R1ii / 14B06: List the mechanisms involved in heat production and loss by the body

14B06: Exam Report

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)

23% of candidates passed this question.

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.

R1ii / 14B06: List the mechanisms involved in heat production and loss by the body. Define Thermoneutral Zone and Interthreshold Range

Define:  Heat = a form of energy determined by how active the molecules of a substance are

Heat Production

  • Heat is a by product of metabolism
  • Most heat generated by deep viscera
  • Rate of heat production determined by
    1. BMR = 40kcal/m2/h
      • 2/3 of energy available from metabolism of nutrients is released as heat
    1. Extrametabolic Activity
      • Muscle activity; voluntary/involuntary
      • Non shivering thermogenesis = all energy of metabolism of brown adipose tissue is released as heat and is an important method of heat production in neonates who have small muscle mass
      • Basal Cell metabolism and temperature of actual cells
      • Breaking of chemical bonds
      • Flow of ions across membranes
      • Hormones (thyroxine, growth hormone) = increase metabolic rate
      • NA, Adr, sympathetic stimulation = increase metabolic rate

Heat Loss

Occurs when body is in contact with external environment

For heat loss to occur:

  1. Transfer heat from core → skin
  2. Transfer heat from skin → environment

Core → Skin

  • Depends on insulation and blood flow
  • Insulation
    • Fat conducts heat 1/3 the rate of other tissue
    • Fat and subcutaneous tissue acts as a heat insulation
    • Body heat is not evenly distributed and peripheral temps are 2-3°C less than core temperature
  • Blood Flow
    • Skin has rich blood vessel network
    • Venous plexus is supplied by skin capillaries and a-v anastomoses
    • Allow heat to bypass the insulator system
    • Therefore blood flow can vary depending on VC/VD
    • Increase blood flow = increase heat transfer to skin = promotes heat loss

Skin → Environment

  1. Radiation 60% heat loss

Heat loss in the form of infrared waves

All objects not 0°C emit infrared waves

VD = increased blood flow to skin

Skin becomes an effective radiator

  1. Conduction 3% heat loss

Skin in direct contact w cooler object transfers its kinetic energy molecules to molecules it is in contact with

  1. Convection 15% heat loss

Removal of heat from body by convesction of air currents

Heat must first be conducted to air then removed by convection

Increase wind = increase convection = increase heat loss

Clothing traps air next to skin thus reducing conduction and heat loss

  1. Evaporation 20% heat loss

Energy is required to change H2O from liquid to vapour

Heat is lost due to the latent heat of water

Water evaporates from skin and airways

Insensible losses  approx. 600ml/day, due to continuous diffusion of water through skin and respiratory surface despite body temperature

Sweating = loss of heat by evaporation of sweat

  1. Behaviour

Less clothing = increase heat loss by radiation

Increase activity – increase heat loss by radiation and evaporation

Touching cool objects – increase heat loss by conduction

Thermoneutral Zone & Interthreshold Range

Humans are homeothermic = actively maintain core body temp w/in narrow range 36.5 – 37.3°C

0.5°C outside of this range activates thermoregulatory mechanisms to restore body temperature

Posterior hypothalamus compares sensory thermal input and initiates response depending on temperature.  Temperature where response is triggered KA Threshold Temperature

Interthreshold Range = range of core temperatures where no thermoregulatory response is triggered.  Normally 0.2-0.5°C from 37°C

Thermoneutral Zone = ambient temperature where we do not have to actively regulate our body temperature.  Therefore metabolic rate and O2 consumption is minimal.

 Adult 25-30°C.  Neonate 32-34°C.