R1iii / 21A09 / 18A19: Explain regulation of body temperature and antipyretic effect of paracetamol

21A09: Exam Report

Outline the mechanisms by which normal body temperature is maintained and regulated

59% of candidates passed this question.

This question was relatively well answered by most candidates. There was significant variation in the temperatures expressed as normal and few candidates mentioned CORE temperature as a concept. Several candidates gave a detailed description of thermo-neutrality for which there were no marks.

18A19: Exam Report

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

52% of candidates passed this question.

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.

R1iii / 21A09 / 18A19: Explain regulation of body temperature and antipyretic effect of paracetamol

Define: Temperature is a measure of the average kinetic energy of individual atoms that make up a substance

Temperature is a measure of the average kinetic energy of individual atoms that make up a substance.

Humans = homeothermic = actively maintain core body temp 36.5-37.3°C

If core body temperature is +/- 0.5-0.8°C outside this range, thermoregulatory mechanisms will activate to restore body temperature

Body heat is not evenly distributed

Core Temp

  • 2/3 of body heat
  • Thoracic & abdominal organs, brain
  • Tightly regulated 36.6-37.4°C
  • Circadian fluctuation; Nadir 0600h, Peak 2000h

Peripheral Temp

  • Limbs, skin, subcut tissue
  • Variable 30-32°C
  • Usually displays a 5-7°C difference w core
  • Maintained by VC to peripheral tissues

Thermoregulation

Controlled by a Negative Feedback System:

Sensors

  1. Hypothalamus
    • Cold & heat sensitive neurons
    • Measure blood temperature
    • 3x heat cf cold receptors
  2. Spinal Cord
  3. Deep Tissue Receptors
    • Viscera, Great veins
    • Increase firing w decreasing core temp
    • Primarily detect and prevent hypothermia
  4. Skin
    • Cold Receptors = Ad fibres, 10-36°C, abundant in peripheries
    • Warm Receptors = C fibres, 30-40°C

Sensory info transmitted via Spinathalamic Tract → synapse in reticular tract of medulla → Hypothalamus

Control

  • Central integration
  • Ant Hypothalamus = NA, 5HT, DA, PGI
  • Ant hypothalamus integrates input and transmits to Post Hypothalamus
  • Post Hypothalamus (Ach) compares input w set point temperature and initiates a response
  • Response depends on threshold temperature

Effectors

 

 

Cold Response

Vasoconstriction

  • Activated at 36.5°C
  • SNS control
  • Constricts vessels penetrating tissues
  • Over a large SA
  • Shunts blood to prevent heat loss

Non Shivering Thermogenesis

  • Brown fat
  • SNS/Catecholamines
  • Brown fat cells release Thermogenin which uncouples oxidative phosphorelation to release heat
  • Very important in neonates (less sk muscle mass and increases brown fat deposits)

Increase Thyroid Hormone Secretion

  • Shivering

Behaviour

Warm Response

Vasodilation

Sweating

Behaviour

Define: Fever = increased body core temperature >38°C

  • Mediated by endogenous pyrogens on hypothalamic set point
  • Adv → activation and augmentation of immune response of bactericidal, phagocytic and chemotactic activation of WBC
  • Disadv: ↑BMR and MVO2
  • ↑hypothalmic set point results in shivering to minimise heat loss
  • When hypothalamic set point is restored, patient stops shivering and starts sweating to facilitation heat loss
  • Paracetamol inhibits COX 3 enzyme, preventing Prostaglandin E2 production which causes fever