2.10. Reflex control of circulation: baroreceptor and chemoreceptor reflexes. Cardiovascular centers.

Question 1

What is the formula of force that drives blood through vessels?

Answer 1

Review:
Force (ΔP) = flow intensity (Q) * Resistance (R)
-> ΔP is the force that drives blood through vessels (Q = ΔP/R)

Question 2

I. Mean arterial (blood) pressure (MAP):
1. What are the characteristics of Mean arterial (blood) pressure (MAP)? How are they controlled?

Answer 2

• Average arterial pressure throughout 1 cardiac cycle (systole + diastole)
• Controlled by reflex regulation. Normally ~93mmHg
  +) if there is no SYM nerve activity, it decreases to 50mmHg
  +) with SYM nerve activity, it increases to 150mmHg

Question 3

I. Mean arterial (blood) pressure (MAP):
2. What does sympathetic nerve innervate?

Answer 3

• resistance vessels: the small arteries, arterioles, metarterioles that all contribute to TPR (arterioles being the biggest contributor)
• capacitance vessels: veins and venules that hold major portion of the blood. Their compliance is decreased with SYM activity
• heart: SA/AV nodes + cardiac muscle

Question 4

II. Neural reflexes originated within the cardiovascular system
1. The route of neural reflexes in cardiovascular system?

Answer 4

1. afferent pathway
2. vasomotor center
3. efferent pathway

Question 5

II. Neural reflexes originated within the cardiovascular system
2. What does afferent pathway contain?

Answer 5

baroreceptors (pressure change), cardiopulmonary receptors, chemoreceptors (chemical change)

Question 6

II. Neural reflexes originated within the cardiovascular system
3. What does efferent pathway contain?

Answer 6

SYM, PARA – that regulates the effector

Question 7

II. Neural reflexes originated within the cardiovascular system
4. What does vasomotor center contain?

Answer 7

pressor zone (RVLM), depressor zone (CVLM), PARA preganglionic neurons

Question 8

II. Receptors
1. List types of receptors involving in reflex control of circulation

Answer 8

(1) high + low pressure Baroreceptors
(2) Chemoreceptors

Question 9

II. Receptors
2. What are the characteristics of baroreceptors?

Answer 9

• Baroreceptors are mechanoreceptors,
  meaning they are sensitive to
  pressure/stretch
• Increase in arterial pressure causes
  increased AP firing rate, and decreased
  arterial pressure causes decreased firing rate
• Baroreceptors are very sensitive to changes in pressure -> strongest stimulus is a rapid change in arterial pressure (as in standing)

Question 10

III. High pressure baroreceptors
1. What are the 2 types of high pressure baroreceptors?

Answer 10

(1) fast-acting baroreceptor
(2) slow-acting baroreceptors

Question 11

III. High pressure baroreceptors - Fast acting baroreceptors
2A. What are the the 2 types of fast acting baroreceptors and where do they locate?

Answer 11

• Stretch receptors are located in the carotid sinuses and in the aortic arch
• 2 types: (1) Carotid sinus baroreceptors and (2) Aortic arch baroreceptors

Question 12

III. High pressure baroreceptors - Fast acting baroreceptors
2B. What are the characteristics of (1) Carotid sinus baroreceptors and (2) Aortic arch baroreceptors?

Answer 12

• Carotid sinus baroreceptors have elastic fibers, so increase in pressure means they expand. Distension -> stretch receptor detection. Sensitive in range of 50-200mmHg
• Aortic arch baroreceptors: similar mechanism. Sensitive to high blood pressure, 100-200mHg

Question 13

III. High pressure baroreceptors - Fast acting baroreceptors
2C. What are the mechanisms of (1) Carotid sinus baroreceptors and (2) Aortic arch baroreceptors?

Answer 13

• The carotid and aortic baroreceptors send information to the medulla -> solitary nucleus
• (carotid sinus receptors -> carotid sinus nerve; aortic arch receptors -> vagus fibers)

Question 14

III. High pressure baroreceptors - Fast acting baroreceptors
2D1. How do Fast-acting baroreceptors response when there is a change in pressure?

Answer 14

If change in pressure
-> PARA or SYM activation response, depending on if pressure is high or low

Question 15

III. High pressure baroreceptors - Fast acting baroreceptors
2D2. What is the mechanism of PARA activation response of fast acting baroreceptors?

Answer 15

PARA: in response to high BP
-> mAChR on SA + AV node, ↓frequency and conductivity (decreases CO)

Question 16

III. High pressure baroreceptors - Fast acting baroreceptors
2D3. What is the mechanism of SYM activation response of fast acting baroreceptors?

Answer 16

SYM: 4 effects (in response to low BP)
- β1-receptors on SA/AV node + ventricular tissue: activation causes ↑chronotropy, dromotropy, inotropy (= ↑CO)
- α1-receptors on arterioles: produces vasoconstriction + ↑TPR (↑venous return = ↑CO)

Question 17

III. High pressure baroreceptors - Slow-acting baroreceptors
3A. Where can you find Slow-acting baroreceptors?

Answer 17

Renin-angiotensin system (RAS)

Question 18

III. High pressure baroreceptors - Slow-acting baroreceptors: Renin-angiotensin system (RAS)
3B. What is the mechanism of Slow-acting baroreceptors?

