Circulation through special regions 2

Prof. Vajira Weerasinghe

Professor of Physiology

Objectives Explain the special features of the following regional

circulations with respect to their functions; A. Coronary

B. Cutaneous

C. Cerebral

D. Skeletal muscle

E. Splanchnic (including liver)

F. Renal

Cerebral circulation Brain least tolerant of organs to ischemia Lack of blood flow for 5 seconds causes loss of

consciousness Lack of blood flow for a few minutes causes

irreversible damage

Two internal carotids Two vertebral arteries

Basilar artery Forms the Circle of Willis No crossing over from R to L (because of equal

pressure) Occlusion of vessel produces ischaemia and

infarction

Circle of Willis

General Characteristics Rest: blood flow of 50-60 ml/min/100 g (750

ml/min)(in contrast Coronary: 70-80 ml/min/100g; 250ml/min)

15% of cardiac output (in contrast Coronary: 4% of CO)

Exercise: blood flow of 750 ml/min Greatest flow goes to grey matter (100 ml/min/100 g) 35% O2 extraction at rest

Circulation is enclosed in a rigid skull Constant volume Brain tissue is incompressible Brain “floats” in a water bath of cerebrospinal fluid High capillary density (3000 - 4000/mm2) Large surface area, short diffusion distances Blood-brain barrier - tight junctions between

endothelial cells prevents circulating vasoactive substances from affecting cerebral blood flow

Local Flow Constant cerebral blood flow is maintained

under varying conditions Factors affecting the total cerebral blood flow

Arterial pressure at brain level Venous pressure at brain level Intracranial pressure Viscosity of blood Degree of active contraction/dilatation of cerebral

arterioles which is controlled by local vasodilator metabolites

Intracranial pressure (ICP) Since the brain is enclosed within the skull the volume of blood,

brain and CSF should remain constant (Monro-Kellie hypothesis)

ICP is normally 0-10 mmHg Whenever ICP increases, cerebral vessels are compressed Change in venous pressure cause a similar change in ICP Rise in venous pressure decreases CBF by compressing the

vessels thereby decreasing perfusion pressure

Autoregulation Pronounced autoregulatory capacity from 50 -

170 mmHg Both myogenic and metabolic mechanisms

involved Sympathetic nervous system activity can shift

the curve to the right