Special circulations Flashcards
Where do left and right coronary arteries arise from?
base of the aorta
How do most coronary veins drain into the atrium?
via coronary sinus into the right atrium
What special adaptations to the circulation are in the coronary arteries?
- high capillary density
- high basal blood flow
- high oxygen extraction
What does high oxygen extraction refer to?
Even under resting conditions, the heart muscle will extract most of the oxygen presented to it (75% compared to 25% whole body average)
What is coronary blood flow controlled by?
intrinsic and extrinsic mechanisms
What are the intrinsic mechanisms for low O2 levels?
- low O2 causes vasodilation of the coronary arteries
- metabolites are released which act as vasodilators
What is the main metabolite responsible for vasodilation in the intrinsic mechanism?
adenosine
What are the extrinsic mechanisms for low O2 levels?
sympathetic stimulation causing vasoconstriction
How does the body override the sympathetic stimulation?
- sympathetic stimulation causes increased HR and SV.
- This causes the heart to work harder
- This leads to decreased O2
- metabolites cause vasodilation
What part does adrenaline play in the extrinsic mechanism?
Sympathetic stimulation causes release of adrenaline which causes vasodilation. This helps to override the vasoconstriction further
What happens to the blood flow in the left coronary artery during systole?
The blood flow becomes almost 0 due to the increased pressure caused by the contraction of the left ventricle
What happens to the blood flow in the left coronary artery during diastole?
The blood flow is normal as there is less pressure
What affect would an increased HR have on oxygenation of the myocardium?
Increased HR leads to shortened diastole. Since most of the blood flow occurs in diastole this causes decreased coronary blood flow, therefore less oxygenation and therefore increased chest pain.
What happens to the blood flow in the right coronary artery during systole and diastole?
Right ventricular pressure is not as high as the left ventricular pressure. Therefore the constriction of the right coronary artery does not occur and the blood flow remains similar in both systole ad diastole.
What part of the brain is very sensitive to hypoxia?
grey matter- consciousness lost after few seconds of ischaemia, irreversible cell damage within ~ 3 minutes
What two arteries form the circle of willis?
basilar and carotid arteries
How is the circle of willis act as protection for the brain?
Major cerebral arteries arise from Circle of Willis
Cerebral perfusion should be maintained even if one carotid artery gets obstructed.
Nevertheless, obstruction of a smaller branch of a main artery would deprive a region of the brain of its blood supply
What classifies as an ischemic stroke?
Atheroma or blood clot (embolus)
What classifies as a Haemorrhage stroke?
damaged vessel wall
What is the most common type of stroke?
ischemic
What does a drop in MAP do to the arterioles in the brain?
causes vasodilation of the arterioles
What does a rise in MAP do to the arterioles in the brain?
causes vasoconstriction of the arterioles
What is autoregulation?
AUTOREGULATION of cerebral blood flow guards against changes in cerebral blood flow
What effect does sympathetic stimulation have on cerebral blood flow?
very little
Is there baroreceptor reflex in the brain
no
What are the limits for autoregualtion?
Autoregulation fails if MABP falls below ~ 60mmHg
or rise above ~ 160mmHg
What occurs in a MABP below 50mmHg?
confusion, fainting, and brain damage if not quickly corrected
What dies increased partial pressure of CO2 cause?
vasodilation
What does decreased partial pressure of carbon dioxide cause?
vasoconstriction
What does hyperventilating do?
By hyperventilating, remove a lot of CO2 causing vasoconstriction thereby reducing blood flow to brain which can lead to fainting
Where does blood flow to in the brain?
Active parts of the brain
What are the components of the skull?
brain (80%)
blood (12%)
cerebrospinal fluid (8%)
What is the normal intracranial pressure in the skull?
8-13 mm Hg
What is the equation for Cerebral perfusion pressure?
Cerebral Perfusion Pressure (CPP) = Mean Arterial Pressure (MAP) - ICP
What will increasing the intracranial pressure do?
will decrease blood flow ie cerebral perfusion pressure
What is the blood brain barrier?
cerebral capillaries with very tight intracellular junctions. These capillaries are very permeable to O2 and CO2 but not to ions
How does glucose pass the blood brain barrier?
By facilitated diffusion using specific carrier molecules
What is the nature of the pulmonary artery BP?
low, typically (20-25)/(6-12) mmHg
What affect does hypoxia have on pulmonary arteries?
Hypoxia causes vasoconstriction in pulmonary arteries so that blood gets diverted from poorly ventilated areas into the non-damaged areas.
Why do absorptive forces exceed filtration forces in pulmonary circulation?
protects against pulmonary oedema
Why does skeletal muscle blood flow increase during exercise?
- metabolites cause vasodilation
- adrenaline causes vasodilation
- increased CO
How does the skeletal muscle pump work?
contraction of muscles causes blood in veins to be pumped up which aids in venous return
The pump reduces the chance of postural hypotension and fainting
What are varicose veins?
Blood pools in lower limb veins if venous valves become incompetent
Why does varicose veins not lead to reduction of CO?
because of chronic compensatory increase in blood volume
