Special Circulations Flashcards
Where do the right and left coronary arteries arise from?
base of aorta
Where does most coronary venous blood drain?
Into right atrium from coronary sinus
What are the special adaptations of coronary circulation?
- high basal blood flow
- high capillary density
- high oxygen extraction
What is the only way extra oxygen van be supplied in coronary circulation?
by increasing coronary flow, which is controlled by intrinsic and extrinsic mechansims
Potent vasodilator
Adenosine from ATP
Intrinsic mechanisms of coronary blood flow
Decrease in oxygen saturation causes vasodilation of coronary arterioles, thus metabolic hyperaemia maths flow to demand
What nerves supply coronary arterioles?
Sympathetic vasoconstrictor nerves
Why does metabolic hyperaemia override sympathetic vasoconstrictive nerve supply?
Due to an increase in HR and SV
Functional sympatholysis
Sympathetic stimulation of heart results in coronary vasodilation (due to being overrides by metabolic hyperaemia) despite direct vasoconstrictor effect
Extrinsic mechanisms
Adrenaline activate B2-ADR-> vasodilation
Metabolic hyperaemia-> overrides sympathetic vasoconstriction-> vasodilation
At what stage does most of the coronary blood flow and myocardial perfusion?
during diastole, when subendocardial vessels from LCA are not compressed
What supplies the brain?
Internal carotids and vertebral arteries
Which arteries anastomose to form the Circle of Willis?
Basilar (formed by two vertebral arteries) and carotid arteries
When does autoregulation come into play?
if MAP changes within the range of around 60-160mmHg (outwith this range, autoregulation will fail and cerebral blood flow will rise/ fall)
What happens if MAP rises in autoregulation of cerebral blood flow?
Automatic vasoconstriction
What happens if MAP falls below approx 50mmHg?
Results in confusion, fainting and brain damage if not corrected quickly
Why can hyperventilation lead to fainting?
a decrease in oxygen saturation causes vasoconstriction
Regional hyperaemia
Blood flow increases to active parts of the brain
Possibly because of rise in K+ conc due to K+ efflux from repetitive active neurones
Increasing PaO2 _____ cerebral blood flow
increases
Normal intracranial pressure
8-12mmHg
How to calculate cerebral perfusion pressure
CPP=MAP-ICP
Effect of increasing ICP on CPP and cerebral blood flow
Decreases CPP and cerebral blood flow
Blood brain barrier
Tight intercellular junctions of cerebral capillaries
What substances is the BBB very impermeable to?
hydrophilic substances eg ions, catecholamines, proteins
What substances is the BBB very permeable to?
Oxygen and carbon dioxide
What helps protect brain neurones from fluctuating levels of ions etc. in blood?
BBB is exceptionally impermeable to hydrophilic substances
Average pulmonary artery BP
20/6-25/12mmHg
how does pulmonary circulation protect against pulmonary oedema?
absorptive forces exceed filtration forces
why does hypoxia cause vasoconstriction of pulmonary arterioles?
It helps divert blood from poorly ventilated areas of lung
Why is resting skeletal muscle flow low?
sympathetic vasoconstrictor tone
Why does skeletal muscle blood flow increase during exercise
- Local metabolic hyperaemia overcomes sympathetic vasoconstrictor tone
- adrenaline causes vasodilation
- increased CO-> increased skeletal muscle blood flow
Varicose veins
Blood pools in lower limbs if venous valves become incompetent, however this doesn’t usually lead to reduced CO due to chronic compensatory increase in blood volume
