2015A06 Describe the role of the baroreceptors in the control of systemic arterial pressure.

 

List:

·         Intro

·         High pressure baroreceptors

·         Low pressure baroreceptors

·         Renal baroreceptors

 

Intro: baroreceptors

Background

·   Ohm’s law: cardiac output = (mAP – CVP) / SVR

·   ↓↓perfusion pressure -> ischaemia e.g. type 2 AMI

·   ↑↑perfusion pressure -> organ damage e.g. haemorrhagic stroke

·   Hence need for rapid negative feedback regulation

Baroreceptors

·   Mechanoreceptor on C fibre general visceral afferent

·   Located in vascular adventitia

·   Vessel stretch -> ? inhibit K+ channel -> depolarisation -> action potential

·   Response degree of stretch, and rate of change in stretch

 

High pressure baroreceptors:

Pathway

Location

·   Carotid sinuses (more sensitive)

·   Aortic sinus (less sensitive)

Stimulus

·   ↓mAP or ↓pulse pressure -> ↓stretch -> ↓afferent

Afferent

·   Visceral afferent C fibre (?)

o CNIX Herings nerve (carotid sinus)

o CNX vagus nerve (aortic sinus)

Integrator

·   Medulla/hypothalamus

o SNS: rostral ventrolateral medulla

o PSNS: input to nucleus tractus solitarius; output from nucleus ambiguuus and dorsal motor nucleus

·   If normal blood pressure:

o Central PSNS cells are inherently quiescent but tonically activated

o Central SNS cells are inherently active but tonically inhibited (allows rapid response)

Neural efferent

·   ↑SNS, ↓PSNS output

·   ↑HR, ↑contractility

·   ↑Venoconstriction -> ↑venous return

·   ↑Vasoconstriction -> ↑SVR

·   ->↑cardiac output, ↑mAP

Hormonal efferent

·   ↑ADH release from hypothalamus (note normally minimal)

·   Aquaporin-2 insertion into apical membrane

·   ↑free H2O reabsorption from collecting ducts

·   ↑blood volume -> ↑mAP

Effective autoregulatory range

·   60-180mmHg

·   Right shift with exercise or chronic hypertension

Response time

·   Seconds (i.e. responds to postural change)

Clinical applications

·   Heart rate used to assess severity of shock

·   Valsalva manoeuvre to assess autonomic function

·   Carotid sinus massage for terminating SVT

 

Low pressure baroreceptors:

Location

·   Atria and great veins

Stimulus

·   ↓Distension -> ↓stretch -> ↓afferent (i.e. volume-sensitive)

Afferent

·   Visceral afferent C fibre (?)

Integrator

·   Medulla/hypothalamus

Neural efferent

·   ↑SNS, ↓PSNS output

Hormonal efferent

·   ↓ANP release by right atrium

·   ↓Dilatation of afferent arteriole -> ↓GFR, ↓washout of medullary interstitium

·   Inhibit NCC in distal convoluted tubule and ENaC in cortical collecting ducts

·   ↑NaCl reabsorption, ↓natriuresis

·   ↑Blood volume, ↑CVP

Response time

·   Minutes

Clinical use

·   Sacubitril -> inhibit neprilysin -> ↑ANP

 

Renal baroreceptors:

Location

·   Walls of afferent arterioles

Stimulus

·   ↓afferent arteriolar pressure -> ↓stretch

Integrator

·   Juxtaglomerular cell

Neural efferent

·   Afferent arteriolar dilatation -> ↑renal blood flow -> ↑GFR

Hormonal efferent

·   ↑Activity of renin-angiotensin system:

·   Angiotensin 2:

o Efferent > afferent arteriolar vasoconstriction

o Peripheral vasoconstriction

o ↑Na+/H2O reabsorption from proximal tubule -> ↑blood volume

o ↑mAP

·   Aldosterone:

o ↑Na+ reabsorption from late distal tubule and collecting ducts

o ↑Na+ reabsorption from colon, sweat

o Indirect H2O reabsorption via ↑ECF osmolality

o ↑Blood volume

o ↑mAP

 

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