Sweating is an important consideration in the area of water and electrolyte balance

Sweating is an important consideration in the area of water and electrolyte balance as this is the one route where there can be extensive losses of water and electrolytes from the body in a healthy individual. If these losses are not replaced, serious consequences can ensue. Eccrine sweat is a clear, watery, odourless substance whose primary function is to promote heat loss by evaporation from the skin surface. When sweat is produced, the daily water losses increase and the intake must be increased or urine production decreased accordingly if euhydration is to be maintained.There is a daily loss on the order of approximately 500 ml of water through the skin. However, when the body is exposed to a heat stress and behavioral and vasomotor mechanisms are insufficient to prevent a rise in temperature, the physiological responses generally include an increase in sweat production in an attempt to prevent hyperthermia; the high latent heat of vaporization of water makes the evaporation of sweat an effective heat loss mechanism (evaporation of 1 l of water from the skin surface will remove 2.4 MJ (580 kcal) of heat from the body). The heat stress may be of external origin (i.e., from the environmental conditions), of internal origin due to muscular work or fever, or from a combination of these factors. In many individuals sweat rates can be in excess of 2 l per hour, especially in situations of exercise undertaken in a warm, humid environment, and these high sweat rates can be maintained for a number of hours. For example, body mass losses in marathon runners have been reported to range from about 1–6% (0.7–4.2 kg) at low (10 C) ambient temperatures to more than 8% (5.6 kg) in warmer conditions. However, when sweat rates are high, a significant fraction of the sweat secreted onto the skin may drip from the body and is therefore ineffective at removing heat.The human body has approximately 2 million sweat glands. The eccrine sweat gland consists of a single tubule, opening onto the epidermis at one end and closed at the other. The proximal half of the tubule is the secretory coil and the distal half the reabsorptive duct. The sweat secreted onto the skin is the original tubular secretion minus the substances which are, further up the tubule, reabsorbed; from the isoosmotic fluid secreted by the coil most of the major electrolytes (Naþ, Cl, HCO3þ, and lactate) are transported out of the duct back into the extracellular fluid in excess of water. The final sweat secreted onto the skin is therefore hypotonic with respect to body fluids.