The Baroreflex Flashcards
Why do we faint if blood pressure falls too low?
Blood supply to cerebral circulation falls. Fainting put us in a supine position that raises blood pressure in the brain.
What is the role of the baroreflex, what’s the effect on cerebral perfusion?
Hold ABP constant, allowing local mechanisms to act independently, sustains cerebral perfusion during postural changes
What are the different types of baroreceptors?
High pressure receptors in aortic arch and carotid sinus
Low pressure receptors in atria and adjacent large veins
What do high pressure baroreceptors do?
Receptors sense degree of stretch of vessel walls as they expand and transmit impulses to NTS in the brain stem (nucleus tractus solitarii).
Describe the course of afferent fibres from aortic arch and carotid sinus baroreceptors
Afferents from the aortic arch baroreceptors travel in vagus nerve (X)
Carotid sinus baroreceptors travel in the carotid sinus nerve which merge with the glossopharyngeal nerve (IX).
What happens to the baroreceptors as the blood pressure rises?
Increased discharge frequency of baroreceptor afferent
What happens as discharge of baroreceptor afferents rises?
Inhibition of sympathetic activity, so force/rate of contraction decreases
Inhibition of vasoconstrictor fibres innervating vessels supplying skeletal muscle, skin, splanchnic circulation etc so vasodilation and decrease TPR so increased blood flow to all parts.
Stimulate vagal activity (slows heart rate and restore bp to baseline)
Why does baroreceptor response have global effects?
Sympathetic activation affects arteries, veins, kidneys etc para affects cardiac output affecting blood flow to all body parts
How does increased baroreceptor activity affect brain stem?
Instructs brain stem to instigate inhibition of medulla vasomotor centre
What are the two classes of carotid baroreceptors?
Type 1
Type 2
What are the main cellular differences between carotid baroreceptor types?
Type 1: large type A myelinated afferent fibers.
Type 2: neurons with unmyelinated or small poorly myelinated type C afferent fibers.
What is the difference between activation thresholds between carotid baroreceptor types?
Type 1 baroreceptors have lower activation thresholds and fire more rapidly upon stimulation.
Type 2 baroreceptors tend to have higher activation thresholds and discharge at lower frequencies.
What are two examples of physiological importance of baroreceptor response?
Haemorrhage or after standing up
What happens in reflex to haemorrhage?
Loss of blood volume so decrease in bp
Inhibit carotid baroreceptor, decreased firing
Increase sympathetic drive and decrease parasympathetic drive
How do baroreceptors adapt to sustained changes in bp, why can this be useful?
Baroreceptor reflex can be reset. i.e. if arterial bp is increased for long time
Useful: reset reflex to higher bp, then high cardiac output can be maintained in exercise with slowing in heart rate due to increased arterial bp
What are long term regulations of arterial bp?
Hormonal and renal
What are the two types of low pressure (cardiopulmonary) receptors?
Type A receptors are activated by wall tension developed by atrial contraction during ventricular diastole
Type B receptors are activated by wall stretch developed by atrial filling during ventricular systole
What are low pressure receptors (cardiopulmonary) receptors for?
Regulate blood volume (blood volume determines bp)
What do low pressure atria receptors respond to?
Respond to increased CVP and cardiac distension
What are the reflexes of atria cardiopulmonary receptor?
1) rise in heart rate and contractility via symp stimulation
2) diuresis (increased salt excretion to regulate plasma volume) (ANP)
Which fibres carry afferent info from atria cardiopulmonary receptors?
Vagus
Which receptors detect a fall in CVP?
Low pressure receptors in large veins and atria
What is the effect of lowering blood volume on atrial receptors?
Signal transmitted to hypothalamus (ANP) to increased ADH secretion, increases water reabsorption and blood volume
What are the cardiovascular changes that drive increase in bp in response to a fall in bp (e.g. change of posture)?
Heart rate increases
Peripheral vascular resistance increases, central veins contract, all of which returns arterial and venous pressure to near normal levels
What happens to both types of receptors as bp falls?
Arterial baroreceptors sileneced
Low pressure receptors fire and detect fall in CVP (increased ADH and less ANP made)
What happens to blood vessels in trunk when you stand, why?
Vasoconstrict, to displace blood to brain and essential organs
How does baroreceptor response further increase perfusion in exercise?
Metabolites themselves act as local vasodilators to increase blood flow to muscle and TPR decreases
Fall in bp detected by baroreceptors which triggers sympathetic activation, further increases perfusion of active muscle
How do kidneys modulate blood volume through reabsorption?
Modulate amount of salt and thus water reabsorbed into distal tubule. Change blood volume and thus bp
How does ADH regulate bp, how does it depend on tissue?
ADH released from posterior pituitary if bp decreases, (osmoreceptors)
More water reabsorped in the nephron
Vasoconstriction to increase bp in most tissues but vasodilation in cerebral + coronary to ensure sufficient blood flow
What is the most important form of long term bp control by kidney?
RAAS
What is RAAS, how is angiotensin 2 made?
Renin-angiotensin-aldosterone system
Kidneys make renin when Na+ in DT low, converts angioteninogen to angiotensin 1, in lungs which is then converted to angiotensin 2.
What does angiotensin 2 do?
Vasoconstricts to increase bp and stimulates aldosterone secretion from adrenal cortex so greater salt and water absorption by DT so increase blood volume
What is ANP’s role, how can it impact organ perfusion?
Relaxes VSM (vasc smooth muscle) by inhibiting effects of catecholamines and receptor mediated elevation of cGMP in VSM
Leads to vasodilation of arterioles, improve organ perfusion
What stimulates ANP production?
Produced by atrial myocytes in response to high cardiac filling pressure (atrial distension)
When are baroreceptors most sensitive?
Within normal physiological ranges of bp
What part of vessel is baroreceptor stituated in?
Muscular and adventitia layers
