What is sweat?
Sweat is mostly water with a varying amount of dissolved substances, such as sodium and chloride. In addition, a little potassium, calcium, iron and other minerals are found in sweat, but the levels of these substances is much lower than sodium and chloride. Sweating is a principal means of getting rid of body heat and keeping the body from overheating. Each liter of sweat can remove 580 calories of heat form the body. Other methods of removing heat from our body include convection, conduction and radiation (Table 7.1)
Why do we sweat?
When “core” body temperature, which is the temperature in and around our vital organs increases, our brain prompts sweating. Sweating is also stimulated by circulating epinephrine, which is released into the blood by our adrenal glands during exercise. This helps us understand why we sweat more when we exercise and why we sweat even more while exercising in warmer climates. Excessive body heat warms the sweat reaching our skin until the water reaches its vapor point. Sweat water changes from a liquid to a vapor and then lifts off into the air, thus taking heat with it.
How much sweat do we produce?
Typically sweating occurs all the time, at least to some degree, even if you are not moving and the temperature seems comfortable. For instance, if you are sitting in your living room, the sweating process is still lightly stimulated by the brain to rid excessive heat. At this low level sweating might only yield two cups a few ounces (100 ml) Therefore, as that produced sweat moves slowly through the tubes, practically all of the sodium and chloride are brought back into our body along with some water. This results in only tiny amounts of water reaching our skin. In fact, you probably do not even realize that you are sweating, but you are. Oppositely, in a warmer environment and/or during exercise, when sweating is more strongly stimulated.
Can sweat composition change?
Not only can sweat vary in how much is produced but it can also vary in composition. By and large the final concentration of sweat depends on how rapidly it is produced. As shown in Figure 7.3, our sweat glands are based pretty deep in our skin. When we sweat, the initial fluid oozing into the tubes leading to our skin surface is concentrated with sodium and chloride and similar to the concentration in our blood (plasma). As that initial sweat flows through the tube, sodium and chloride can be absorbed back into our body along with some of the water. What is most important in determining the final amount and composition of the sweat reaching our skin surface is how rapid the sweat flows through the tubes, which itself is related to how much stimulation. When the flow of sweat through the tubes is faster, more sweat reaches the skin and less and less sodium, chloride, and other factors are reabsorbed. As the sweat evaporates it leaves the once-dissolved substances on our skin which can cake on a drier day.
Is it possible to increase the amount we sweat?
Because sweating is such an important means of removing heat, distance runners and other endurance athletes become “better sweaters.” This means that their sweat glands and tubes have adapted during the athlete’s training to produce larger volumes of sweat but containing less sodium and chloride. This helps keep them from overheating but at the same time it keeps them from losing excessive amounts of the key electrolytes in sweat. A well-trained endurance athlete may sweat 2 to 3 liters per hour of exercise. That’s more than 8 to 12 cups.