Coronary, cerebral and cutaneous circulation Flashcards
(37 cards)
What is a-vO2 difference?
The amount of O2 a tissue extracts to meet its metabolic demands.
A large a-vO2 shows a high demand
How does coronary circulation maintain a secure O2 supply?
Alters local flow via FUNCTIONAL HYPERAEMIA. It can increase by 4-5 times when CO increases due to the CORONARY RESERVE.
What is the a-vO2 difference in coronary circulation?
Very large at 120ml.L extracting almost maximum at rest as it cannot withstand anaerobic conditions.
As the coronary tissue extracts almost max O2 at rest, how is an increase in O2 demand met?
Increase flow
How does coronary flow vary throughout the cardiac cycle?
The flow to muscle is intermittent as in order for it to flow Pa>Pv AND Pin>Pout. A pressure difference must be present to keep the vessel open.
What is the flow to coronary vessels on the left side of the heart?
Flow to LV ceases during SYSTOLE as Pout>Pin. The vessels are compressed from the high ventricular pressure.
Most flow occurs during DIASTOLE.
Aortic pressure determines flow, with max flow being reached early in diastole allow for a shortened diastole at high HR.
What happens to the left coronary vessels during systole?
The contracting myocytes collapse the vessels, forcing blood backwards towards the aorta = EXTRAVASCULAR COMPRESSION.
How do coronary vessels penetrate the myocardium?
At right angles
What is the flow to the coronary vessels on the right side of the heart?
The RV produces a lower pressure during systole, as they only need to open the pulmonary valve, so the flow is CONTINUOUS throughout the cycle. Pin>Pout. Most of the flow is received during SYSTOLE.
What happens during metabolic hyperaemia of the coronary muscle?
Adenosine is released from metabolising muscle to DILATE the arterioles for increased flow. PGs, low O2, NO and K can all cause vasodilation.
Also have myogenic autoregulation between 60-180mmHg.
What is the sympathetic influence on coronary vessels?
Overruled by local control and hyperaemia.
What results from a reduced flow through the coronary arteries?
Angina e.g. in exercise
How does cerebral circulation ensure a secure O2 supply?
Via myogenic autoregulation and local flow can be altered according to brain activity via functional hyperaemia.
What results from a low cerebral perfusion pressure?
Loss of consciousness as cannot tolerate anaerobic conditions. Supply lost for >4min causes neuronal damage.
How is the cerebral circulation structurally adapted to ensure perfusion is maintained if an artery becomes blocked?
Circle of Willis.
Short arterioles and dense capillary network at high vascular resistance with blood arriving from the ICA or vertebral A.
Why are the coronary capillaries not leaky?
BBB limits the passage due to its tight junctions. Lipophilic pass unaided but AA require protein transport and ions need channels. Needs to maintain a constant environment to protect the neurons.
What are the adaptations of the cerebral circulation?
- High basal flow = 15% of CO
- Peripheral vasoconstriction maintains pressure by shunting from other organs
- myogenic autoregulation within 60-170mmHg.
- Vessels are responsive to hypercapnia to cause vasodilation. Less responsive to PO2 with only severe hypoxia causing dilation
- Functional hyperaemia
- Little ANS control with sympathetic only contributing to 20-30% increased resistance and little baroreceptor effect.
What is myogenic autoregulation?
A change in BP is met by a change in resistance to maintain the flow.
How does systemic hypoxia present?
Hypoxia evokes hyperventilation so becomes masked by HYPOcapnic vasoconstriction.
What causes raised ICP and what results from it?
Bleeding, oedema, tumour = collapses veins and reduces CPP.
CPP =
Mean ABP - ICP
What causes postural syncope?
Due to an impairment in the baroreceptor reflex or autonomic activity. Linked to age
What causes cerebral ischaemia?
Ischaemic or haemorrhagic stroke
What can vasodilation in cerebral circulation result in?
Headaches or migraines
