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

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)

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

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

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)