Heat Loss by Evaporation OccursWhen the Water in Sweat, Saliva, and Respiratory Secretions Is Converted into WaterVapor
The evaporation of 1 L of water into water vapor requires 580 kilocalories (kcal). If the body provides this heat, evaporation can be a major form of heat loss. Some evaporative heat loss occurs continuously by the diffusion of water through the skin and by loss of water vapor from the respiratory tract.
This water loss is obligatory, but under thermal stress, evaporative cooling can be increased greatly because sweat glands are activated or the animal begins to pant. Evaporative heat loss becomes increasingly important as the ambient temperature approaches body temperature; it is the only form of heat loss available once ambient temperature exceeds body temperature. The effectiveness of evaporation is reduced as the relative humidity increases, that is, as the air becomes more saturated with water vapor.Sweating occurs from two types of coiled, tubular sweat glands located in the dermis. Apocrine glands produce a proteincontaining secretion, whereas eccrine glands produce an aqueous secretion. All placental mammals except rodents and Iagomorphs have sweat glands, but in the dog and pig these glands are poorly developed and of little use in thermoregulation. Apocrine glands produce the thermoregulatory sweat of hoofed animals, whereas in primates, sweat is produced by the eccrine glands. Secreted sweat has an ionic composition similar to that of plasma. As it passes to the skin surface along the duct, its composition is altered by the reabsorption of ions. If secretion rates are low, almost all the sodium and chloride, along with water, is absorbed. Therefore the sweat reaching the skin is a concentrated solution of urea, lactic acid, potassium ions, and, in the case of hoofed mammals, protein. When secretion rates are high, less sodium and chloride are absorbed, more water is lost, and the other constituents are consequently diluted. In hot environments, acclimatization increases the sweating rates, and because of increased secretion of aldosterone, most of the sodium and chloride is reabsorbed before the sweat reaches the skin.
In most species, sweating is under the control of sympathetic cholinergic nerve fibers, but in the horse, control is through β2 adrenoceptors activated by catecholamines originating from sympathetic nerves or the adrenal medulla.Panting is one mode of increasing evaporation from the respiratory tract. Small tidal volumes are moved at rapid frequency (200 breaths/min) over the respiratory dead-space. The rate of panting is close to the resonant frequency of the respiratory system, and thus the work of breathing is minimized and does not add to the heat load. In the panting animal, two mechanisms act to elevate heat loss through evaporation: (1) vascular engorgement of the respiratory and oral mucosa and (2) increased salivation. By ventilating primarily deadspace, severe hyperventilation and respiratory alkalosis are avoided. In birds,gular flutter is another method of increasing airflow over the respiratory dead-space. Even in mammals that do not pant, such as the horse, evaporative heat loss from the respiratory tract probably increases during prolonged exercise because dead-space ventilation increases.
Mammals vary in the relative importance of different modes of evaporative heat loss. In horses and cattle, sweating is the major form of evaporative heat loss. Sheep sweat, but panting is also of considerable importance. The dog relies almost totally on panting. Even small rodents, which neither pant nor sweat, increase evaporative heat loss by smearing saliva or water on their fur.