Thermoregulation Physiology

I. Concepts and mechanisms of thermoregulation

1. Concept:

• Thermoregulation is the ability to maintain a stable body temperature within a narrow range, despite changes in the environmental temperature.

2. Body temperature:

• Body temperature is the internal temperature of the body.
• Core body temperature: The temperature of the internal organs, usually stable within the range of 36 – 37.5°C.
  • Locations for measuring core body temperature: rectum, mouth, armpit (in order of closest to core body temperature).
• Peripheral body temperature: The temperature of the extremities, unstable and different in all locations.
  • Example: The temperature at the forehead is higher than the palm because the forehead is closer to the core, and has higher metabolism.

3. Factors affecting body temperature:

• Age: Body temperature decreases with age.
• Circadian rhythm: Body temperature is higher in the evening, lower in the early morning.
• Menstrual cycle: Body temperature in women is higher in the second half of the menstrual cycle.
• Pregnancy: Body temperature slightly increases during pregnancy.
• Children: The thermoregulation system is not yet fully developed, making them susceptible to environmental temperature changes.

4. Processes of heat production and heat loss:

• Heat production:
  • All energy-consuming processes produce heat.
  • Shivering is an important heat-producing mechanism because the efficiency of muscle contraction is only 25%, the rest is converted into heat.
• Heat loss:
  • Methods of heat exchange:
    • Direct heat transfer: Through contact with an object of a different temperature.
    • Convective heat transfer: Through the movement of fluids or air, depending on the wind speed.
    • Radiant heat: Emitted from the body surface, depending on the temperature of the emitting object and the color of the receiving object.
    • Evaporation of water: Through the skin and respiratory tract, depending on the humidity and wind.
  • Sweat is the most important heat-loss mechanism because evaporation of water through the skin and respiratory tract is insufficient when the environmental temperature > 36°C.
  • Note:
    • Evaporation of water through the skin only occurs through sweat, there is no insensible water evaporation through the skin.
    • The amount of insensible water evaporation through the skin is 0.5 liters per day.
    • The amount of water evaporation through the respiratory tract depends on the volume of pulmonary ventilation.

5. Heat balance:

• Heat balance is the equilibrium between heat produced and heat lost.
• Heat balance = Metabolic heat – Heat of water evaporation +- Radiant heat +- Convective heat.
• When heat balance is positive, heat is retained in the body, leading to an increase in body temperature.
• When heat balance is negative, heat is lost from the body, leading to a decrease in body temperature.

6. Note:

• Sweat dripping does not have any effect on heat loss.
• The body’s “comfortable temperature” is within the range of 0 – 50°C.

II. The mechanism of body temperature regulation

1. Thermoregulation center:

• Located in the hypothalamus.
• Receives information from thermoreceptors in the skin, internal organs, and blood.
• Compares actual temperature with the set point (37°C) to regulate body temperature.

2. Transmission pathway of the thermoregulatory reflex arc:

• Afferent pathway:
  • From thermoreceptors in the skin, internal organs, and blood through sensory nerves to the dorsal horn of the spinal cord => spinothalamic tract – thalamus, spinoreticular tract – reticular formation => thalamus, reticular formation of the brainstem => cerebral cortex.
• Efferent pathway:
  • Through nerves:
    • Anti-heat reflex arc:
      • Stimulates the anterior part of the hypothalamus, causing anti-heat responses:
        • Vasodilation of skin blood vessels, increased sweating, increased pulmonary ventilation, decreased heat production.
    • Anti-cold reflex arc:
      • Stimulates the posterior part of the hypothalamus, causing anti-cold responses:
        • Vasoconstriction of skin blood vessels, piloerection, shivering, increased chemical heat production, increased thyroxine secretion.
    • Both anti-heat and anti-cold reflex arcs pass through the sympathetic centers in the lateral horn of the spinal cord, causing vasoconstriction or vasodilation of the skin, and pass through the motor neurons in the anterior horn of the spinal cord, causing muscle contraction or relaxation.
  • Through the humoral pathway:
    • The hypothalamus secretes TRH, CRH => pituitary gland secretes TSH, ACTH => thyroid gland, adrenal cortex secretes hormones => changes in tissue metabolism.

