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

· Intro

· High pressure baroreceptors

· Low pressure baroreceptors

· Renal 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

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

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

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