13. Short Term Control of BP Flashcards
What is the equation for calculating MAP?
MAP= CO x TPR
What can low MAP cause?
fainting (syncope_
What can high MAP cause?
hypertension; overtime damages capillary beds leading to cardiovascular disease, stroke etc
Where are baroreceptors found? (2)
- at the carotid sinus (at internal carotid arteries)
2. at the aortic arch
What effect does pressure in arteries have on vessel stretchability?
Makes arteries more stretchable when high pressure is detected during bigger APs
What is resting arterial pressure?
-90mmHg
Where do baroreceptors send their impulses to?
to the medulla of the brain (cardiovascular centre) which compares the blood pressure
What are baroreceptors?
- stretch receptors
- fire more APs if detect high pressure and stretch artery walls
What is the carotid sinus?
area where internal and external carotid arteries split (bifurcate)
What is the AP firing rate throughout the cardiac cycle?
Most APs fired at systole and least APs fired during diastole
What nerve sends signals from the carotid sinus baroreceptors to the medulla?
glassopharyngeal nerve
What nerve sends signals from the aortic arch baroreceptors to the medulla?
vagus nerve
What 2 nerves travel AWAY from the medulla towards the heart?
- sympathetic nerves
2. parasympathetic (vagus) nerves
What do sympathetic and parasympathetic nerves from the medulla innervate in the heart?
the SA node
What receptors do sympathetic nerves act on?
alpha 1 receptors
What is the effect of the sympathetic nerves on the heart?
- cause blood vessel constriction
- cardiac output and stroke volume will increase
- increase in TPR
What are other inputs to the medullary cardiovascular centres? (5)
- cardiopulmonary baroreceptors
- central chemoreceptors
- chemoreceptors in muscle
- joint receptors
- higher centres
What is another name for cardiopulmonary baroreceptors?
low pressure baroreceptors
What happens to baroreceptors firing rate during hypertension and hypotension?
Hypertension; increase
Hypotension; decrease
What does decreased sympathetic tone do to lower heart rate? (4)
i.e. increased vagal tone
- reduces veno/arteriolar constriction
- reduce adrenaline production in adrenal medulla
- reduce contractility
- reduce heart rate
What does decreased vagal tone do to increase heart rate? (4)
i.e. increased sympathetic tone
- increases veno and arterial constriction ( increases venomotor tone)
- increases adrenaline production in adrenal medulla
- increases contractility
- increases heart rate
How do signals travel from the medullary CV centres to the veins/arteries smooth muscle?
sympathetic nerves
What carries signals from the medullary CV centres to the adrenal medulla?
sympathetic nerves
From which spinal level do sympathetic nerves supplying the heart arise from?
T5-8
What do chemoreceptors respond to?
- respond to Co2 changes which cause pH changes in the CSF
- CSF is closely related to blood
- induces heart to pump more O2 around body in hypoxic situations
When are joint receptors activated?
activated during physical activity (during repeated movement)
How does activating central chemoreceptors affect MAP?
Activated when high CO2; means increased heart rate and contractility and therefore increased cardiac output and MAP
When do chemoreceptors specifically activate?
during exercise
What group of receptors are classified as “feedback receptors”?
- cardiopulmonary baroreceptors ( low pressure baroreceptors)
- central chemoreceptors
- chemoreceptors in muscle
How do higher centres (hypothalamus and cortex) affect MAP before exercise?
- act as feed-forward systems
- tells CV system that something is about to happen and pre-prepares it to increase the CO
- gets CV system working before stimulus occurs to be prepared for increase in heart rate in anticipation for blood to muscles
Can arterial baroreceptors control and regulate BP in the long run?
No, they can’t
What does regulation of BP in the long run revolve around?
revolves around blood volume
What are the main sensors which regulate BP in the long run?
cardio-pulmonary baroreceptors
In cardiopulmonary baroreceptors, what do their effect tend to be produced as?
hormonal effect
What structures does hormonal effect mainly influence? (2)
- blood vessels
2. kidneys
Why is arterial baroreceptor reflex important when standing up?
Prevents body from fainting when all the blood rushes to the feet
How does standing affect MAP?
Lower venous return as blood pools in legs causing venous distension.
- lower EDV
- lower preload
- lower SV
- lower CO
- lower MAP
Define valsalva manoeuvre.
Forced expiration against a closed glottis
What does valsalva mamoeuvre cause first in the thorax?
increase in thoracic pressure
How does increased thoracic pressure affect aortic pressure?
