Skin, Hair, and Nails - Sweat glands and blood vessels

While everybody is aware that the amount we perspire is related to the temperature around us, not everybody is aware that the countless tiny blood vessels in our skin—some 15 feet of them coursing beneath every square inch of skin—also react to changes in outside temperatures. Working together, and both controlled by an automatic “thermostat” in our brain, sweat glands and blood vessels have the all-important role of keeping our internal organs near their normal 98.6° Fahrenheit temperature.

The trick in maintaining an internal body temperature near normal is to conserve body heat when it is colder outside and to lose heat when it is warmer. Blood circulating near the surface of the skin is warmed (gains heat) or cooled (loses heat) according to the outside temperature.

The skin's myriad blood vessels constrict when the outside temperature is colder. This means that less blood can come into contact with the colder outside air, and therefore the overall temperature level of the blood remains warmer than if the blood vessels had not become constricted. On the other hand, when the body needs to lose heat—for example, during and after a vigorous tennis match—the skin's blood vessels dilate. This accounts for the “heat flush” or reddening of skin that light-skinned people exhibit when very heated.

Sweat glands aid in temperature regulation by secreting moisture, which, evaporating on the skin's surface, cools the skin and therefore the blood flowing beneath it. Moisture that does not evaporate but remains as liquid on the skin or runs off in rivulets is not efficient in cooling. Humid air tends to prevent evaporation, while moving air or wind aids it. Sweat that evaporates as soon as it reaches the skin's surface usually goes unnoticed. Fresh sweat has no odor; but if it remains without evaporating, bacteria begin to give it the odor known medically as bromhidrosis .

There are some two million sweat glands in the skin. Each consists of a coiled, corkscrewlike tube that tunnels its way up to the surface of the skin from the dermis or from the deeper subcutaneous layer.