Thermoregulation and Heat Balance Quiz

Thermoregulation and Heat Loss Mechanisms

• Radiative heat loss decreases as the temperature gradient between the skin and ambient air decreases
• At temperatures above 36ᵒC, heat loss occurs primarily through evaporation, regulated by sweat production
• 1 liter of water evaporated from the skin surface removes 580 kcal of heat
• Sweat glands are innervated by sympathetic cholinergic nerves and can deliver up to 6L fluid/hr to the skin surface
• Evaporation rate depends on ambient temperature and humidity
• The thermoregulatory center is located in the anterior hypothalamus and determines the temperature set point
• The hypothalamic control of thermoregulation involves cholinergic and adrenergic mechanisms
• The thermoregulatory responses to changes in core body temperature involve heat-loss and heat-promoting centers in the hypothalamus
• Hyperthermia can result from prolonged exposure to heat, high ambient humidity, or physical exertion
• Hypothermia can be mild, moderate, or severe, with severe hypothermia below 28ᵒC
• Heat stroke can result from thermoregulatory failure, leading to CNS dysfunction and even death
• The primary heat loss mechanism at room temperature is evaporation of fluid from the skin

Thermoregulation and Heat Balance

• Core body temperature (Tb) is homeostatically regulated at 37ᵒC (98.6 ᵒF) with a range of 36 - 37.5 ᵒC (97 - 99.5 ᵒF).
• Tb varies with activity and ambient temperature, increasing during exercise to ~40 ᵒC and decreasing in extreme cold weather to ~35 ᵒC.
• Skin temperature (Ts) varies widely with changes in ambient temperature and cutaneous blood flow.
• Rhythmic changes in body temperature include circadian and monthly rhythms, with a 0.5 ᵒC increase during the luteal phase of the menstrual cycle.
• Heat balance is maintained when heat gain equals heat loss, with gain > loss leading to increased heat storage and gain < loss leading to decreased heat storage.
• The body's high heat capacity, primarily due to its water content, requires approximately 70 kcal of heat energy to increase the body temperature of a 70 kg individual by 1 ᵒC.
• Heat production is mainly through metabolism, with basal metabolic rate (BMR) accounting for 50-70% of daily energy expenditure and influenced by factors such as age, sex, hormones, and digestive state.
• Muscular activity and non-shivering thermogenesis in brown adipose tissue are other mechanisms of heat production.
• Heat loss occurs through radiation, conduction, convection, and evaporation, with evaporation being the primary mechanism at high ambient temperatures and during strenuous physical activity.
• Heat transfer between the core and the environment involves convection and conduction, with heat dissipation occurring from areas of high temperature to areas of lower temperature.
• Regulation of skin temperature is controlled by the autonomic nervous system, with sympathetic activity leading to vasoconstriction and heat storage, and decreased sympathetic tone leading to vasodilation and heat loss.
• Naked individuals in rooms with different ambient temperatures lose heat predominantly by radiation to surrounding solid objects, with increased evaporative heat loss at higher temperatures.