CVS – Responses

Master answer

Normal circulating Volume ~ 5L

2L Haemorrhage ~ 40% loss without resuscitation – Major Haemorrhage

Immediate:

Baroreceptor Response

• HP BR
  • Stretch receptor: ↑ distension of vessel → ↑ discharge rate
  • Threshold > 60mmHg → normally has baseline tone.
  • Located at Carotid Sinus and Aortic Arch
  • Carotid sinus and aortic arch receptors
    • Detects > 5-10 % change in plasma volume
    • ↓ Plasma volume → Increased central SNS tone and decreased PSNS tone
    • ↑’d distension → ↓’d HR/Contractility/SVR
  • Provides strict negative feedback to Δ’s in CO
• LP BR
  • Located at junction of return vessels and atria, vetricular walls, pulmonary vessles
  • Throughout the peripheral vasculature (esp kidney)
  • Detect > 10% decrease in plasma volume as decreased atrial stretch
  • Reduced ANP release
  • Increase SNS output
  • 2 types
    • A receptors → fire at atrial contraction (a wave)
    • B type → fire at atrial filling (v wave)
    • 1. Medulla afferents cause
      • ↓ SNS (NA) and ↑PSNS (RVLM) outflow → peripheral vascular vaso and venodilation
      • ↓ SNS activity to kidney → ↓ Na/H2O conservation
      • ↑ SNS activity to sinus node
    • 2. Hypothalamic afferents cause
      • → ↓ ADH release and ↓ Thirst
  • Outcome
    • ↓ PL → ↓ CO → ↓BR discharge rate
      • → ↓ PSNS medullary outflow
      • → ↑ RVLM outflow
    • ↑ contractility/HR
    • ↑ SVR → autotransfusion and ↑ perfusing pressure (but ↑AL)

Chemoreceptor response

• Peripheral CR
• “Carotid bodies” (more important)
  • at bifurcation point of common carotid arteries
  • supplied by CN IX
• “Aortic bodies” (less important)
  • above and below the aortic arch
  • supplied by CN X
• PCRs found in “Glomus cells”
  • ↓ PaO2 – via inhibition of O2 sensitive K-channels
  • ↑ PaCO2 and/or ↓pH – via effect on pH sensitive K-channels
  • Type I cells (rich in NAd, DA, ACh)
    • Hypoxia causes release of NTs
    • NAd/ACh – ↑ AP firing rate of AB or CB afferent fibres
    • DA – Damping of type 2 cell responses
  • Type II cells (rich in capillary supply)
    • ↓ PaO2, ↑ PaCO2 and/or ↓pH 
    • ↓ IC [ATP] which leads to ↑ NT production and release
    • ↑ AP firing rate of AB or CB afferent fibres
• CCRs  located Below the ventral surface of the medulla
  • ↑ ECF [H+] stimulates ↑ ventilation
  • CO2 rapidly diffuses into CSF
  • Low CSF protein and HCO3 (c.f. plasma)
    • Poor buffering capacity
    • ΔCSF pH per ΔPaCO2 is GREATER than that of blood
• Cause
  • ↑’s SVR due to
    • ↓ hypothalamic perfusion pressure
    • ↓ Hct
  • ↑’d RR → ↑ venous return
  • ↑ ADH secretion (intermediate term response)

Intermediate term response

• Chemoreceptor stimulation (↓’d perfusion → ↑pCO₂:pO₂)
  • ↑ ADH secretion
• ↑’d RAAS activation
  • ↓ renal perfusion → ↑ renin → ↑AT2 and Aldosterone
  • ↑ADH → ↑Na/H2O retention and vasoconstriction
• ↓Hydrostatic pressure
  • altered starlings forces
  • autotransfusion of 0.25mL/kg/min ~ 1L/hr

Long Term Response

• ↑ RAAS → ↑ Na/H2O retention
• Osmoreceptor response
  • Anterior hypothalamic receptors
  • Detect 1% decrease from normal (280-300mOsm/kg)
  • Threshold 280mOsm/kg
  • Promotes release of ADH from posterior pituitary
  • ↑thirst
• EPO release
  • From adrenal medulla → ↑ RBC mass
• ↑ hepatic plasma protein synthesis

Gladwin 2016

Answer by Gladwin