Question 19

Explain the mechanisms by which normal body temperature is maintained and regulated.

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College Answer

The best answers were systematic, using a sensor, integrator, effector approach, while also mentioning physiological variations i.e. diurnal, with ovulatory cycle etc. Few candidates raised the concept of central and peripheral compartments. The differentiation of the concepts of set point, interthreshold range and thermoneutral zone was often confused.

Discussion

There is no mention of the  interthreshold range and thermoneutral zone in the stem, which means the candidates must have pulled this out and then tripped over it. 

  • Thermoregulation is required to maintain temperature within a narrow range (the interthreshold range: the range between core temperature at the onset of shivering and that at the onset of sweating, usually between 36.5 ºC and 37.5 ºC).
  • Temperature sensors can be divided into peripheral and central
    • Peripheral sensors are nociceptive neurons that express temperature-activated transient receptor potential (TTRP) cation channels; report to the hypothalamus via the lateral spinothalamic tract
    • Central sensors are temperature-sensitive neurons in the preoptic area of the hypothalamus, which sense core temperature 
  • The hypothalamus is the integrator for temperature sensation
    • Specifically the preoptic area of the hypothalamus
  • Thermoeffectors  are skin, skeletal muscle, sweat glands, and brown adipose tissue  
    • Non-shivering thermogenesis by muscle and brown adipose tissue mostly due to futile proton leak through the inner mitochondrial membrane which uncouples oxidative phosphorylation from ATP synthesis 
    • Shivering, involuntary muscle contractions which produce heat through the hydrolysis of ATP
    • Hypeventilation (and panting in animals), to increase evaporative heat loss via the upper respiratory tract 
    • Behavioural changes, eg. shelter or warmth-seeking, exercise 
    • Thermoeffector functions of the skin are the quantitatively important:
      • Cutaneous vasodilation and vasoconstriction is the mechanism for regulating the convective heat exchange between the core and the periphery.
      • By vasodilating, skin can increase to 50-70% of total cardiac output, increasing the convective heat exchange
      • This increases the efficiency of heat loss by convection, radiation, and evaporation of sweat.
      • Sweating  is quantitatively the most important - using the latent heat of vaporisation of sweat (2.4 kJ per gram of sweat at 30°C)
        • At maximum sweat production (~2000ml/hr), up to 1700W of heat can be dissipated under ideal conditions
      • Piloerection is of minimal importance in humans, but in furred mammals increases the thickness of the insulating air layer  

References

Kuht, James, and Andrew D. Farmery. "Body temperature and its regulation." Anaesthesia & Intensive Care Medicine 15.6 (2014): 273-278.

Romanovsky, Andrej A. Thermoregulation Part I: From basic neuroscience to clinical neurology. Elsevier, 2018.

Romanovsky, Andrej A. Thermoregulation Part II: From basic neuroscience to clinical neurology. Elsevier, 2018.