Thermoregulation Mechanisms

Regulation of Body Temperature

• Two mechanisms to regulate body temperature:
  • Decrease heat gain: by inhibiting mechanisms that cause heat production and behavioral responses such as apathy and anorexia
  • Increase heat loss: through vasodilatation of skin blood vessels and sweating

Sweating

• Sweat is a hypotonic secretion of NaCl
• Center: preoptic nuclei in the anterior hypothalamus
• Supply: eccrine sweat glands are supplied by sympathetic cholinergic fibers (blocked by atropine)
• Sweat secretion: an active process in which the acini secrete isotonic sweat, but NaCl is gradually reabsorbed by the ducts to make it hypotonic
• Cooling effect of sweat: each 1ml of evaporated sweat removes 0.6 K Cal
• Sweat starts at environmental temperature of 32°C
• Dribbling alone without evaporation does not lead to loss

Acclimatization of Sweating

• Acute exposure to hot weather: a person sweats 700 ml/h and loses 15-30 gm NaCl/day
• After exposure to hot weather for 6 weeks: a person sweats 2000 ml/h and loses 3-5 gm NaCl/day
• Cold sweat: an emotional sweating even with cold and vasoconstriction

Disorders of Temperature Regulation

• Fever (pyrexia): hyperthermia caused by resetting of the setpoint of the hypothalamus to a higher level
• Counter-current heat exchanger: directs blood to deep veins, which run parallel to the arteries, to conserve heat

Behavioral Responses

• Putting on heavy clothes
• Curling the body to decrease surface area
• Erection of hair as an insulator for cold (sympathetic effect)

Increase in Heat Production

• Shivering: involuntary rhythmic contractions of the skeletal muscle to produce large amounts of heat
• Center: posterior hypothalamus in the primary motor center for shivering
• Shivering can be prevented by curare (neuromuscular blocker)

Hormonal Thermogenesis

• Adrenaline:
  • Stimulates the metabolic rate
  • Causes cutaneous vasoconstriction
  • Stimulates glycogenolysis
  • Stimulates lipolysis (of depot fat)
• Thyroxin: stimulates the Thyroid Stimulating Hormone (TSH) to secrete thyroxin hormone, which increases the basal metabolic rate (BMR)
• Cortisol: stimulates the blood glucose and metabolic rate

Effect of Exposure to Heat

• Exposure to heat stimulates the hypothalamus to regulate body heat balance
• Loss of fluids and sweat may lead to circulatory shock and tissue degeneration
• Treated by: immediate cooling of the body, sponge with alcohol, and antipyretic drugs

Sun Stroke

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

Physiological Changes Associated with Hyperthermia

• Central Nervous System:
  • At first, hyperthermia stimulates the CNS leading to tremors and convulsions
  • Above 41°C, malfunction of CNS occurs leading to loss of reflexes and coma
• Cardio Vascular System:
  • Increase heart rate by 10 beats/min for each 1°C
  • Increase in body temperature due to direct stimulation of SAN or cardio-accelerator centre
  • Increase cardiac output due to vasodilatation of peripheral arterioles
  • Increase systolic blood pressure and decrease diastolic blood pressure
• Respiration:
  • Increased respiratory rate by stimulation of the central and peripheral chemoreceptors

Hypothermia

• 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: exposure to extreme cold weather leading to freezing in lobes of ears and digits of hands and feet
  • Impaired regulation of body temperature by the hypothalamus, leading to sleepiness and even coma