Physio - Regulation of arterial pressure

Question 1

What is the most important mechanism for regulating arterial pressure?

Answer 1

Basics:

• The most important mechanisms for regulating arterial pressure are:
  
  • fast, neurally mediated baroreceptor mechanism
  • slower, hormonally regulated renin–angiotensin–aldosterone mechanism
• Sympathetic regulation of blood flow

Question 2

How does Baroreceptor Reflex relate to Regulation of Arterial Pressure?

Answer 2

Basics:

• Fast, neural mechanism
• Negative feedback system
  
  • responsible for the minute-to-minute regulation of arterial BP
• Baroreceptors = stretch receptors located w/in the walls of the carotid sinus
  
  • near the bifuraction of the common carotid arteries

Steps in baroreceptor reflex:

1. DECREASE in arterial pressure = DECREASE stretch on walls
   
   • receptors = most sensitive to changes in arterial pressure
   • rapid decreaing pressure = greatest response
2. DECREASE stretch = DECREASE firing rate of carotid sinus nerve
   
   • Hering’s nerve (CN IX)
     
     • carries information to the vasomoto center in brain stem
3. Set point for mean arterial pressure in the vasomotor center = 100mmHg
   
   • If arterial pressure = LESS than 100mmHg –> series of AUTONOMIC responses coordinate
   • Changes attempt to INCREASE BP –> normal
4. Decrease in BP leads to….
   
   • DECREASE parasympathetic (vagal) outflow to the heart
     
     • ↑ heart rate
   • INCREASE sympathetic outflow to the heart & blood vessels
     
     • ↑ heart rate
     • ↑ contractility & stroke volume
     • ↑ vasoconstriction of arterioles
     • ↑ vasocontriction of veins = ↓ unstressed volume = ↑ venous return = ↑ cardiac output by Frank-Starling mechanism
5. Increase in BP leads to…
   
   1. INCREASE parasympathetic
      
      • ​↑ freq of afferent nerve AP
      • ↑ firing of baroreceptors
      • ​↓ HR (negative chronotropic)
      • ↓ SV (negatiave inotropic)
   2. DECREASED sympathetic
      
      • ​↓ HR (negative chronotropic)
      • ↓ SV (negatiave inotropic)
      • ↑ arteriolar vasodilation & ↑ compliance of veins
        
        • blood shift toward venous side

Question 3

What is the Valsalva Maneuver?

Answer 3

Basics:

• integrity of the baroreceptor mechanism can be tested = Valsalva maneuver
• expiring against a closed glottis
  
  • causes an ↑ intrathoracic pressure –> ↓ venous return
  • ↓ venous return = ↓ cardiac output + Pa

If baroreceptor reflex works…

• ↓ Pa is sensed by baroreceptors –> ↑ sympathetic outflow to the heart + blood vessels
• ↑ in heart rate noted

When person stops maneuver…

• Rebound ↑ venous return + cardiac output + Pa
• ↑ Pa is sensed by baroreceptors –> ↓ heart rate noted

Question 4

What are 2 important aspects of the Baroreceptor reflexes?

What are the 2 Variations of Barorecptor reflexes?

Answer 4

Importance:

1. Minute to minute BP regulation system
2. Reflex control center has a set point of ~100mmHg
   
   • any changes cause reflex responses

Variations:

1. During sleep:
   
   • Pa falls (10-20mmHg) yey the vasomotor center makes no attempt to correct for it
2. Sedentary people w/ chronic HTN:
   
   1. aerobic exercise can REDUCE the set point back to normal

Question 5

What happens to BP to cause this?

Answer 5

Decreased blood pressure

Question 6

What happens to BP to cause this?

Answer 6

Increased blood pressure

Question 7

How does the Renin–angiotensin–aldosterone system affect arterial pressure?

