special circulations

Question 1

what are the special adaptions of coronary circulation?

Answer 1

• high capillary density
• high basal blood flow
• high oxygen extraction under resting conditions
• this means extra O2 cant be supplied by increasing coronary blood flow
• can only be supplied by increasing coronary blood flow
• coronary blood flow is controlled by intrinsic and extrinsic mechanisms

Question 2

how do the intrinsic mechanisms of coronary blood flow work?

Answer 2

• decreased pO2 causes vasodialtion of the coronary arterioles
• metabolic hyperaemia matches flow to demand

Question 3

what is a potent dilator?

Answer 3

adenosine

Question 4

what are the extrinsic mechanisms of coronary blood flow?

Answer 4

• coronary arterioles are supplied by sympathetic vasoconstrictor nerves but over-ridden by metabolic hyperaemia as a result of increased heart rate and stroke volume
• so sympathetic stimluation of the heart results in coronary vasodilation despite direct vasocontrictor effect
• circulating adrenaline activates B2 adrenergic receptors which cause vasodilation

Question 5

what when peak low coronary flow occur?

Answer 5

• during diastole

Question 6

when does coronary blood flow and myocardial pefrusion occur?

Answer 6

• in diastole when the subendocardial vessels form the left coronary artery are not compresses

Question 7

what are the special adaptions of cerebral circulation?

Answer 7

• basilar and carotid arteries anastomose to form circle of willis
• major cerebral arteries arrise from circle of willis
• cerebral perfusion should be maintained even if one carotid artery gets obstructed
• autoregulation of cerebral blood flow

Question 8

when does a stroke occur?

Answer 8

• cut off of blood supply to a region of the brain

Question 9

what is autoregulation of cerebral blood flow?

Answer 9

• if MABP rises, resistance vessels automatically constrict to limit blood flow
• autoregulation fails of MABP falls below 60mmHg or above 160mmHg
• MAPB below 50mmHg = fainting, confusion etc

Question 10

what other important regulations of cerebral blood flow are there?

Answer 10

• increased PCO2 causes cerebral vasodilation

- blood flow increases to active parts of the brain (regional hyperaemia)

Question 11

what is the skull filled with?

Answer 11

• 80% brain
• 12% blood
• 8% cerebrospinal fluid

Question 12

what is normal intracranial pressure?

Answer 12

8-13mmHg

Question 13

how is cerebral perfusion pressure calculated?

Answer 13

mean arterial pressure - intracranial pressure

Question 14

what happens when ICP is increased ie due to head injury or brain tumour?

Answer 14

• decreases CPP and cerebral blood flow

Question 15

what is the blood brain barrier?

Answer 15

cerebral capilaries which have very tight intercellular junctions

Question 16

what are cerebral capillaries highly permeable to?

Answer 16

O2 and CO2

Question 17

what cross the BBB and how?

Answer 17

glucose by facilitated diffusion using specific carrier molecules

Question 18

what is the BBB impermeable to and why is it important?

Answer 18

• hyprophillic substances
• eg ions, catecholamines, proteins etc
• importnat becasye it helps protect the brain neurones from fluctuating levels of ions etc in blood

Question 19

what are the special adaptions of pulmonary circulation?

Answer 19

• pulmonary capillary pressure is low compared to systemic capillary pressure
• absorptive forces excees filtration forces - protects against pulmonary oedema
• hypoxia causes vasoconstriction of pulmonary arteries

Question 20

why is resting blood flow low?

Answer 20

because of sympathetic vasconstrictor tone

Question 21

what happens to skeletal muscle blood flow during exercise?

Answer 21

• during exercise, ocal metabolic hyperaemia overcomes sympathetic vasoconstrictor activity
• circulating adrenaline causes vasodilation
• increased cardaic output during exercise increases skeletal muscle blood flow