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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