Special ciculations Flashcards
Where do the right and left coronary arteries arise from?
The ascending aorta
Where does most of the coronary venous blood drain?
Via the coronary sinus into the right atrium
What are the special adaptations of coronary circulation?
High capillary density, high basal blood flow and high oxygen extraction
What is the difference between oxygen extraction by the myocytes compared with other cells in the body?
Myocytes are able to extract around 75% of oxygen (the whole body average in around 25%)
How does the percentage of extraction affect the way in which oxygen supply is increased to the heart?
As the extraction of oxygen cannot be increased any further, the only way to increase oxygen supply to the heart is to increase coronary blood flow
What intrinsic mechanisms increase coronary blood flow?
Decreased PO2 causes vasodilation of the coronary arterioles
Metabolic hyperaemia matches flow to demand
Adenosine (from ATP) is a potent vasodilator
What nerves supply the coronary arterioles?
The sympathetic vasoconstrictor nerves
What are the sympathetic nerves over-ridden by?
Metabolic hyperaemia as a result of increased heart rate and stroke volume
What hormone causes vasodilation and by action on what receptor?
Adrenaline activates B2 receptors causing vasodilation
Explain the affect of symoathetic circulation on coronary blood flow
Sympathetic stimulation has a vasoconstrictor affect on the coronary arterioles. But it also causes an increased SV and HR, increasing CA which causes vasodilation (increasing coronary blood flow)
During what phase of the cardiac cycle does peak left coronary blood flow happen?
Ventricular diastole as during systole the vessels are squeezed
Explain how trachycardia can result in chest pain
During trachycardia ventricular diastole is shortened. This means there is less time for coronary blood flow and the oxygen demand of the heart is not met
Why is there not a great difference in cornonary blood flow between systole and diastole on the right side of the heart?
They pressure in the right ventricle is lower
What is the Circle of Willis
Part of the cerebral circulation. Internal carotid arteries + vertebral arteries Composed of the: R&L anterior cerebral arteries Anterior communicating artery L&R internal carotid artery L&R posterior cerebral artery L&R posterior communicating artery
What is the advantage of the Circle of Willis?
Cerebral perfusion should be maintained even if one carotid artery gets obstructed
What causes a stroke?
Interruption/cut-off of blood supply to a region of the brain
What is the purpose of autoregulation of cerebral blood flow?
It guards against changes in cerebral blood flow if mean arterial pressure changes within a range (around 60-160mmHg)
What happens to resistance vessels is the MABP rises?
They automatically constrict to limit blood flow
What happens if MABP falls?
The resistance vessels automatically dilate to maintain blood flow
What happens is MABP falls below 50mmHg
Confusion, fainting and brain damage (if not quiclky corrected)
What is the affect of increased PCO2 on cerebral blood flow?
Cerebral vasodilation which increases blood flow
What is the affect of decreased PCO2 on cerebral blood flow?
Causes vasoconstriction
What is normal intracranial pressure (ICP)?
About 8-13mmHg
What is the formula for cerebral perfusion pressure (CPP)?
CPP = mean arterial pressure - intracranial pressure (ICP)
