Special Circulations: Coronary, Cerebral, Pulmonary & Skeletal Muscle Flashcards
Where do the left and right coronary arteries arise from?
The base of the aorta
Where does coronary venous blood drain to and via?
Drains into the right atrium via the coronary sinus
What are the special adaptations of the coronary circulation?
High capillary density
High basal blood flow
High oxygen extraction (75%)
What are the intrinsic mechanisms of coronary blood flow regulation?
- Decreased PO2 causes vasodilatation
- metabolic hyperaemia matches flow to demand
- adenosine (from ATP) is a potent vasodilator
What are the extrinsic mechanisms of coronary blood flow regulation?
- Coronary arterioles are supplied by sympathetic vasoconstrictor nerved BUT this is overriden by metabolic hyperaemia as a result of increased HR and SV
- sympathetic stimulation of the heart results in coronary vasodilatation despite direct vasoconstrictor effect (functional sympatholysis)
- circulating adrenaline activates B2 adrenergic receptors which causes vasodilatation
What is coronary blood flow like during systole?
in the left coronary artery it is low due to the constriction of the sub-endocardial arteries when the ventricle contracts
What is coronary blood flow like during diastole?
left coronary blood flow is increased and most occurs during diastole due to the high pressures of the left ventricle
What happens if you shorten diastole?
tachycardia shortens the left coronary artery flow period, causing chest pain although the don’t have ischaemic heart disease
When does right coronary flow peak?
Peaks in systole but occurs in diastole too
What supplies the brain?
brain is supplied by internal carotids and vertebral arteries
Why does the brain need a secure supply of oxygen?
Grey matter is very sensitive to hypoxia, consciousness will be lost after a few seconds and permanent damage occurs in minutes
What are the special adaptations of the cerebral circulations?
Basilar & carotid arteries anastomose to form circle of willis
major cerebral arteries arise from circle of willis
cerebral perfusion should be maintained even if one carotid artery gets obstructed
obstruction of a smaller branch would deprive a smaller region
What are the two types of stroke?
Haemorrhage
Ischaemic
What does auto regulation of cerebral blood flow do?
guards against changes in cerebral blood flow as mean arterial blood pressure changes within a range
What has little affect on cerebral blood flow?
direct sympathetic stimulation
baroreceptor is negligible
What happens to resistance vessels if MABP rises?
automatically constrict to limit blood flow
What happens to resistance vessels if MABP falls?
automatically dilate to maintain blood flow
What is the range of MABP auto regulation allows for?
below 60- above 160mmHg
What happens if MABP drops bellow 50mmHg
confusion, fainting, and brain damage if not quickly corrected
What is special about the brain during ischaemic?
blood flow increases to active parts of the brain (regional hyperaemia).
mechanism is unknown. may be due to rise in [K+]O due to potassium efflux from repeatedly active firing neurones?
What is contained within the skull?
brain 80%, blood 12% and CSF 8%
What is normal intracranial pressure?
8-13mmHg
What is cerebral perfusion pressure?
Mean arterial pressure (MAP) - ICP
What does increasing ICP cause?
decrease CPP and cerebral blood flow
if regulation fails then injury
What is the blood brain barrier?
cerebral capillaries have very tight intercellular junctions- the blood brain barrier
they only allow certain substances to go into the brain
What are cerebral capillaries permeable to?
O2 and CO2
How does glucose enter the brain?
facilitated diffusion using specific carrier molecules
Where does the pulmonary flow come from?
Right ventricle
What must meet the oxygen requirements of the airways?
Systemic Bronchial circulation
What is pulmonary resistance like in comparison to systemic resistance?
-10%
What is pulmonary artery BP typically?
20-25/6-12 mmHg
What are the special adaptations of the pulmonary circulation?
pulmonary capillary pressure is low (-8-11 mmHg) compared to systemic capillary pressure (17-25mmHg)
absorptive forces exceed filtration forces- protects against pulmonary oedema
hypoxia causes vasoconstriction of pulmonary arterioles
What % of body mass is skeletal muscle?
approx 40
What regulates resting blood flow to skeletal muscle?
Sympathetic vasoconstrictor tone
When is sympathetic vasoconstrictor activity overriden in skeletal muscle?
during exercise
What overcomes sympathetic vasoconstrictor activity during exercise?
- circulating adrenaline causes vasodilatation (B2 adrenergic receptors)
- increased CO during exercise, these could increase skeletal muscle blood flow by many folds
What are the overall changes to BP and pulse pressure during exercise?
systemic blood pressure will increase but diastolic BP may not change or even drop
pulse pressure will be wide during steady state exercise
What is the skeletal muscle pump?
- large veins in limbs lie between skeletal muscles
- contraction of muscle aids venous return
- one-way valves allow blood to move towards the heart
What does the skeletal muscle pump reduce the chance of?
postural hypotension & fainting
What happens when the venous valves become incompetent?
blood pools in lower limb veins- varicose veins
What will varicose veins due to cardiac output?
because of compensatory increase in blood volume