Answer 7

Basics:

• slow, hormonal mechanism
• long-term BP regulation
  
  • via adjustment of blood volume
• Renin = enzyme
• Angiotensin I = inactive
• Angtiotension II = physiologically active
  
  • degraded by angiotensinase
    
    • one of the peptide fragements (angiotensin III) has some of the biologic activity of angiotensin II

Steps in the renin–angiotensin–aldosterone system

1. DECREASE in renal perfusion pressure –> juxtaglomerular cells of afferent arteriole to secrete renin
   
   • Renin = enzyme that converts angiotensinogen –> angiotensin I (in plasma)
2. Angiotensin-converting enzyme (ACE) catalyzes conversion of angiotensin I –> angiotensin II (in lungs)
   
   • ACE inhibitors (captopril) block this conversion = ↓ BP
   • Angiotensin receptor (AT₁) antagonists (losartan) block action of angiotensin II at its receptor = ↓ BP
3. Angiotensin II has 5 effects:
   
   1. stimulates synthesis & secretion of aldosterone (in adrenal cortex)
      
      • Aldosterone = ↑ Na+ resorption by renal distal tubule = ↑ ECF, blood volume, P_a
   2. ↑ Na+/H+ exchange; Cl- resorption; K+ excretion; H2O retention (in proximal convoluted tube)
      • angiotensin II directly ↑ Na+ resorption, complementing indirect stimulation noted ^^
      • angiotensin II –> contraction alkalosis
   3. ↑ thirst & water intake
      
      1. ↑ ADH secretion (in post. pituitary) = H20 absorption from collecting duct in kidneys
   4. causes vasoconstriction of arterioloes –> ↑ TPR & P_a
   5. _​↑ sympathetic activity

_{TPR = total peripheral resistance (or temp, pulse, respiration)}

Question 8

What is the importance of RAAS & ADH Compensatory Mechanisms?

Answer 8

• Body’s most important long-term blood pressure
  regulation system

Question 9

What is Atrial natriuretic peptide (ANP)?

Answer 9

Basics:

• released (from atria) in response to ↑ blood volume & P_a___​
  
  • also prompted by:
    
    • Atrial distension
    • Sympathetic stimulation
    • Angiotensin II
    • Endothelin
• other regulatory mechanism

Effects of Atrial Natriuretic Peptide:

• relaxation of vascular smooth muscle
• dilation of arterioles
• ↓ TPR
• ↑ excretion of Na+ & H2O in kidney
  
  • reduces blood volume & attempts to bring down BP
• inhibits renin secretion
  
  • ​↓ Aldosterone + ↓ Angiotensin II –> Natriuresis Diuresis = ↓ blood volume

Question 10

What is the importance of ANP?

Answer 10

• Counter-regulatory system for RAAS

Question 11

What is Brain Natriuretic Peptide (BNP)?

Answer 11

1. Synthesized in the ventricles
   
   • (as well as in the brain where it was first identified)
2. Probably released by same mechanisms that release ANP
3. End result similar to ANP
4. Clinical diagnostic marker for heart failure

Question 12

What is the importance of chemoreceptors in the carotid & aortic bodies?

Answer 12

Basics:

• Located near bifurcation of common carotid arteries, along aortic arch
• high rates of O2 consumption
  
  • very sensitive to decrease in partial pressure of O2 (P_O2)

​Central chemoreceptors (in medulla)

• Main stimulus = HIGH P_CO2
  
  • ​ex: stagnant hypoxia –> acidosis

Peripheral chemoreceptors

• Main stimulus = LOW P_O2
  
  • _​ex: MAP falls below 60mmHg

Decreases in P_O2:

• activate vasomotor centers
  
  • produce arteriolar vasoconstriction
  • ↑ in TPR
  • ↑ P_a

Question 13

What is the importance of Chemoreceptor Regulation?

Answer 13

• Important for extreme situations & chronic situations
  
  • Less important in regulating normal cardiac function
• Overall effect = Arteriolar Vasoconstriction

Question 14

What is the Importance of Bainbridge Reflex?

Answer 14

• Physiological counterbalance to baroreceptor reflex
  
  • disputed, but is an important reg system when CO & VR are mismatched!