3. Response organs of the thermoregulatory reflex arc:

• All cells in the body, especially:
  • Muscles: Heat production, shivering.
  • Blood vessels: Vasoconstriction or vasodilation of skin blood vessels.
  • Sweat glands: Sweat secretion.

4. Mechanisms of anti-heat and anti-cold:

• Anti-heat:
  • Sweat secretion.
  • Increased ventilation.
  • Vasodilation of skin blood vessels.
  • Decreased heat production.
• Anti-cold:
  • Vasoconstriction of skin blood vessels.
  • Piloerection.
  • Shivering.
  • Chemical heat production.
  • Increased thyroxine secretion.

5. Note:

• Sweat gland fluid differs from plasma because it lacks protein.
• Sweat gland fluid passing through the straight duct undergoes partial reabsorption of Na+ and Cl-.
• People living in hot environments have adapted sweat:
  • Greater amount.
  • Increased reabsorption of Na and Cl- due to the effect of aldosterone from the adrenal cortex.
• Brown adipose tissue is a non-shivering thermogenic tissue.
• The purpose of rapid, shallow breathing is to avoid increasing alveolar ventilation, and avoid disrupting acid-base balance.
• To dilate skin blood vessels, the core temperature acts on the blood distribution center and inhibits the sympathetic center in the hypothalamus.

III. Fever

1. Fever:

• Is a condition of increased body temperature above normal, due to a change in the body’s “set point”, controlled by the thermoregulation center.
• Causes: Due to infection, inflammation, cancer, drug effects, etc.
• Mechanism: Pyrogens act on the hypothalamus, raising the “set point”.

2. Sequence of chills:

• Shivering: Due to an increase in the “set point”, the body is cold, producing heat through shivering.
• Shivering: Vigorous, prolonged shivering, releasing heat.
• Fever: Body temperature rises to the new “set point”.
• Sweating: When the body reaches the new “set point”, the thermoregulation center lowers the “set point”, the body loses heat by sweating.

3. Decreased heat production:

• Decreased heat production by inhibiting shivering and inhibiting chemical heat production due to catecholamine hormones.

4. Note:

• Fever should be lowered when it is high and prolonged, because it can cause damage to the body.
• Heatstroke is a condition in which the body cannot lose heat due to high temperature and humidity, leading to an increase in body temperature to 41 – 42°C.
• Sunstroke is a condition in which sunlight affects the thermoregulation center, causing symptoms similar to heatstroke.
• When body temperature falls below 30°C, the hypothalamus loses its thermoregulatory function.

IV. Artificial hypothermia

1. Effects:

• Decreased metabolism.
• Reduced oxygen and nutrient requirements.
• Increased tolerance to anemia.

2. Note:

• Whole-body hypothermia is not usually applied because it can be dangerous due to decreased respiration and circulation.

V. Effects of increased body temperature

• Increased metabolism.
• Increased number of white blood cells and phagocytic capacity of white blood cells.
• Increased complement.

Note:

• Caring for children under 3 years old: Do not wrap them too tightly in blankets, but do not let them get cold because the thermoregulation mechanism of children under 3 years old is not yet fully developed.
• A sufficient area of skin is needed (a large number of receptors) for the skin to sense cold or heat.

VI. Additional information

• Applying the principle of radiant heat:
  • In the summer, wear light-colored clothing to reflect light and reduce heat absorption.
  • In the winter, wear dark-colored clothing to absorb light and increase heat.
• Artificial antipyretics act on the efferent pathway of the thermoregulatory reflex arc, mainly anticholinergic drugs.
• Stimulation of the sympathetic nervous system increases metabolism.

Hopefully, this article helps you better understand thermoregulation physiology.