Reflex Control Of Circulation

Question 1

Give examples of excitatory inputs and what does stimulation of these reflexes cause?

Answer 1

E.g. arterial chemoreceptors, muscle metaboreceptors (work)

Stimulation of reflexes causes an increase CO, TPR and BP -> PRESSOR response

Question 2

Give examples in inhibitory inputs and what does stimulation of these reflexes cause?

Answer 2

E.g. arterial baroreceptors, cardiac-pulmonary receptors

Stimulation of reflexes causes decrease CO, TPR, and BP -> DEPRESSOR response

Question 3

What is function of arterial baroreceptors?

Answer 3

BP sensors in walls of the carotid arteries/aorta inform brain of pressure changes in these key feeder vessels
Sensors detect arterial wall stretch -> BARORECEPTORS

Question 4

Where are baroreceptors found?

Answer 4

aortic baroreceptors -> found on aorta

some baroreceptors are found on carotid sinus

Question 5

What do baroreceptors respond to?

Answer 5

respond to changes in pressure -> are sensitive to pressure changes so every heart beat they fire as we get change in pressure.
In the face of continued high or low pressure, threshold for baroreceptor activation can change -> adaptation to a new normal

Question 6

What is the effect of increased BP on baroreflex?

Answer 6

termed loading (e.g. stress or exercise)

• depressor reflex
• pulse pressure falls (decreased SV)
• vasodilation decreases TPR + BP
• decreased sympathetic nerve activity
• increased vagus nerve activity

Question 7

What is the effect of decreased BP on baroreflex?

Answer 7

termed unloading (e.g. haemorrhage)

• Increased sympathetic activity & decreased vagus activity
• Increased HR and force of contraction so increased CO
• Arteriolar constriction = increased TPR
• Venous constriction increases CVP and so by Starlings law SV & CO
• This all maintains BP therefore blood flow to vital organs.
• Also: Adrenaline secretion, ADH secretion & stimulation of RAAS (i.e. Angiotensin II increases Na+/H2O absorption in kidneys raising blood volume -> indirectly affects BP
• Vasoconstriction decreases capillary pressure which increases absorption of interstitial fluid which also increases blood volume.

Question 8

What are examples of mechanoreceptors in heart?

Answer 8

ventricular mechanoreceptors

veno-atrial mechanoreceptors

Question 9

What stimulates veno-atrial mechanoreceptors?

Answer 9

stimulated by increase in cardiac filling/CV, increased sympathetic activity, tachycardia

Question 10

What stimulates ventricular mechanoreceptors?

Answer 10

stimulated by over distension of ventricles - depressor response. Weak reflex - mild vasodilatation, lower BP and preload, protective

Question 11

What stimulates nociceptive sympathetic afferents?

Answer 11

stimulated by K+,H+ (lactate), bradykinin durin ischaemia

Question 12

Where are arterial chemoreceptors located?

Answer 12

in carotid and aortic bodies

Question 13

What stimulates arterial chemoreceptors?

Answer 13

stimulated by low O2 (hypoxia), high CO2 (hypercapnia), H+ and K+

Question 14

What is function of arterial chemoreceptors?

Answer 14

regulate ventilation and drive cardiac reflexes during asphyxia (low O2/high CO2) shock (systemic hypotension ) and haemorrhage)
When BP drops below range of baroreflex (maximally unloaded), the chemoreceptors are still active and may compensate

Question 15

Is arterial chemoreceptors have depressor or pressor response?

Answer 15

a pressor response

• increased sympathetic activity
• tachycardia, increased selective arterial/venous constriction
• increased CO + BP

Question 16

What are muscle metaboreceptors and where are they located?

Answer 16

‘work receptors’

sensory fibres in Group IV motor fibres located in skeletal muscle

Question 17

What activates muscle metaboreceptors?

Answer 17

activated via metabolites, K+, lactate and adenosine

Question 18

What is function of muscle metaboreceptors?

Answer 18

important during isometric exercise

• e.g. weight lifting/handgrip
• higher BP drives blood into contracted muscle to maintain perfusion
• these muscles undergo metabolic hyperaemia allowing blood flow to contracted tissue

Question 19

central role of nucleus tractus solitarius (NTS)

Answer 19

signal from baroreceptors afferent fibres enter NTS
this sends info out to CVLM
CVLM sends inhibitory info to RVLM -> results in inhibition of sympathetic efferent nerves to heart and vessels
less sympathetic efferent signals result in reduction in HR, less vasoconstriction, lower BP etc
RVLM sends signal to SNS causing it to contract
CVLM sends signal there to inhibit that reducing sympathetic tone leading to slight vasodilation

Question 20

Limbic stimulation of cardiac vagal activity

Answer 20

limbic system (emotional centre) stimulates nucleus ambiguous causing increased activity of vagal nerve and depressor effect on AV and SA nodes 
can lead to fainting - vasovagal attack 
also syncope (fainting) caused by decreased cerebral blood flow (reduced O2 delivery) due to sudden drop in arterial CO and BP