Control of Body Temperature

Thermoregulatory System

• The thermoregulatory system is composed of sensory receptors (thermoreceptors), central integrator (centre), and effector organs.
• Peripheral thermoreceptors are located in the skin and detect cold and warmth, with more cold receptors present.
• Deep body temperature receptors are present in abdominal viscera and the spinal cord to detect body core temperature.

Central Thermoreceptors

• The anterior hypothalamus and the preoptic area contain a large number of heat and cold sensitive neurons.
• These receptors are sensitive to core temperature (brain and blood temperature).

Thermoregulatory Center (Thermostat)

• The thermoregulatory center is present in the hypothalamus.
• It receives impulses from the thermoreceptors and compares them with its specific standard reference temperature (set-point) of approximately 37.1°C.
• If body temperature is higher than the set-point, the anterior hypothalamus is stimulated, leading to heat loss.
• If body temperature is lower than the set-point, the posterior hypothalamus is stimulated, leading to heat production and reduced heat loss.

Effector Organs

• Skin (blood vessels and sweat glands)
• Skeletal muscle
• Endocrine glands

Effects of Exposure to Cold

• Decrease in heat loss:
  • Vasoconstriction of skin blood vessels
  • Counter-current heat exchanger
  • Behavioral responses (putting on heavy clothes, curling the body, and erection of hair as an insulator)
• Increase in heat production:
  • Shivering
  • Hormonal thermogenesis (adrenaline, thyroxin, and cortisol)
  • Behavioral responses (increased appetite)

Effects of Exposure to Heat

• Decrease heat gain:
  • Inhibition of mechanisms that cause heat production
  • Behavioral responses (apathy and anorexia)
• Increase heat loss:
  • Vasodilatation of skin blood vessels
  • Sweating
  • Behavioral responses (drinking large amounts of water)

Sweating

• Sweat is a hypotonic secretion of NaCl.
• The center for sweating is the preoptic nuclei in the anterior hypothalamus.
• Sweat glands are supplied by sympathetic cholinergic fibers.
• Sweat secretion is an active process in which the acini secrete isotonic sweat, which is then modified to become hypotonic sweat.
• Cooling effect of sweat: each 1ml of evaporated sweat removes 0.6 KCal.

Acclimatization of Sweating

• In acute exposure to hot weather, a person sweats 700ml/h and loses 15-30gm NaCl/day.
• After exposure to hot weather for 6 weeks, a person sweats 2000ml/h and loses 3-5gm NaCl/day.

Disorders of Temperature Regulation

Fever (Pyrexia)

• Definition: hyperthermia caused by resetting of the set-point of the hypothalamus to a higher level.
• Mechanism of fever:
  • Toxins of bacteria and degenerated tissue act on monocytes and macrophages, releasing interleukin (IL-1) and Tumor Necrosing Factor (TNF).
  • Endogenous pyrogens (IL-1) reach the hypothalamic thermosensitive neurons, causing fever within 10 minutes.
• Control of fever:
  • PGE2 has negative feedback on interleukin I.
  • Down regulation of interleukin I receptors.
  • Glucocorticoids as cortisol inhibit interleukin I.
  • Aspirin as an antipyretic drug inhibits the synthesis of PGE2 from arachidonic acid.

Heat Stroke

• Cause: exposure to hot humid weather or high fever.
• Mechanism: excessive sweating leads to dehydration and salt loss, depression of the heat regulating center, and irreversible denaturation of tissue proteins.
• Clinical picture: dizziness, loss of fluids and sweat, and circulatory shock.
• Treatment: immediate cooling of the body by immersion in ice cold water, sponge with alcohol, and antipyretic drugs.

Sun Stroke

• Beside sweating and dehydration, damage to brain tissue by direct sun rays leads to severe fever.
• Treatment: immersion in ice bath and drinking saline.

Physiological Changes Associated with Hyperthermia

• Central Nervous System:
  • Stimulation of the CNS leading to tremors and convulsions.
  • Malfunction of the CNS leading to loss of reflexes and coma.
• Cardiovascular System:
  • Increase in heart rate by 10 beats/min for each 1°C.
  • Increase in cardiac output due to vasodilatation of peripheral arterioles.
  • Increase in systolic blood pressure and decrease in diastolic blood pressure.
• Respiration:
  • Increased respiratory rate by stimulation of the central and peripheral chemoreceptors.

Hypothermia

• Definition: a drop of body temperature to a low level with slow metabolic and physiologic processes.
• Causes:
  • Exposure to extreme cold water (ice water) for 20 minutes.
  • Frost bite.
  • Artificial hypothermia induced by strong sedatives or cooling the patient with ice.
• Consequences: impaired ability of the hypothalamus to regulate body temperature, leading to sleepiness, coma, and even heart stop.

