Lecture 8.2a Part 2 - Special Circulations PDF

Cerebral circulation: ◦Brain has very high oxygen demand ◦Brain is very intolerant to reduced blood supply ◦Interruptions - unconscious within 5 seconds, irreversible damage after 4 minutes ◦Adaptations: ‣ High capillary density - many blood vessels in brain...

Cerebral circulation: ◦Brain has very high oxygen demand ◦Brain is very intolerant to reduced blood supply ◦Interruptions - unconscious within 5 seconds, irreversible damage after 4 minutes ◦Adaptations: ‣ High capillary density - many blood vessels in brain ‣ Short diffusion distance (~10 um) ‣ High basal flow rate ‣ High oxygen extraction (from the blood) Cerebral circulation - how secure blood flow is maintained: ◦Structural adaptations: ‣ Circle of Willis - anastomoses between basilar and internal carotid arteries Allows the brain to have constant blood supply in case one side fails ◦Functional adaptations to maintain constant blood flow: ‣ Myogenic autoregulation - change in blood pressure leads to change in vessel diameter Increase in blood flow -> increase BP -> constriction Decrease in blood flow -> decrease BP -> vasodilation ‣ Metabolic autoregulation - change in pCO2 leads to change in vessel diameter Metabolically active tissue -> Increase CO2 -> Increase pCO2 -> hypercapnia (high CO2 levels) -> vasodilation Low pCO2 -> hypocapnia -> vasoconstriction Cushing's reflex: ◦Pathological increase in intracranial pressure -> compress brain ◦Increase in ICP -> impair blood flow to brain stem -> increase in sympathetic activity -> increase in BP -> increase blood flow ‣ Increase in BP triggers baroreceptors, so stimulates parasympathetic NS, which decreases HR ◦But if this fails, Cushing's triad: ‣ Triad consisting of bradycardia, irregular respiration and systolic hypertension Skeletal muscle circulation: ◦Functions: ‣ Meet metabolic need during exercise ‣ Many of the capillaries are closed off from the rest of the circulation due to the contraction of pre-capillary sphincters (therefore less blood flow to capillary bed ‣ This results in a higher vascular tone and the constriction of vessels ‣ This allows to cope with the increased workload during exercise ‣ Metabolic hyperaemia Cutaneous circulation: ◦Relatively less metabolically active, main role in temperature regulation ◦Blood supply different from other tissues ◦Skin has arteriovenous anastomoses (AVAs) instead of capillaries ◦AVAs play a role in temperature regulation ‣ High temperature -> decrease sympathetic NS -> vasodilation -> increase blood flow -> increase heat loss -> decrease temperature ‣ Low temperature -> increase sympathetic NS -> vasoconstriction -> decrease blood flow-> decrease heat loss -> increase temperature ◦Sympathetic nervous system influences blood flow through AVAs