Control of Body Temperature
In mammalian (homeotherm) species, the temperature of the deep tissues of the body—the core—remains within a narrow range (within ±1° F), even when exposed to extreme ambient temperature variations.
In contrast, the skin temperature rises and falls with the temperature of the surroundings, giving it an important role in core temperature regulation. However, because the temperature regulatory mechanisms are not perfect, the core body temperature of even healthy animals can sometimes rise temporarily to as high as 104° F (40° C) when excessive body heat is produced (such as during a strenuous exercise) or, conversely, temporarily fall below 97° F (36° C) when the body is exposed to extreme cold. Maintenance of body temperature is under neuronal control in a negative feedback system. The first mechanism involves warm- and cold-sensitive neurons within the preoptic region of the anterior hypothalamus (POAH), which act similarly to a thermostat with a desired “set point.”1 Temperature sensors are also present in the skin and deep tissues of the body such as the abdominal viscera and around the great veins of the abdomen and thorax.1 Most of the temperature-sensitive neurons in the skin respond to cold so that environmental temperature changes are detected before they threaten core temperature.2 It is also for that reason that shivering occurs after exercise as the skin rapidly cools during sweat evaporation, despite the fact that core temperature may be normal or slightly elevated. Those temperature signals produced by the central and peripheral receptors are transmitted to the POAH, which then integrates them and initiates mechanisms to either increase or decrease heat loss or production (Fig. 4.1).3 It is also important to note that the temperature “set point” varies between species and individuals and that fluctuations in body temperature occur normally throughout the day due to circadian rhythm.1,4 For example, pigs have a higher temperature “set point” than do sheep, as the former lack hair or fiber. Conversely, dairy cattle that have a high milk production have a lower temperature “set point,” as heat production increases with their increased metabolic demand. Temperature “set points” can also be affected by acclimatization to certain ambient temperatures. As such, with heat acclimatization, there is a lower body temperature threshold for sweating, and sweat gland sensitivity and capacity 4improve.4
Most of the heat produced in the body is generated in the skeletal muscles and deep organs. Muscle activity may range from inapparent contractions to generalized shivering and vary according to the need. Digestion of food also contributes significantly to heat production, and the amount of heat produced per gram of food differs between food categories (carbohydrates, fat, and proteins), with fat producing more than twice as much heat as carbohydrates and proteins.1 Heat conservation occurs from adrenergic autonomic stimuli to decrease peripheral circulation (vasoconstriction) and cause piloerection. Behavioral means of heat conservation include adopting a “huddled” posture, group aggregation, and seeking a sheltered environment.
Heat loss occurs from conduction, convection, and radiation from body surfaces and by evaporation. Sympathetic vasodilation of cutaneous vessels contributes to surface cooling. As ambient temperature rises, evaporative heat loss becomes more important. In ruminants, evaporative heat loss is mostly confined to the respiratory system, and respiratory rate increases concurrently with temperature. In horses, sweat production and subsequent evaporation have the most important role in heat loss, followed by respiratory tract evaporation. Behavioral responses that contribute to heat loss include seeking shade and wind currents and wading into water.