Lecture 8.2a Part 2 - Special Circulations PDF
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Aston University
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This document covers special circulations, such as cerebral, skeletal muscle, and cutaneous. It explains the high oxygen demand of the brain and how blood flow is maintained, highlighting adaptations like the circle of Willis and autoregulation. It also discusses metabolic and myogenic factors influencing blood vessel diameter. The document details the temperature regulation role of cutaneous circulation via arteriovenous anastomoses (AVAs).
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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