It’s transmitted through the aorta where it’s added to aortic pressure; therefore MAP rises
How does increased thoracic pressure affect venous return?
Increased thoracic pressure reduces venous return because it squashes the veins near the heart preventing blood flow.
How does lowered venous return affect MAP?
As less blood gets back to the heart EDV lowers. Hence CO lowers and MAP falls. This happens steadily so you get a gradual reduction in MAP back towards normal.
What is the bodies response to lowered MAP?
When MAP falls below normal (due to reduced VR) it triggers the baroreceptor reflex which makes MAP rise again
When the valsalva manoeuvre ends the decreased thoracic pressure is transmitted to the aorta, what effect does this have on the MAP?
Reduces MAP significantly (large drop below resting level)
How does returning back to normal venous return after the valsalva manoeuvre affect MAP?
- After the valasalva manoeuvre thoracic pressure returns to normal so venous return does too.
- EDV returns to normal and MAP quickly rises again.
- Since the baroreflex is still active MAP rises well above normal
What happens to BP after the valsalva manoeuvre after the baroreflex wears off?
BP returns to normal
What is the risk to elderly who go through a valsala manoeuvre?
Their CVS can’t cope with the variations in MAP, particularly the sudden hypotension when thoracic pressure returns to normal (common for elderly to faint or have heart attacks)
What hormone system regulates MAP in the long term?
renin-angiotensin- aldosterone system
What effect on arterioles does angiotensin II have?
It causes constriction and therefore increase in TPR
What effect does aldosterone have on cells? (2)
- increase Na reabsorption
- increase in plasma volume
What effect does vasopressin (antidiuretic) hormone have on arterioles? (4)
- causes arteriolar constriction
- increase in TPR
- increased water permeability of collecting duct
- increase in plasma volume
What effect do atrial natriuretic peptide and brain natriuretic peptide have on arterioles? (4)
- cause arteriolar dilation
- decrease in TPR
- increase in Na (natriuresis)
- decrease in blood volume
In summary what 3 hormone groups are involved in long term control of BP?
- renin-angiotensin- aldosterone system
- vasopressin (antidiuretic)
- atrial natriuretic peptide and brain natriuretic peptide
What are 2 ways in which posture affects the BP?
- effect of standing
2. the reflex response
What effect does standing have on the hydrostatic pressure?
Increased hydrostatic pressure causes pooling of blood in veins/ venules of feet/ legs (naturally decreases BP)
What effect does standing have on: VR, EDV, preload, SV, CO, MAP, baroreceptor firing rate
decrease VR decrease EDV decrease preload decrease SV decrease CO decrease MAP decrease baroreceptor firing rate
What is the reflex response?
naturally responds to standing (adrenaline will have the same effect to it; wakes up the circulatory system by increasing the dropping BP)
What does a DECREASE in vagal tone cause in reflex response? (putting the parasympathetic system to sleep) (2)
- decrease in heart rate
- decrease in cardiac output
What does an INCREASE in sympathetic tone cause in a reflex response? (in terms of; HR, CO, SV, VR, EDV, TPR)
increase heart rate increase cardiac output increase stroke volume increase venous return increase end diastolic volume increase TPR (through vasoconstriction)
What happens to the pressure in thorax during valsalva manoeuvre? Why does it affect MAP ultimately?
Increased thoracic pressure is transmitted through aorta which affects MAP ultimately
What effect does increased thoracic pressure have on;
VR, EDV, SV, CO and MAP
decrease VR decrease EDV decrease SV decrease CO decrease MAP
What is decrease in MAP detected by? (during a valsalva manoeuvre) What do these initiate?
detected by baroreceptors; which initiate reflex response
What effect does reflex response have on CO and TPR?
- increase CO
- increase TPR
Why does the valsalva manoeuvre graph continue on after BP has been stabilised and stroke volume increases once more to normal?
because reflex effects have not worn off
Why can the valsalva manoeuvre be used to test a patient’s arteriobaro reflex?
it will continue to fall without stabilising in a patient with poor baroreflex
In summary what are the main features of the pumping ability of the heart? (4)
- structure
- electrical activity
- cardiac cycle
- control of HR and SV
What are 2 controls of arterioles and what do they control?
- Local: to meet needs of tissues
2. Central: to regulate MAP
What is the central control of MAP?
arterial baroreflex system
