Chapter 24.6 Thermoregulation and Heat Exchange

Thermoregulation

  • Thermoregulation is the process by which the body tightly regulates its temperature, maintaining it within certain boundaries even when the surrounding temperature differs significantly.
  • The core body temperature remains steady at around 36.5–37.5 °C (97.7–99.5 °F).
  • During ATP production, about 60% of the energy is produced as heat, which helps maintain body temperature.
  • Thermoregulation is an example of negative feedback.

Hypothalamus: The Thermostat

  • The hypothalamus in the brain acts as a thermostat to regulate the body’s core temperature.
  • If the temperature is too high, the hypothalamus initiates processes to lower it, such as:
    • Increasing blood circulation to the body's surface for heat dissipation through the skin.
    • Initiating sweating to cool the skin through water evaporation.
  • If the temperature falls below the set core temperature, the hypothalamus initiates shivering to generate heat.
    • Shivering increases energy use and heat production.
    • Thyroid hormone stimulates more energy use and heat production by cells.

Thermoneutral Environment

  • An environment is thermoneutral when the body doesn't expend or release energy to maintain its core temperature.
  • For a naked human, this is around 84 °F ambient air temperature.
  • When wearing clothes or in higher temperatures, the body uses cooling mechanisms.
  • The body loses heat through heat exchange mechanisms.

Hypothalamus Controls Thermoregulation

  • If body temperature is low:
    1. Temperature receptors in the hypothalamus stimulate heat-producing mechanisms.
    2. Superficial arteries are constricted, reducing heat loss to the air.
    3. Blood flow to the digestive system decreases.
    4. Shivering increases aerobic respiration in muscles, releasing heat.
    5. The thyroid stimulates cells to increase metabolic heat production.
    6. Body temperature increases.
  • If body temperature is high:
    1. Temperature receptors initiate heat-releasing mechanisms.
    2. Superficial arteries are dilated, causing flushing and increasing heat loss to the air.
    3. Blood flow is not diverted away from the digestive system.
    4. Sweating is initiated in the skin.
    5. The thyroid stimulates cells to decrease metabolic heat production.
    6. Body temperature decreases.
  • Temperature homeostasis: 36.5−37.5°C36.5-37.5°C

Mechanisms of Heat Exchange

  • When the environment isn't thermoneutral, the body uses four mechanisms of heat exchange to maintain homeostasis: conduction, convection, radiation, and evaporation.
  • Each mechanism relies on heat flowing from a higher to a lower concentration and varies in rate according to environmental temperature and conditions.
Conduction
  • Conduction is the transfer of heat by two objects in direct contact.
  • Example: Holding a glass of ice water or warming hands around a hot mug.
  • Only about 3% of the body’s heat is lost through conduction.
Convection
  • Convection is the transfer of heat to the air or water surrounding the skin.
  • Warmed air/water rises and is replaced by cooler air/water.
  • Example: Body losing heat to water when the water temperature is lower than the body’s temperature, potentially leading to hypothermia.
  • About 15% of the body’s heat is lost through convection.
Radiation
  • Radiation is the transfer of heat via infrared waves between two objects with different temperatures.
  • Example: A radiator warming a room or the sun warming the skin.
  • About 60% of the heat lost by the body is lost through radiation.
Evaporation
  • Evaporation is the transfer of heat by the evaporation of water.
  • Evaporating water (sweat) takes a great deal of energy from the skin.
  • The rate of evaporation depends on relative humidity.
  • Sweating is the primary means of cooling during exercise.
  • At rest, about 20% of the heat lost by the body occurs through evaporation.

Metabolic Rate

  • The metabolic rate is the amount of energy consumed minus the amount of energy expended by the body.
  • The basal metabolic rate (BMR) describes the amount of daily energy expended by humans at rest in a neutrally temperate environment while in the postabsorptive state.
  • It measures how much energy the body needs for normal, basic, daily activity.
  • About 70% of all daily energy expenditure comes from the basic functions of the organs.
  • Another 20% comes from physical activity, and the remaining 10% is for thermoregulation.
  • BMR is higher if a person is more active or has more lean body mass.
  • As you age, the BMR generally decreases as the percentage of muscle mass decreases.