Body temperature and thermal balance in the body. Mechanisms of thermoregulation.
Core Body Temperature-
- The normal core body temperature is 37°C (98.6°F).
- This temperature fluctuates by about 1°C (1.8°F) over a 24-hour period, with the lowest point typically occurring in the morning.
Heat Production and Loss-
- Cellular processes generate heat as a byproduct of metabolism.
- More active cells produce more heat.
- The brain, skeletal muscle, and visceral organs are responsible for most of the body's heat generation.
- Heat production must be balanced with heat loss to the environment to maintain a stable core temperature.
Mechanisms of Heat Exchange-
- The skin and subcutaneous fatty tissue provide insulation between the internal organs and the external environment.
- The skin's rich network of blood vessels facilitates heat exchange.
- Capillary networks in the skin, particularly in the extremities, can be closed off or dilated to adjust heat loss.
- In cooler temperatures, capillaries in the hands and feet constrict to reduce heat loss; the opposite occurs in warm conditions.
- Sympathetic nerves control the arterioles, regulating vasodilation and vasoconstriction to adjust blood flow and heat exchange.
Heat Loss Mechanisms-
Heat may be lost from the body via:
- Radiation: Loss of heat in the form of infrared waves (about 60% of heat loss).
- Conduction: Transfer of heat between objects in direct contact (a small amount of heat loss, increased by wind chill).
- Convection: Transfer of heat by the movement of fluids (air or water).
- Evaporation: Loss of heat through the evaporation of sweat (2.4 MJ of heat lost for each liter of water that evaporates).
Thermoregulation is the process of maintaining a stable core body temperature.
Role of the Nervous System-
- Thermoreceptors located throughout the body detect temperature changes.
- Some receptors respond to warmth, while others respond to cold.
- The hypothalamus is the primary control center for thermoregulation.
- It receives input from peripheral thermoreceptors in the skin and central thermoreceptors in the hypothalamus, spinal cord, and abdominal organs.
- Central thermoreceptors provide the negative feedback necessary to maintain a constant body temperature.
- The hypothalamus coordinates responses to cold and heat exposure.
Responses to Cold-
In cool conditions, the body initiates mechanisms to conserve heat and increase heat production:
- Cutaneous Vasoconstriction: Reduces blood flow to the skin, minimizing heat loss.
- Mediated by sympathetic stimulation, releasing norepinephrine, which binds to alpha-adrenergic receptors on blood vessels.
- The "hunting reaction" is a safety mechanism that causes temporary vasodilation during prolonged cold exposure to prevent frostbite.
- Shivering: Involuntary muscle contractions generate heat.