Question 16

College Answer

(a) The definition of temperature and heat varies considerably in texts, so reasonable leeway was granted in marking this part.

(b) In this section the following information was expected; body temperature range; thermoneutral zone; sensors (including receptors, fibres, and tracts); central processing (the role of the hypothalamus including anterior and posterior portions); interthreshold range; and a detailed description of the efferent responses for either increasing or decreasing body temperature.

Discussion

The leeway offered by the CICM examiners to the candidates presumably extends also to the the Commission for Thermal Physiology of the International Union of Physiological Sciences, who define heat and temperature in their  Glossary of Terms in 2001:

Temperature: A measure of the mean kinetic energy of the molecules in a volume.

Heat: see Energy.

Energy: Energy may occur either as chemical, electromagnetic, or mechanical energy. [Ws = J]. Synonymous to mechanical energy (force times length, height or distance) is work, synonymous to thermal energy is heat. Work or heat per unit time (→ work rate or → heat flow) constitute physically → power, and are used as components in the body heat balance equation.

Thermoregulation:

• 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).
  • 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.

• 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  

Commission for Thermal Physiology of the International Union of Physiological Sciences: Glossary of terms for thermal physiology, 3rd ed. Japn J Physiol 2001; 51:245-280

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.