PHYS - Neural Reflexes and Regulation of MAP

Question 1

ARTERIAL BLOOD PRESSURE

Answer 1

• Physiological factors –> physical factors –> arterial BP
  
  • Physiological = Mean Arterial Pressure
    
    • CO (= SV*HR)
    • TPR
  • Physical = Pulse Pressure
    
    • Arterial blood volume
    • Arterial compliance
• Can change/regulate arterial blood pressure by changing HR, SV, or vessel diameter

Question 2

BARORECEPTORS

Answer 2

• Sense acute changes in BP
• Unmyelinated afferent nerve fibers located within the carotid sinus near the bifurcation of the common carotids
  
  • Baroreceptor activity is INCREASED by sudden increase in BP and DECREASED by sudden decrease in BP
  • Respond to absolute BP change and how rapidly it changes
• High pressure baroreceptors
  
  • Carotid baroreceptors (50-200 mmHg)
  • Aortic arch baroreceptors (100-200 mmHg)
  • Sense STRETCH (resulting from change in BP or vessel tension)
• Low pressure = cardiopulmonary baroreceptors
  
  • Pulmonary artery
  • Junction of atria and vena cava
  • Junction of atria and pulmonary vein
  • Atria and ventricles
  • Sense STRETCH (resulting from changes in LVEDP, VR, and blood volume) → regulate renal Q and blood volume

Question 3

MEDULLAR CARDIOVASCULAR CENTER

Answer 3

• Medulla oblongata (vasomotor center)
• Receives info from sensory fibers to regulate and maintain systemic arterial BP at a set point
• Also receives information from the hypothalamus, limbic center, and cerebral cortex which can override homeostasis of the CV system

Question 4

BARORECEPTOR REFLEX RESPONSE

Answer 4

• Decreased MAP
  
  • Decreased stretch on carotid baroreceptors
  • Decreased firing of carotid sinus nerve
  • Decreased parasympathetic tone
    
    • Increased HR
  • Increased sympathetic tone
    
    • (β1)Increased contractility and HR
    • (α1)Increased arteriolar vasoconstriction (TPR)
    • (α1)Increased venous vasoconstriction
      
      • Increased venous return
  • Increase in MAP to normal
• Increased aortic pressure
  
  • Increased aortic baroreceptor activity
  • Increased parasympathetic tone (decreased HR)
  • Decreased sympathetic tone
  • Decreased aortic BP to normal

Question 5

BARORECEPTOR CONCEPTS

Answer 5

• Massaging the carotid arteries → perceived increase in BP by baroreceptors → HR decreases
• Loss of arterial baroreceptor nerves (cut) → no change in MAP because there are several modes of BP regulation outside the ANS
  
  • BUT, fluctuations around the mean would increase and BP would be less stable
• Loss of arterial and cardiopulmonary baroreceptors → increased MAP and BP fluctuation around the mean
  
  • New set point for MAP
• In chronic HT, a new set point for MAP at a much higher BP is created
  
  • Because baroreceptors only respond to sudden changes, not chronic
• Baroreceptors = acute BP regulation (short term)

Question 6

ARTERIAL CHEMORECEPTORS

Answer 6

• Peripheral chemoreceptors
  
  • Only stimulated in low BP situations (below 80 mmHg)
• Buffer acute changes
  
  • Cardiopulmonary = pCO2, pH
    
    • Regulate breathing
    • Increased CO2 or H+ → stimulate breathing = hyperventilation
    • Decreased CO2 or H+ → inhibit breathing = hypoventilation
• Carotid/Aortic = pO2 (hypoxia), pCO2 (hypercapnia), and pH (acidosis)
  
  • Respond to severe hypoxia (ex: hemorrhagic hypotension)
    
    • Chemo receptors have high rates of O2 consumption → sensitive to O2 decreases → stimulate vasoconstriction → increased TPR → increased arterial pressure
    • pO2 < 60 mmHg will initiate hyperventilation
  • Decreased O2, increased CO2, and decreased pH
    
    • Decrease parasympathetic tone = HR increases
    • Increase sympathetic tone = increase HR, SV, and vasoconstriction
  • Asphyxia (fixed or prevented ventilation) → bradycardia via vagal nerve stimulation, renal vasoconstriction, and increased respiratory rate and depth (increased tidal volume)

Question 7

RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM

Answer 7

• Renin → Angiotensin I –ACE→ Angiotensin II
  
  • Adrenal Medula → aldosterone release → increased Na+ reabsorption
  • Renal proximal tubule → increase Na+ reabsorption
  • Peripheral arterioles → increase vascular resistance
• Triggered by a decrease in renal perfusion pressure
• Slow, utilizes hormones → long-term BP regulation via changes in volume and vascular resistance

Question 8

OTHER MECHANISMS OF ARTERIAL BP REGULATION

Answer 8

• CNS Ischemic Response
  
  • CNS ischemia → increased pCO2
    
    • Increased sympathetic tone to BVs → increased SV, vasoconstriction (HR ish) → systemic arterial BP
    • Increased parasympathetic tone to heart → decreased HR
  • Cushing Response
    
    • Increased cranial P → compression of cerebral arteries → decreased cerebral Q = ischemia → CNS ischemic response → increased Q to medulla
• Vasopression (ADH)
  
  • Released in response to decreased blood volume (hemorrhage) and activated low P baroreceptors
  • Vasoconstriction
  • H2O reabsorption
• Increased extracellular fluid volume
  
  • Increased blood volume, increased CVP, increased VR, increased CO, autoregulation, increased TPR, increased mean arterial pressure
• Natriuretic Peptides (ANP, BNP)
  
  • Released by atria in response to increased pressure
  • Vasodilation → decreased TPR
  • Increased Na+/H2O filtration
  • Inhibition of renin secretion

Question 9

COMPARISON OF BP REGULATION SYSTEMS

Answer 9

• Speed of regulation
  
  • Baroreceptors/Chemoreceptors/Ischemic response = fast
    
    • Baro within a heart beat
  • Renin-angiotensin = slow, minutes to hours
  • Aldosterone = slowest, days
• Pressure of regulation
  
  • Ischemic response = below 75 mmHg
  • Baroreceptors = 50-200 mmHg
  • Chemoreceptors = 250-115 mmHg
  • Aldosterone = 25+ to 200+ mmHg
  • Renin-angiotensin = 50-125 mmHg