feet to smell better?

Answer

Everybody is familiar with this phenomenon. Most of us have a friend or relative who can clear out a room when they kick off their shoes. And even the sweetest smelling person can do a decent job stinking up a pair of sh
oes by running a few miles in them. So what's going on here? Why do your feet have a stronger odor than the rest of you does? The main thing that feeds foot smell is sweat. With more than 250,000 sweat glands each, your feet are among the most perspiring parts of the body. In one day, each foot can produce more than a pint of sweat! Sweat is basically just salt and water, though, so it doesn't have a distinctive smell of its own. The smell is actually caused by bacteria on our skin that eats the sweat and excretes waste that has a strong odor. It's perfectly normal to have bacteria on your skin, and it doesn't ordinarily produce a noticeable smell, but sweat attracts bacteria and gives them a whole lot to feed on.

Of course we sweat all over -- our hands have a comparable number of sweat glands, for example -- and most of the rest of our body doesn't particularly stink (the armpits being a notable exception. See How Sweat Works for details.) So what's different about our feet? The answer is our socks and shoes. The sweat our feet excrete can't easily escape into the air like the sweat our hands excretes -- it all collects on our skin and in our socks. The bacteria love this dark, damp feast and have a sort of feeding frenzy. When you take off your shoes, the smell that hits you is all the bacteria excretion that's collected on your feet and in your socks and shoes.

The main reason some people's feet (or more precisely, some people's socks and shoes) smell worse than other people's is that some people sweat more than other people. This is just one of the many variable physiological qualities of human beings. This is also why sometimes your feet smell much worse than at other times -- it all has to do with how much you sweat.

So, since foot odor is caused by bacteria digesting sweat, there are two main ways to reduce the stink. You can:
decrease the amount of bacteria on your feet
decrease the amount of sweat that collects on your feet and in your shoes
Reducing the level of bacteria is really a matter of cleanliness. To control the bacteria population on you feet, you should:
wash your feet with strong anti-bacterial soap
wear clean socks
don't wear the same shoes everyday - give a pair of shoes 24 hours or more to air out before wearing them again. To reduce the amount of sweat that collects in your shoes, you should:
wear well-ventilated shoes instead of very constrictive shoes, such as boots
always wear socks, preferably made of cotton or other absorbent materials that absorb a lot of the sweat so the bacteria can't feed on it
change your socks a few times a day
buy some absorbent Odor-Eater type shoe inserts
apply an antiperspirant to your feet
If your foot odor is really bad and these solutions don't help much, then you should see a doctor. There are a number of prescription drugs that can treat serious foot odor, some by killing bacteria and some by reducing foot sweat.

Question

What is in an antiperspirant that stops sweat?

Answer

Most people put on either deodorant or antiperspirant before leaving the house. There are some products that perform both functions. These products are used to control sweat and odor in our underarms. There are two types of glands in our underarms, apocrine and eccrine. The eccrine glands are by far the most numerous sweat glands and are responsible for producing most of the sweat in our underarms, as well as in our entire body. Some people wear deodorants to cover up underarm smells, but if you sweat a lot, you probably need an antiperspirant to slow down the production of underarm sweat. Our bodies are constantly producing sweat, but there are certain times when they produce a lot more. Additional sweat is produced to cool down our bodies when we are exposed to heat, physical exertion, stress or nervousness. When the sweat gland is stimulated, the cells secrete a fluid that travels from the coiled portion of the gland up through the straight duct and out onto the surface of our skin.

Solid antiperspirants are made with several ingredients, including wax, a liquid emollient and an active-ingredient compound. It's the active ingredient that gives antiperspirants their sweat-blocking power. All antiperspirants have an aluminum-based compound as their main ingredient. If you look at the back of an antiperspirant container, the aluminum-based compound is always the first ingredient listed. Here are a few of the common active ingredients:
Aluminum chloride
Aluminum zirconium tricholorohydrex gly
Aluminum chlorohydrate
Aluminum hydroxybromide

The aluminum ions are taken into the cells that line the eccrine-gland ducts at the opening of the epidermis, the top layer of the skin, says dermatologist Dr. Eric Hanson of the University of North Carolina's Department of Dermatology. When the aluminum ions are drawn into the cells, water passes in with them. As more water flows in, the cells begin to swell, squeezing the ducts closed so that sweat can't get out.
Each cell can only draw in a certain amount of water, so eventually, the concentrations of water -- outside and inside the cells -- reach equilibrium. When this happens, the water inside the cell begins to pass back out of the cell through osmosis, and the cell's swelling goes down. This is why people have to re-apply antiperspirant. For those who suffer from excessive sweating, hyperhydrosis, aluminum chloride in high concentrations can prolong the swelling and may ultimately shrink the sweat gland, decreasing the amount of sweat it can produce. According to Hanson, an average over-the-counter antiperspirant might have an active-ingredient concentration of about 1 to 2 percent. For those who have excessive underarm sweating, there are prescription products that contain concentrations as high as 40 times those of over-the-counter antiperspirants.