Medical undergraduate lecture

1. Circulation through
special regions 2
Prof. Vajira Weerasinghe
Professor of Physiology

2. 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

3. 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

4.  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

5. Circle of Willis

6. 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

7.  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

9. 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

10. 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

11. 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