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

1. Arterioles
2. 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