2001A05 Explain briefly the role of the skin in maintaining a normal body temperature.

 

List:

·       Intro

·       Skin as sensor

·       Skin for heat conservation

·       Skin for heat loss

 

Intro:

Skin

·  Body’s outer covering

·  Outer epidermis: multiple layers of keratinized cells

·  Inner dermis: rich in arteries, veins, capillaries

·  Highly variable blood flow rate, avg 600mL/min

Temperature control

·  Sensors: ambient (skin), core (spinal cord, thorax, abdomen)

·  Integration: hypothalamus

·  Efferents: behavioural and autonomic systems

·  Tight control of core tempeature ~37°C

·  Heat production = heat loss (except short term)

 

Skin sensors: for ambient temperature

Warm receptors

·  Ruffini endings via C fibres and spino-thalamic tracts

·  Become active >30°C

Cold receptors

·  Krause bulbs via Aδ fibres and spinothalamic tracts

·  Become active <25°C

 

Skin effectors: heat conservation

Vasoconstriction

·  Of arteries, veins, A-V anastomoses, pre-capillary sphincters

·  ↑Cutaneous SNS from hypothalamus -> ↑α1 adrenoceptor activity

·  ↓skin blood flow up to 10x

·  Skin temp ↓ to near ambient temp

·  ↓temp gradient between cool skin and cooler surrounds

·  ↓loss by radiation, convection, conduction

·  (Inhibited by general anaesthetic, vasodilators)

Insulation

·  Physical contact required for heat loss by convection, conduction

·  Skin and subcut fat separate internal organs from cold environment

·  Prevents radiation, convection, conduction, evaporative loss from core

·  (Laparotomy -> direct contact with air -> ↑↑heat loss)

Countercurrent exchange

·  Cutaneous arteries lie next to cutaneous veins

·  Temperature gradient is established: ↓from internal to external

Piloerection

·  Unimportant in man

 

Skin effectors: heat loss

Vasodilation

·  Of arteries, veins, A-V anastomoses, pre-capillary sphincters

·  ↓Cutaneous SNS from hypothalamus -> ↓α1 adrenoceptor activity (and ↑mAChR?)

·  ↑skin blood flow up to 30x

·  ↑skin blood volume in cutaneous venous plexuses

·  Skin ↑ to near core temp

·  ↑temp gradient between warm skin and cooler surrounds

·  Also ↑flow of warm blood through cool skin

·  ↑loss by radiation, convection, conduction (only if temp body > temp surrounds)

Sweating

·  ↑SNS activity at mAChR

·  Secrete water + electrolyte onto skin

·  Evaporation up to 2L/h

·  Latent heat of vapourisation 2.26MJ/kg (i.e. high)

·  Evaporation rate ∝ skin temperature / ambient humidity

·  Active even when temp body < temp surrounds (doesn’t require gradient)

Insensitive loss

·  Transepidermal diffusion

·  400mL/day

·  Not under direct control

·  Latent heat of vapourisation at 2.24MJ/kg

·  Evaporation rate ∝ skin temp / ambient humidity

·  Active even when temp body < temp surrounds (doesn’t require gradient)