Coronary, Cerebral and Cutaneous Circulations Flashcards
What is a-vO2 difference?
Where is a large a-vO2 difference found?
Where is a small a-vO2 difference found?
The difference in O2 between the arteries and veins
It represents the amount of O2 that a particular tissue extracts
Large difference in heart
Small difference in kidney/skin
What are the 2 main tasks of the coronary circulation?
- maintain secure O2 supply to coronary muscle (low capacity for anaerobic metabolism)
- alter local flow according to activity - functional hyperaemia (metabolic regulation)
What is meant by the coronary reserve?
flow can increase 4-5x when CO increases
What is a typical CO in the following situations?:
- rest
- moving around house
- walking briskly
- light jogging
- strenuous running
- 5 L/min
- 10
- 15
- 20
- 25
In the coronary circulation there is a very large a-vO2 difference.
What consequences does this have e.g. in angina?
The heart is extracting almost maximum amount of O2 possible at rest
Increased demand for O2 must be met by large flow increase
Why is flow to coronary muscle is intermittent?
For blood to flow through a coronary vessel
- Pa > Pv this to leads to directional flow
- Pi > Po so that vessels remain open
Pi = pressure in aorta
Po = pressure in ventricles
So if ventricular pressure > aortic pressure, blood will not flow
Describer coronary blood flow on the left side of the heart.
SYSTOLE = vessels are compressed by high pressure in the ventricle
Most blood flows to left myocardium during diastole
Aortic pressure during diastole determines flow
Max during early diastole
At high HR, diastole is shortened and reduces time for perfusion.
Draw graphs for coronary blood flow in the left side of the heart
flow may reverse during systole
greatest flow during diastole
Explain when blood might travel backwards in the coronary circulation
During systole
Extravascular compression in the left ventricular wall
Contracting myocutes collapse vesssle
Arterial blood is foes backward toward aorta
Describe blood flow in the right side of the heart
Right coronary flow highest during systole
They is constant flow to the right ventricle because ventricular pressure never exceeds aortic pressure
In what range do coronary arterioles exhibit myogenic regulation?
How else is coronary flow controlled?
60-180mmHg
- some sympathetic control but overridden by local control
- adenosine, prostaglandins, low O2, NO, K+,
What are the 2 tasks of the cerebral circulation?
- Maintain totally secure O2 supply to brain tissue (myogenic autoregulation)
- Alter local flow according to activity functional hyperaemia (metabolic regulation)
What are the structural adaptions of the cerebral circulation?
- Circle of Willis - ensures supply even if one artery becomes blocked
- short arterioles, dense capillary network
- relatively high vascular resistance
- cerebral perfusion maintained if carotid artery obstructed
What is the cellular basis of the blood brain barrier?
Why is this important?
- brain capillary endothelial cells connected by tight junctions
- controls bulk flow
- no vesicular transport
- protects neurones
- maintains environment
- responsible for long-lasting effects of heroine
What can pass through the BBB?
lipophilic solutes - O2, CO2, alcohol, nicotine, caffeine amino acids via transport proteins glucose Na+ via Na+K+ATPase ion channels
What are the 7 adaptations of the cerebral circulation?
- High basal flow
- Regulation of other organs safeguards cerebral circulation
- Autoregulation
- Cerebral vessels are VERY responsive to arterial CO2
- Cerebral vessels are less response to arterial O2
- Neuronal activity-evoked functional hyperaemia
- Nervous control not important in determining cereal flow
- High basal flow
- What % of CO?
15% of CO
high extraction of O2
- Regulation of other organs safeguards cerebral circulation
- peripheral vasoconstriction (except heart) can maintain arterial pressure
- Autoregulation
- What is it?
- What is the autoregulatory range?
- a change in blood pressure is met by a change in resistance to maintain perfusion
- 60-170mmHg
- Cerebral vessels are VERY responsive to Co2
Hypercapnia (>40mmHg) –> vasodilation (endothelial NO, fall in myocyte pH, H+ rather than CO2 per se
Hypocapnia (<40mmHg) –> vasoconstriction
lose ability to autoregulate high PaCO2
This is responsible to dizziness during hyperventilation
- Cerebral vessels are less responsive to levels arterial O2
Moderate hypoxia evokes little change in cerebral flow
Severe hypoxia leads to vasodilation: adenosine, K+, NO
- systemic hypoxia evokes hyperventilation so hypoxic vasodilation often masked by hypocapnic vasoconstriction
- Neuronal activity-evoked functional hyperaemia
- What factors are important in coupling tissue metabolism to local flow?
- CO2
- increased interstitial [K+] due to increased K+ permeability
- adenosine
- nitric oxide
- Is nervous control importnant in determining cerebral flow?
- maximal sympathetic stimulation increases resistance by only 20-30%
- baroreceptors have little influence on cerebral flow
- sympathetic stimulation shifts autoregulatory curve to right
REMEMBER TO LOOK OVER GRAPHS IN LECTURE
remember to look over graphs in lecture
