Question 6

List the mechanisms involved in heat production and loss by the body. (80% of marks) Define thermoneutral zone and inter-threshold range. (20% of marks)  

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College Answer

Many candidates did not read the question carefully and misinterpreted what was being asked. Candidates often digressed into a discussion of thermoregulation. Several candidates wrote 
about body's response to cold and heat rather than mechanisms of heat production and loss as  was asked. There was confusion between mechanisms of endogenous heat production and measures to 
conserve heat. "Behaviour" only attracted marks in relation to voluntary muscle activity for heat production. Changing clothes or seeking a warm environment does not increase heat output by the body. Behaviour can reduce heat loss. Many candidates did not specify ambient or core body temperature. The definitions of Thermoneutral Zone and Interthreshold Range were not clear. Knowledge generally lacked detail and this was most evident when precise definitions were 
asked.

Discussion

One may counter that the candidates likely read the question very carefully, which still did not protect them, because surely a discussion of the mechanisms involved in heat loss would touch on such thermoregulatory strategies as sweat evaporation. Still, it should be possible to answer without specifically mentioning anything thermoregulatory. For example:

  • Mechanisms of heat production are metabolic chemical reactions that yield heat.
    • Breaking of high energy binds, eg. hydrolysis of ATP into ADP and Pi, yields chemical energy which is used to drive endothermic chemical reactions, but it is not utilised with 100% efficiency, and the residual energy is dissipated as heat.
  • Cellular reactions which produce heat, that are quantitatively important, are mostly localised to the mitochondria, and include:
    • Oxidation of NADH
    • ATP synthesis and the activity of the electron transport chain
    • Proton leak through mitochondrial inner membrane
    • Na+/K+ ATPase activity
    • Actin/myosin cross bridge cycling
  • Sites of thermogenesis are: 
    • Organs and tissues with a high baseline metabolic rate
      • Brain (60 kJ/hr)
      • Liver (60 kJ/hr)
      • Intestine (170 kJ/hr)
      • Heart (50 kJ/hr at rest) 
    • Organs and tissues with uncoupled mitochondria or scalable metabolic activity, which can increase their metabolic rate on demand:
      • Muscle (80 kJ/hr at rest, but up to five times more with shivering, and up to twenty times more with strenuous exercise) 
      • Brown adipose tissue (from 9 to 60 kJ/100g/hr)
  •  Heat loss occurs by:
    • Radiation -  heat transfer by emission of IR-spectrum electromagnetic radiation (50%)
    • Convection - heat transfer by conduction to a moving gas or liquid, eg. air (30%)
    • Evaporation (20%), loss of heat energy to the latent heat of vapourisation of water, where
      • Respiratory heat transfer usually contributes about 10% (2% heating of inspired air, and 8% latent heat of vapourised lung water)
      • The rest is contributed by sweat evaporation
    • Conduction, transfer of heat energy to a lower-temperature object by direct surface contact (usually 0%)
  • Thermoneutral zone is the range of ambient temperatures where the body can maintain its core temperature solely through regulating dry heat loss by skin blood flow; 28-32 °C for a nude human and 14.8 °C - 24.5 °C for lightly clothed.
  • 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.

References

Romanovsky, Andrej A. "The thermoregulation system and how it works." Handbook of clinical neurology 156 (2018): 3-43.

Houdas, Y., et al. "Man and his environment." Human Body Temperature: Its Measurement and Regulation (1982): 57-80.

Hardy, James D., Eugene F. Du Bois, and G. F. Soderstrom. "Basal metabolism, radiation, convection and vaporization at temperatures of 22 to 35 C.: Six figures." The journal of nutrition 15.5 (1938): 477-497.

Kakitsuba, Naoshi, Igor B. Mekjavic, and Tetsuo Katsuura. "Individual variability in the peripheral and core interthreshold zones." Journal of Physiological Anthropology 26.3 (2007): 403-408.

Kingma, Boris, Arjan Frijns, and Wouter van Marken Lichtenbelt. "The thermoneutral zone: implications for metabolic studies." Frontiers in Bioscience-Elite 4.5 (2012): 1975-1985.