Answer 18

Detects a drop in pressure in renal artery via mechanoreceptors in afferent arteriole of kidney:
- prorenin -> renin (enzyme) converts angiotensinogen to ANGI (in lungs)
-> in kidney + lungs, angiotensin-converting enzyme (ACE) converts ANGI to ANGII

Question 19

III. High pressure baroreceptors - Slow-acting baroreceptors: Renin-angiotensin system (RAS)
3C. What are the effects of ANGII?

Answer 19

ANGII has several effects: (main concept: ↑blood volume -> ↑ in BP)

1. ANGII acts on adrenal cortex zona glomerulosa to stimulate aldosterone
   -> acts on Distal tubule (DT) + Collecting duct (CD) to ↑Na+-absorption = ↑water reabsorption = ↑blood volume = ↑BP
2. ANGII acts on HT to stimulate thirst + secrete ADH
   -> drink more water + water reabsorption in CT = ↑blood volume

Question 20

IV. Low pressure baroreceptors
1. Where can you find low pressure baroreceptors ?

Answer 20

cardiopulmonary receptors within the venous system:
- Receptors located close to heart: SVC, IVC, pulmonary veins before entering atria + both atria

Question 21

IV. Low pressure baroreceptors
2. What is the mechanism of Low pressure baroreceptors?

Answer 21

• An increase in blood volume (e.g. IV infusion)
  -> increase in right atrial pressure
  -> receptor stimulation
  ->increased HR + hormone regulation (↑ANP, ↓RAS) to increase the excretion of Na+ in an effort to lower the blood volume
  -> lead to 3 effects which are…
  1) secretion of ANP (atrial natriuretic peptide)
  2) Inhibition of ADH production
  3) Renal vasodilation

Question 22

IV. Low pressure baroreceptors
2. Low pressure baroreceptors lead to secretion of ANP (atrial natriuretic peptide). -> Consequences?

Answer 22

secreted by atria in response to ↑atrial pressure
-> vasodilation = relaxes VSM
-> ↓BP. Kidney: ↑Na+ + H2O excretion
-> ↓blood volume decreases = ↓BP decreases

Question 23

IV. Low pressure baroreceptors
3. Low pressure baroreceptors lead to Inhibition of ADH production -> Consequences?

Answer 23

atrial pressure projects to the HT and inhibits secretion of ADH
-> ↓H2O-reabsorption

Question 24

IV. Low pressure baroreceptors
4. Low pressure baroreceptors lead to Renal vasodilation -> Consequences?

Answer 24

inhibition of SYM vasoconstriction in arterioles = ↑filtration = ↑Na+ + H2O excretion

Question 25

V. Chemoreceptors
1. What are the characteristics of Chemoreceptors?

Answer 25

• Regulates the respiration, stimulates the respiratory centers and increases ventilatory drive
• Sensitive to hypoxia (low O2 – high CO2 levels)
• Also to K+, pH, osmolarity

Question 26

V. Chemoreceptors
2. What are the 2 types of chemoreceptors?

Answer 26

1. Peripheral chemoreceptors
2. Central chemoreceptors

Question 27

V. Chemoreceptors
3A. What are the characteristics of Peripheral Chemoreceptors?

Answer 27

• Peripheral chemoreceptors are the aortic bodies (aortic arch) and carotid bodies (bifurcation of CCA)
• Sensitive primarily to changes in pO2, but also of some ↑ of pCO2 + ↓ in pH

Question 28

V. Chemoreceptors
3B. What is the mechanism of Peripheral Chemoreceptors?

Answer 28

When pO2 decreases, there is an ↑ firing rate of sensory nerves that activate the SYM vasoconstrictor centers
-> arteriolar vasoconstriction in skeletal muscle (to conserve oxygen for places that need it like the brain) and vasodilate in needed places

Question 29

V. Chemoreceptors
3C. What are the characteristics of Central Chemoreceptors?

Answer 29

• Responsible for resting breathing drive
• Located in the medulla in the reticular formation
• Most sensitive to pCO2 and also pH, less to O2
• Responds to changes in the pH of the CSF

Question 30

V. Chemoreceptors
3D. What is the mechanism of central chemoreceptors?

Answer 30

Mechanism:
↓cerebral blood flow
-> immediate ↑ in pCO2 and ↓in pH (due to bicarbonate and proton formation)
-> SYM activation
-> ↑in SYM outflow
-> intense arteriolar vasoconstriction
-> ↑ in TPR
-> blood flow redirected towards brain

Question 31

V. Chemoreceptors
4A. Explain Cushing reflex

Answer 31

Cushing reflex explains how central chemoreceptors work:
Intracranial pressure (ICP↑)
-> compression of cerebral arteries
-> decreases cerebral perfusion and activates chemical chemoreceptors
-> peripheral vasoconstriction + cerebral vasodilation

Question 32

V. Chemoreceptors
4B. Explain Molecular mechanism of Cushing reflex

Answer 32

• ↑ arterial BP due to ↑ ICP -> arterial baroreceptors detect that -> PARA activation -> HR↓
• In short: ↑ICP -> HR + ↑BP