2008A10 Define ‘thermoneutral zone’. Briefly explain how the body regulates temperature when the ambient temperature exceeds the thermoneutral zone.

 

List:

·       Thermoneutral zone

·       If ambient temp exceeds thermoneutral zone

·       Heat loss effectors in detail

 

Thermoneutral zone:

Definition

·  Range of environmental temperatures over which the metabolic cost of thermoreg is minimal and is effected by alteration in skin blood flow alone.

Range

·  Clothed adult 18-22°C

·  Naked adult 25-30°C

·  Neonates 32-34°C

·  Premature neonate even higher

Diagramme

Purpose

·  Core temperature must stay ~37°C

·  Preserves function of enzymes, ion channels etc

·  Heat loss must equal heat gain (except in short term)

 

If ambient temperature exceeds thermoneutral zone:

Sensors

·  Ambient temp: skin

o Warm receptors: Ruffini endings via C fibres (active >30°C)

o Cold receptors: Krause bulbs via Aδ fibres (active <25°C)

·  (Core temp: spinal cord, deep thorax and abdomen)

·  Afferents via spinothalamic tracts

Integrator

·  Hypothalamus: anterior and pre-optic areas

Effectors

·  Responses: (see below)

o Behavioural

o Autonomic

·  Mechanisms of heat loss:

o Radiation 40% ∝ (T1)4 - (T2)4  (T in Kelvin)

o Convection 30% ∝ (T1-2)

o Evaporation 15% (no gradient required)

o Respiration 10%

o Conduction 5% ∝ (T1-2)

 

Heat loss effectors in detail:

Behavioural

·  Rest (↓heat production)

·  Hide from sun (↓radiation gain)

·  Take off clothes (↑convection)

·  Pant (↑evaporation) – unimportant in man

·  Lie on a cold surface (↑conduction)

Vasodilatation

·  i.e. cutaneous arteries, veins, A-V anastomoses, precapillary sphincters

·  ↓SNS α1 activation, ? ↑SNS mAChR activation

·  ↑skin blood flow up to 30x

·  ↑skin blood volume in cutaneous venous plexuses

·  ↑skin temp

·  ↑temp gradient between skin and surrounds

·  ↑radiation, conduction, convection

·  Only effective if core temp > ambient temp

Sweating

·  ↑SNS muscarinic activation

·  Secretion of water and solute onto skin

·  Evaporation up to 2L/h

·  Latent heat 2.24MJ/kg

·  i.e. very effective

·  Less effective if high relative humidity

·  Still effective when ambient temp > core temp

 

 

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