25A16: Exam Report

  1. Define the interthreshold range of body temperature and the thermoneutral zone and provide the normal values (25% of marks).

  2. Explain how the body detects (25% of marks) and defends (50% of marks) against cold exposure, including why neonates and the elderly may be more vulnerable to hypothermia.

76% of candidates passed this question.

  1. The first part of this question required clear definitions of interthreshold range (ITR) and thermoneutral zone (TNZ), relating each to the body and environment respectively. Normal values were a common omission.
  2. The second part of the question was best answered by explaining how the body detects and defends against a fall in environmental temperature using a sensor-controller-effector mechanism. Effector responses include mechanisms to reduce heat loss vs. those which generate heat and candidates were expected to explain vasomotor changes, shivering and non- shivering thermogenesis as a minimum. Discussion of other mechanisms (such as behavioral responses) attracted additional marks. Finally, candidates needed to explain how each component of the defense may be impaired in the neonate or elderly. High marks were awarded for answers that relate these explanations to the concepts of ITR and TNZ. Information about the response to warm environments or the physiological consequences of hypothermia was not required.

R1iii / R1iv / 25A16: Define the interthreshold range of body temperature

a) Define the interthreshold range of body temperature and the thermoneutral zone and provide the normal values

ITR

  • Range of core body temperatures over which no autonomic thermoregulatory responses occur.
  • 37 +/- 0.4 degrees Celsius in the non-anaesthetised state.
  • Higher thresholds (+/- 0.5 degrees) in women.
  • Core body temperature defined as deep body temperature of main internal organs.
    • Obtained via rectal, IDC or arterial (via PICCO) temperature measurements.
Graph of thermoregulatory thresholds at different temperatures.

TNZ

  • Range of ambient temperatures over which metabolic rate (and oxygen consumption) is minimal and steady.
  • 25 to 30 degrees Celsius in a naked person
    • Lower limit is the critical temperature.
    • Below this, the metabolic rate rises as temperature decreases.
    • Metabolic rate rises again once above the upper limit of the TNZ.
Graph showing metabolic rate vs environmental temperature.

a) Explain how the body detects and defends

Detection and Defence

Sensor

  • Cold Receptors (bulbs of Krause) in skin sense ambient temperature.
    • In dermis.
    • Transmit impulses to hypothalamus via Aδ fibres.
    • Increase in discharge below 25 degrees Celsius.
  • Temperature sensors also in spinal cord, hypothalamus and in deep tissue (viscera and blood vessels).

Afferent Pathway

  • Via lateral spinothalamic tract in the anterior spinal cord.
  • Synapses in the reticular system of the medulla.
  • Connects to the posterior hypothalamus.

Central Regulation

  • Posterior hypothalamus.
    • Receives cold afferents.
  • Generates a temperature set point (via Na+ and Ca++ ions).
  • Signalling via Ach neurotransmitter.

Efferent

  • Sympathetic efferents.

Effector Responses

↑ Increased Metabolic Heat Production

  • Shivering:
    • Involuntary contraction of muscles to increase BMR.
      • ATP hydrolysis → heat.
    • Non-shivering thermogenesis:
      • Via brown fat.
      • Heat production activated by β3-AR agonism (SNS/catecholamines).
      • Lipolysis of brown fat yields FFA which is used as an E source for liberation of heat.
        • BMR increases 2-3-fold.
      • VERY IMPORTANT IN NEONATES.
    • Thyroid hormones:
      • Via nuclear receptors.
      • Regulates activity of Na+/K+ ATPase.
      • Increases BMR.
      • Also increases response of brown fat to catecholamines (above).
    • Behavioural:
      • Eating food → increased metabolic heat production.

↓ Decreased Heat loss

  • Vasoconstriction:
    • SNS control (α1 agonism) of cutaneous vessels.
      • Over a large SA.
    • Decreases heat loss via radiation at skin.
      • Severe cold can lead to paradoxical vasodilation due to paralysis of peripheral blood vessels.
    • Behavioural:
      • Seek warm shelter.
      • Curling into ball (decrease SA for heat loss).
      • Add on warm layers of clothing.
    • Increased insulation
      • Goose pimples
        • From piloerector muscles of hairs.
        • Increased insulation by trapping of air next to skin.
        • Decreases convectional heat loss.
        • Less important in humans c.f animals.

Paediatric and Elderly Considerations

Paediatric

  • Increased SA:V ratio → increased SA for heat loss via radiation.
  • Nil appropriate behaviours (dependent on carer) → nil clothing, shelter etc → heat loss via radiation, convection, conduction.
  • No shivering response → decreased heat production
  • Very reliant on non-shivering thermogenesis from brown fat.

Elderly

  • Physiological
  • Behavioural
    • Reduced income, high heating costs – may not insulate appropriately
  • Dementia leads to decreased appropriate behaviour.
    • Increased heat loss from lack of shelter, clothing.

Sources:

  • Kam and Power, 4th Edition

Author: Alex Fagarasan