special circulations (CVS13) Flashcards
1
Q
physiology of coronary circulation (blood supply to myocardium)
A
- right and left coronary arteries arise from the base of the aorta
- most coronary venous blood drain via the coronary sinus into the right atrium
- oxygen demand of cardiac muscle is high especially during exercise
- coronary circulation requires special adaptations
2
Q
coronary heart disease
A
- if left coronary artery is blocked at the beginning of the artery before any branches, then the full left ventricle will be deprived of blood supply
- if left coronary artery is blocked at one of its terminal branches, then only the area surrounding that branch will be deprived of blood supply
3
Q
special adaptations of coronary circulation
A
- high capillary density
- high basal blood flow
- high oxygen extraction (~75% compared to 25% whole body average) under resting conditions
- this means that extra O2 (when required) cannot be supplied by increase O2 extraction
- > can only be supplied by increasing coronary blood flow
- coronary blood flow is controlled by intrinsic and extrinsic mechanisms
4
Q
control of coronary blood flow (intrinsic mechanisms)
A
- decreased PO2 causes vasodilation of the coronary arterioles
- metabolic hyperaemia matches flow to demand
- adenosine (from ATP) is a potent vasodilator
5
Q
hyperaemia
A
hyperemia is the increase of blood flow to different tissues in the body. It can have medical implications, but is also a regulatory response, allowing change in blood supply to different tissues through vasodilation
6
Q
control of coronary blood flow (extrinsic mechanisms)
A
- coronary arterioles supplied by sympathetic vasoconstrictor nerves BUT
- over ridden by metabolic hyperanaemia as a result of increased heart rate and stroke volume
- so sympathetic stimulation of the heart results in coronary vasodilation despite direct vasoconstrictor effect (functional sympatholysis)
- circulating adrenaline activates beta2 adrenergic receptors, which causes vasodilation
7
Q
sympatholytic
A
tending to oppose the physiological results of sympathetic nervous activity
8
Q
control of coronary blood flow
A
- sympathetic stimulation:
- > increases circulating adrenaline (which activates beta2 adrenergic receptors, causing vasodilation which increases coronary blood flow)
- > has a negative effect itself on coronary blood flow as it causes vasoconstriction (however this is over ridden by the metabolic hyperanaemia which causes vasodilation)
- > increases SV and HR (causing an increase in CO which increases coronary blood flow)
- > the increase in SV also increases cardiac work (which increases metabolism)
- the increase in metabolism:
- > decreases PO2 (which causes vasodilation and increases adenosine which is a potent dilator to achieve this, therefore increases coronary blood flow)
- > increases adenosine which causes vasodilation and increases coronary blood flow
- > produces metabolites (eg. K+, PCO2 and H+) which increase coronary blood flow
9
Q
metabolites
A
a substance formed in or necessary for metabolism
10
Q
coronary flow during the cardiac cycle (graph)
A
- the peak of the left coronary flow occurs during diastole
- diastole is shortened eg. by a very fast heart rate which decreases coronary flow
11
Q
physiology of cerebral circulation (blood supply to the brain)
A
- the brain is supplied by the internal carotids and vertebral arteries
- the brain needs a secure supply of oxygen
- the grey matter is very sensitive to hypoxia (deficiency in the amount of oxygen reaching the tissues) therefore consciousness is lost after a few seconds of ischaemia (an inadequate blood supply to an organ or part of the body) and irreversible cell damage occurs within ~3 minutes of it
- > as a result of this, special adaptations of the cerebral circulation is needed to ensure the brain always has secure supply of O2
12
Q
special adaptations of cerebral circulation (circle of willis)
A
- basilar artery (formed by two vertebral arteries) and carotid arteries anastomose to form circle of willis
- major cerebral arteries arise from circle of willis
- cerebral perfusion (process of a body delivering blood to a capillary bed in its biological tissue) should be maintained even if one carotid artery gets obstructed
- nevertheless, obstruction of a small branch of a main artery would deprive a region of the brain of its blood supply
13
Q
stroke
A
- stroke is caused by interruption/cut-off blood supply to a region of the brain
- main types of stroke:
- > haemorrhagic (bleeding) stroke (blood leaks out of artery wall which is damaged) - happens to branch of vertebral artery
- > ischaemic stroke (formed due to a blood clot which forms on atheroma/the fatty material which forms deposits in the arteries, on the artery wall or comes from another part of the body and gets stuck, blood cannot flow past the clot) - occurs in internal carotid branches
14
Q
special adaptations of the cerebral circulation (regulation)
A
- autoregulation of cerebral blood flow guards against changes in cerebral blood flow if mean arterial blood pressure changes within a range (~60-160mmHg)
- direct sympathetic stimulation has little effect on overall cerebral blood flow (therefore only decreases cerebral blood flow slightly)
- participation of the brain in baroreceptors reflex is negligible, which is just as well!
- increased PCO2 causes cerebral vasodilation increasing cerebral blood flow (cause of hypercapnia), decreased PCO2 causes vasoconstriction (this is why hyperventilation could lead to fainting, because when you hyperventilate you are trying to get more O2 into the body, therefore CO2 decreases causing vasoconstriction, so the lack of CO2 causes the vessels to constrict and less O2 reaches the brain as a result)
- blood flow increases to active parts of the brain (regional hyperaemia), mechanism is unknown, may be due to rise in [K+]o as a result of K+ efflux from repetitively active neurones?
15
Q
autoregulation of the cerebral blood flow
A
- autoregulation of cerebral blood flow guards against changes in cerebral blood flow if mean arterial blood pressure changes within a range (~60-160mmHg)
- if MABP rises, resistance vessels automatically contstrict to limit blood flow
- if MABP falls, resistance vessels automatically dilate to maintain blood flow
- > autoregulation fails if MABP falls below ~60mmHg (cerebral blood flow falls), or rises above ~160mmHg (cerebral blood flow rises)