Thermoregulatory System

• The thermoregulatory system is composed of sensory receptors (thermoreceptors), central integrator (centre), and effector organs.
• Peripheral thermoreceptors are located in the skin and detect cold and warmth, with more cold receptors present.
• Deep body temperature receptors are present in abdominal viscera and the spinal cord to detect body core temperature.

Central Thermoreceptors

• The anterior hypothalamus and the preoptic area contain a large number of heat and cold sensitive neurons.
• These receptors are sensitive to core temperature (brain and blood temperature).

Thermoregulatory Center (Thermostat)

• The thermoregulatory center is present in the hypothalamus.
• It receives impulses from the thermoreceptors and compares them with its specific standard reference temperature (set-point) of approximately 37.1°C.
• If body temperature is higher than the set-point, the anterior hypothalamus is stimulated, leading to heat loss.
• If body temperature is lower than the set-point, the posterior hypothalamus is stimulated, leading to heat production and reduced heat loss.

Effector Organs

• Skin (blood vessels and sweat glands)
• Skeletal muscle
• Endocrine glands

Effects of Exposure to Cold

• Decrease in heat loss:
  • Vasoconstriction of skin blood vessels
  • Counter-current heat exchanger
  • Behavioral responses (putting on heavy clothes, curling the body, and erection of hair as an insulator)
• Increase in heat production:
  • Shivering
  • Hormonal thermogenesis (adrenaline, thyroxin, and cortisol)
  • Behavioral responses (increased appetite)

Effects of Exposure to Heat

• Decrease heat gain:
  • Inhibition of mechanisms that cause heat production
  • Behavioral responses (apathy and anorexia)
• Increase heat loss:
  • Vasodilatation of skin blood vessels
  • Sweating
  • Behavioral responses (drinking large amounts of water)

Sweating

• Sweat is a hypotonic secretion of NaCl.
• The center for sweating is the preoptic nuclei in the anterior hypothalamus.
• Sweat glands are supplied by sympathetic cholinergic fibers.
• Sweat secretion is an active process in which the acini secrete isotonic sweat, which is then modified to become hypotonic sweat.
• Cooling effect of sweat: each 1ml of evaporated sweat removes 0.6 KCal.

Acclimatization of Sweating

• In acute exposure to hot weather, a person sweats 700ml/h and loses 15-30gm NaCl/day.
• After exposure to hot weather for 6 weeks, a person sweats 2000ml/h and loses 3-5gm NaCl/day.

Disorders of Temperature Regulation

Fever (Pyrexia)

• Definition: hyperthermia caused by resetting of the set-point of the hypothalamus to a higher level.
• Mechanism of fever:
  • Toxins of bacteria and degenerated tissue act on monocytes and macrophages, releasing interleukin (IL-1) and Tumor Necrosing Factor (TNF).
  • Endogenous pyrogens (IL-1) reach the hypothalamic thermosensitive neurons, causing fever within 10 minutes.
• Control of fever:
  • PGE2 has negative feedback on interleukin I.
  • Down regulation of interleukin I receptors.
  • Glucocorticoids as cortisol inhibit interleukin I.
  • Aspirin as an antipyretic drug inhibits the synthesis of PGE2 from arachidonic acid.

Heat Stroke

• Cause: exposure to hot humid weather or high fever.
• Mechanism: excessive sweating leads to dehydration and salt loss, depression of the heat regulating center, and irreversible denaturation of tissue proteins.
• Clinical picture: dizziness, loss of fluids and sweat, and circulatory shock.
• Treatment: immediate cooling of the body by immersion in ice cold water, sponge with alcohol, and antipyretic drugs.

Sun Stroke

• Beside sweating and dehydration, damage to brain tissue by direct sun rays leads to severe fever.
• Treatment: immersion in ice bath and drinking saline.

Physiological Changes Associated with Hyperthermia

• Central Nervous System:
  • Stimulation of the CNS leading to tremors and convulsions.
  • Malfunction of the CNS leading to loss of reflexes and coma.
• Cardiovascular System:
  • Increase in heart rate by 10 beats/min for each 1°C.
  • Increase in cardiac output due to vasodilatation of peripheral arterioles.
  • Increase in systolic blood pressure and decrease in diastolic blood pressure.
• Respiration:
  • Increased respiratory rate by stimulation of the central and peripheral chemoreceptors.

Hypothermia

• Definition: a drop of body temperature to a low level with slow metabolic and physiologic processes.
• Causes:
  • Exposure to extreme cold water (ice water) for 20 minutes.
  • Frost bite.
  • Artificial hypothermia induced by strong sedatives or cooling the patient with ice.
• Consequences: impaired ability of the hypothalamus to regulate body temperature, leading to sleepiness, coma, and even heart stop.

This quiz covers the thermoregulatory system, including peripheral thermoreceptors, sensory receptors, central integrator, and effector organs. It also explores the pathways and sites involved in detecting body temperature.