Question 6

College Answer

It was expected candidates would describe that the maintenance of body temperature is a balance of heat loss and heat production and outline the series of controls that are important in this process.

An overview of the reflexes involved was expected with some comments on temperature receptors in skin for hot and cold, the hypothalamic integration and then the effector being skin blood flow (vasodilation / vasoconstriction and A-V shunts).

Answers were expected to include some comment on the fact that heat exchange occurs via radiation / conduction / convection/evaporation and requires a heat gradient and that skin blood changes can facilitate or impair this exchange. Using examples such as; if body needs to loose heat there is vasodilation and sweating often added clarity. Extra credit was given for discussing sweat production and the principle of how that will allow increased heat loss with further credit given for discussing the impact of humidity.

Credit was also given for mentioning piloerection, particularly if candidates went on to explain why this might be useful (even though not really applicable in man).

Discussion

As the question specifically asked for the role of the skin, and the marking rubric clearly contained a variety of distantly related elements from mechanisms for heat transfer and thermoregulation, it is remarkable that 57% of the candidates passed. To write a model answer for this is therefore challenging; as one does not know whether to answer the SAQ as it is worded, or the examiner's expectations as they are expressed in the college comments. A balance of both hopefully follows:

Skin is involved in temperature sensation and thermoeffector responses to heat and cold

Sensor function:

• Nociceptive neurons that express temperature-activated transient receptor potential (TTRP) cation channels
• Conduct to the central integrator (hypothalamus) via the lateral spinothalamic tract

Thermoeffector function:

• Cutaneous vasodilation and vasoconstriction is ​​​​​​​the mechanism for regulating the convective heat exchange between the core and the periphery.
• This is mediated by: 
  • Centrally driven α1-mediated noradrenergic vasoconstriction
  • Translocation of  α2c adrenoceptors to the smooth muscle surface membrane, which is a local temperature-mediated  effect
  • Locally driven α1-mediated noradrenergic vasoconstriction (nerves talking to nerves, bypassing the central nervous system - this is a form of reflex neurogenic vasoconstriction)
  • Nitric oxide synthase inhibition, which occurs largely due to the effects of decreased temperature on that enzyme
• By vasodilating, skin can increase its blood flow 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 - usuing 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, which would balance the heat [production of a human performing heavy exercise
  • At rest, heat production is only ~ 100W, and is balanced with dry (conductive, convective, radiative and respiratory) heat loss, with no need for sweating.
• Piloerection is of minimal importance in humans, but in furred mammals increases the thickness of the insulating air layer  

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Colin, Jean, and Yvon Houdas. "Experimental determination of coefficient of heat exchanges by convection of human body." Journal of Applied Physiology 22.1 (1967): 31-38.

Chaplin, George, et al. "The role of piloerection in primate thermoregulation." Folia primatologica 85.1 (2013): 1-17.