Skin blood flow
[Ref: BL8:p241-245]
- Oxygen and nutrient requirement small relative to blood supply.
- Primary function of skin circulation is thermoregulation.
- Countercurrent mechanism present to aid in heat conservation.
Two types of vessels
- Arterioles
- AV anastomoses
1. Arterioles
- Some basal tone
- Dual-control: by sympathetic nervous system and autoregulation
- Neural factors dominate over local factors
Sympathetic nervous system
- Vasoconstriction can be caused by
* Noradrenergic stimulation
* Circulating catecholamine
- Denerved skin shows enhanced sensitivity to circulating catecholamine
- Skin in certain regions are under influence of higher centres of CNS
* head, neck, shoulder, upper chest
* Mechanism of blushing
NB:
- Cholinergic fibres of sympathetic system stimulates sweat glands
--> Bradykinin produced (?? by an enzyme in sweat)
--> Local vasodilation
Autoregulation
- Myogenic mechanism mainly
- Also show reactive hyperemia
2. AV anastomoses
- No basal tone
- Not under metabolic control
- No autoregulation
- No reactive hyperemia
- Highly sensitive to catecholamine
- Regulation is by CNS in thermoregulation or by higher centres
Effect of temperature
- Cold temperature causes vasoconstriction
- Prolonged cold exposure causes secondary vasodilation
Regulation centres
[WG22:p254]
Posterior hypothalamus control the reflex response activated by cold
- Posterior controls heat production.
Anterior hypothalamus control the reflex response activated by warmth
NB:
- Hypothalamic thermoregulatory reflexes in skin overrides all other reflexes that influence skin perfusion.
- (Probably a mistake) [BL8:244]
Thermoregulation center in anterior hypothalamus causes vasoconstriction in response to cold.
--> Should've been posterior??
Skin blood flow
Some skin facts
[WG21:p615]
- 3.6kg by weight
- Blood flow ~ 9% of cardiac output
= 450mL of blood/min
- Oxygen consumption ~ 5% of body oxygen consumption
= 12.5mL O2/min
Some sweat facts
[KB2:p21-22]
[Na+] of sweat varies from 30 to 65 mmol/L
* Decreased by aldosterone
Daily Na+ loss in sweat
= 5 to 350 mmol/day
