special circulations Flashcards
where do the right and left coronary arteries arise from?
the base of the aorta
where does most coronary venous blood drain via?
the coronary sinus into the right atrium
Oxygen demand of cardiac muscle is ___ especially during ___. Coronary circulation requires ______
Oxygen demand of cardiac muscle is high especially during exercise
Coronary circulation requires special adaptations
what are three special adaptations of the coronary circulation?
High Capillary Density
High Basal Blood Flow
High Oxygen Extraction (~75% compared to 25% whole body average) under resting conditions
what does the high oxygen extraction of the coronary arteires under resting conditions mean when further O2 is required?
This means extra O2 (when required) cannot be supplied by increasing O2 extraction
Can only be supplied by increasing coronary blood flow
what is the only way of increased the supply of oxygen to the coronary circulation?
Can only be supplied by increasing coronary blood flow
what are the two types of mechanisms for coronary blood flow control?
intrinsic and extrinsic
what three intrinsic mechanisms are there to increase coronary blood flow?
- decrease Po2 causes vasodilatation of the coronary arterioles
- Metabolic hyperaemia matches flow to demand decreased PO2
- Adenosine (from ATP) is a potent vasodilator
what mechanisms are involved in the extrinsic control of coronary blood flow?
- the coronary arterioles are supplied by sympathetic nerves (causing vasoconstriction)
- also circulating adrenaline activates B2 adrenergic receptors found on cardiac smooth muscle arterioels causing vasodilatation
what intrinsic mechanism over-rides the sympathetic innervation to the coronary arterioles? and how is this done?
metabolic hyperaemia over-rides sympathetic as a result of increased hr and SV.
The effect of sympathetic innervation on the heart causes:
- Decreased local PO2
- Increased local PCO2
- Increased local [H+] (decreased pH)
- Increased extra-cellular [K+]
- Increased osmolality of ECF
- Adenosine release (from ATP)
so
So sympathetic stimulation of the heart results in coronary vasodilatation/constriction despite direct vasoconstrictor effect (functional sympatholysis)
vasodilatation
what are the receptors for sympathetic neurotransmitter signals in the coronry vascular SM? and what is the neurotransmitter?
noradrenaline acting on alpha 1
what does adrenaline act on in coronary SMc?
beta receptors
where are the adrenaline beta receptors found in the body?
in the skeletal and cardiac smooth muscle arterioles
where are the alpha adrenaline receptors found in the body? and what does this cause>
found in the gut, skin, and kidney arterioles and cause vasoconstriction
look at the table in khogali’s special circulations lecture
x
when does most of the left coronary artery filling occur?
during diastole when the subendocardial vessels from the left coronary artery are not compressed
what causes a short diastole period?
a very fast heart rate - so get reduced coronary flow
which main arteries supply the brain?
the internal carotids and the vertebral arteries
why does the brain need a secure supply of oxygen?
Grey matter VERY sensitive to hypoxia - consciousness lost after few seconds of ischaemia, irreversible cell damage within ~ 3 minutes
what forms the basilar arteries?
two vertebral arteries and the carotid arteries
what do the basilar arteries do to improve circulation?
they anastamose to form the circle of Willis
the circle of willis gives rise to?
major cerebral arteries
what does anastamoses ensure?q
Cerebral perfusion should be maintained even if one carotid artery gets obstructed.
what could happen if a smaller branch in the cerebral circulation is occluded?
obstruction of a smaller branch of a main artery would deprive a region of the brain of its blood supply
what are the two types of stroke?
haemorrhagic and ischamic
what happens in a haemorrhaguc stroke?
blood leaks out of artery wall which is damaged
what happens in ischaemic stroke?
blood clot forms on atheroma on artery wall or comes from another part of the body and gets stuck
what is the main special adaptation of the cerebral circulation to ensure secure blood flow?
autoregulation
whihc two mechanisms that are noramlly important in maintaining blood pressure have little effect on the brain?
Direct sympathetic stimulation has very little effect in overall cerebral blood flow
Participation of the brain in baroreceptor reflexes is negligible,
what happens in the cerebral circulation in response to a rise in MABP?
resistance vessels automatically constrict to limit blood flow
what happens in the cerebral circulation in response to a fall in MABP?
resistance vessels automatically dilate to maintain blood flow
what are the limits for autoregulation of cerebral blood flow?
Autoregulation fails if MABP falls below ~ 60mmHg (cerebral blood flow falls) or rise above ~ 160mmHg (cerebral blood flow rises)
what does a MABP below 50 mmHg result in?
confusion, fainting, and brain damage if not quickly corrected
what is the effect of increased PCO2 on cerebral circulation?
↓ Pco2 causes vasoconstriction, this is why hyperventilation could lead to fainting.
what is regional hyperaemia in the brain?
Blood flow increases to active parts of the brain (regional hyperaemia). Mechanism unknown. May be due to rise in [K+]O as a result of K+ efflux from repetitively active neurones?
what is the normal intracranial pressure?
8 - 13 mmHg
how is the cerebral perfusion pressure calculated?
MAP- ICP
what does increased ICP (due to head injury or brain tumour) do to CPP?
decreases CPP and cerebral blood flow
what pathological processes can lead to failure of autoregulation of cerebral blood flow
Some conditions which increase ICP
what is responsible for the blood brain barrier
cerebral capillaries have very tight intercellular junctions - THE BLOOD BRAIN BARRIER
which two molecules are cerebral capillaries highly permeable to ?
oxygen and CO2
how does glucose cross the BBB?
by facilitated diffusion using specific carrier molecules
Brain has obligatory requirement for glucose
what is the BBB exceptionally impermeable to?
hydrophilic substances such as ions, catecholamines, proteins
what is the protective function of the BBB by not letting hydrophilic substances contact neurones?
This helps protect brain neurones from fluctuating levels of ions etc in blood
what is the normal range of pulmonary artery BP
20 - 25/ 6-12 mmHg
the absorptive forces in the pulmonary arteries exceed the filtration forces because of the low pressure. What does this healp to protect against
pulmonary oedema
what is the effect of hypoxia on the systemic arterioles
the arterioles dilate
what is the effect of hypoxia on the pulmonary arterioles?
they vasoconstrict
why do the pulmonary arterioles vasoconstrict during hypoxic conditions?
to help divert blood away from poorly ventilated areas of the lung.
the resistance of skeltal muscle vascular bed has a large impact on….
blood pressure
why is the resting blood flow low in skeletal muscle
because of the sympathetic vasoconstrictor tone
what overcomes the sympathetic vasoconstrictor tone during exercise?
metabolic hyperaemia
what does circulating adrenaline do to skeletal vascular smooth muscle?
causes vasodilatation through beta 2 adrenergic receptors
what effect does adsrenaline have on the heart rate?
activates B1 adrenergic receptors in nodal and cyocyte cells in the heart causing increase in HR
what does the skeletal muscle pump prevent?
hypotension and fainting as it aids venous return
what causes varicose veins?
Blood pools in lower limb veins if venous valves become incompetent
why do varicose veins not usually lead to a reduction of CO?
because things happen over time and through hormones and things - the CO is compensated for
