Short term control of BP Flashcards
What is MAP?
The driving force pushing blood through the circulation
MAP = CO x TPR
What happens if MAP is too low?
Tissue aren’t adequately perfused
- Affects brain = fainting
What happens if is MAP too high?
Hypertension
- Inc long term risk of stroke and coronary heart disease
Where are baroreceptors located?
One in aortic arch
One in carotid sinus
- At bifurcation of carotid internal and external arteries
What do baroreceptors do?
Detect changes in pressure indirectly = Are stretch receptors
What is a baroreceptors response to increase in pressure/stretch?
Increase firing rate of action potentials
When (normally) is pressure increase and AP firing increases?
During early systole when theres a rapid rise in pressure
What nerve are signals from baroreceptors transmitted up?
Aortic arch baroreceptors = vagus nerve
Carotid sinus baroreceptors = glossopharyngeal nerve
Where is the integrating centre for baroreceptor signals?
Medullary cardiovascular centres
What are cardiopulmonary baroreceptors?
- Sensing central blood volume
- Stretch receptors
- Increase firing rate of AP with increased stretch (volume)
What do central chemoreceptors sense?
Arterial pCO2 and pO2
- Detect increase in CO2 and decrease in O2
What response does central chemoreceptors have?
Stimulate respiratory drive
- Increase the amount of O2 getting into the lungs
- Increase MAP
What do chemoreceptors in muscle sense?
Metabolite concentration
What do chemoreceptors in joints sense?
Joint movement
What does the hypothalamus do?
Coordinating vascular response for thermoregulation
Coordinating fight or flight
What does the cerebral cortex do?
Strong emotional stimuli can trigger CVS response
Vasovagal syncope = fainting at sight of blood or a needle
What responses can the medullary CV centre have to bring MAP back to normal values?
- Parasympathetic outflow via vagus nerve innervating the sinoatrial node in the heart.
- Sympathetic nerves innervating sinoatrial node in heart
- Sympathetic nerves innervating muscle of ventricle
- Release of adrenaline and noradrenaline into the blood
What nerve fibres carry parasympathetic innervation and where do they innervate?
Vagus nerve
- Sinoatrial node in heart
What neurotransmitter is released by Parasympathetic fibres and what does it bind to?
Acetylcholine - muscarinic receptors on pacemaker cells
What is the effect of parasympathetic innervation of the SA node?
Hyperpolarisation of pacemaker cells
- means they take longer to reach threshold
Decreased HR (bradycardia)
No effect on contractility (as not innervating muscle)
What neurotransmitter is released by sympathetic nerves innervating the SA node and where does it bind?
Noradrenaline - B1 receptors
What affect does noradrenaline binding to B1 receptors have?
Pacemaker cells depolarise faster
Increased HR
What is the effect of sympathetic innervation of the myocardium?
Increased contractility and thus stroke volume
How is contractility increased by sympathetic innervation?
Increased release of calcium
- More actin and myosin cross bridges forming
- Increased strength of contraction
What does adrenaline and noradrenalin released into the bloodstream bind to?
B1 receptors
A1 receptors on smooth muscle of blood vessels
What is the result of noradrenaline and adrenaline in bloodstream binding to B1 receptors?
Increased HR and increased contractility
What is the result of noradrenaline and adrenaline in bloodstream binding to A1 receptors?
Smooth muscle contraction = vasoconstriction
- Pushes more blood back to the heart giving increased EDV & preload, stronger fore of contraction, larger SV, increased CO and thus MAP
(arteriolar constriction = resistance vessels = increases TPR = increases MAP)
What is the Valsalva manoeuvre?
Forced expiration against a closed glottis
What is the Valsalva manoeuvre used for (physiological)?
Defecation
Lifting heavy things
Popping ears
What happens to BP throughout the Valsalva manoeuvre?
Increases immediately
Slow decrease
Reduced BP
Drop in BP
Massive rebound increase in BP
What happens in phase 1 of the Valsalva manoeuvre?
Increased thoracic pressure is transmitted through to aorta - hence immediate jump
What happens in phase 2 of the Valsalva manoeuvre
Increased thoracic pressure reduced the filling pressure from the veins (making it harder for blood to get back to heart) which therefore:
- Reduces venous return, EDV and preload
- Starlings law = reduced preload then less efficient excitation-contraction coupling, fewer cross bridges forming, reduced force of contraction so stroke volume falls.
- SV & CO fall
- MAP will fall
What happens in late phase 2 of the Valsalva maneuovre?
The reduced MAP will be detected by baroreceptors in carotid sinus and aortic arch which initiate a reflex increase in CO and TPR
How is a reflex increase in CO and TRP brought about in the Valsalva manoeuvre?
- Increases sympathetic tone
- Increased release of noradrenaline from sympathetic nerves that bind to beta 1 receptors o sinoatrial node
- Increases HR
- Increased contractility and so you get vasoconstriction (More blood is pushed back to heart and EDV increases)
- Arteriolar constriction = increases TPR
What happens in phase 3 of the Valsalva manoeuvre?
At end of manoeuvre, the decrease in thoracic pressure is transmitted through to the aorta, hence the drop in BP
What happens in phase 4 of the Valsalva Manoeuvre?
Massive increase in BP (rebound)
- When doing manoeuvre blood was stopped from getting back to heart so was accumulating in veins
- When stop manoeuvre blood comes flooding back
What happens in phase 5 of the Valsalva manoeuvre?
Venous return restored/increased
- Massive increase in SV
- Now tries to decrease Map via increased vagal tone
- More acetylcholine released from vagus nerve
- Acting on cholinergic muscarinic receptors and slow down sinoatrial node
- Decrease in HR
- Leading to decreased CO
What is the clinical significance of the Valsalva manoeuvre?
Used as a test of autonomic function
When can Valsalva manoeuvre be reduced?
Older people
- BP is not restored by baroreflex
Autonomic neuropathy
- e.g. damage due to diabetes
- Reduced baroreflex
