Q1vi: Explain the physiological consequences of acute and chronic anaemia

Definitions & Normal Values

  • Anaemia = ↓Hb content & ∴O2 carrying capacity of blood
  • WHO: Hb → M < 130g/L, F <120g/L
  • Chronic, not well defined, but ongoing anaemia

CaO2 & DO2

  • CaO2 = O2 bound Hb + O2 dissolved

          = ([Hb] x SpO4 x 1.34) + (0.03 x PO2)]

  • Most of O2 is transported bound to Hb
  • ∴↓Hb = ↓↓O2 carrying capacity of blood

→ Each Hb has 4 heme moieties

→ ∴can transport 4 x O2

→ +ve cooperativity

→ Design of RBC to accommodate ~300million Hb molecules

→ ∴showing how important it is as O2 carrier

DO2 = CaO2 x CO

∴↓CO2 = ↓delivery of O2 to organs

Compensatory Mechanisms

  1. ↑CO
  2. ∆ Distribution CO
  3. ↑O2 extraction
  4. ∆ Hb affinity
  5. Renal effects
  6. Resp effects

↑CO

  • CO = HR x SV
    • PreL
    • After L
    • Contractility
  • 2 mechanisms of ↑CO → ↓blood viscosity + ↑symp stimulation

→ Viscosity affects preL & afterL

→ Symp affects C & HR

Viscosity

  • Loss of RBC = ↓viscosity of blood
Viscosity
  • ∴↓viscosity = ↑flow
  • Flow is slowest in post cap venules (where viscosity is highest)
  • In anaemia → ↓viscosity → ↑venule flow → ↑VR → ↑PreL → via F – S → ↑CO

Sympathetic Stimulation

  • ↓O2 delivery
  • ↑anaerobic metabolic products (↑H+, K+, lactate)
  • ↓SVR
  • MAP = CO x SVR

∴↓MAP

  • Detected by high P BaroR
  • Vasomotor Centre in Medulla → ↑symp & ↓parasymp outflow
    • H = ↑HR, ↑FoC
    • Resistance Vessels → VC = ↑SVR → restore MAP
    • Capacitance vessels → VC → ↑VR
  • OVERALL = ↑ CO

∆ Distribution CO

  • Blood is redistributed to tissues with high ER (brain, H)
  • CBF can ↑600%!
  • Skin, kidneys, splanchnic are main donors
  • Kidney has high perfusion in excess of metabolic requirements → also ↓GFR → stigma → ↓intravascular volume loss

↑O2 Extraction

Fick’s Equation: VO2 = Q (CaO2 – CvO2)

  • ∴↓CvO2 = ↑VO2 (O2 extraction)
  • H+ accumulation
  • Conformational ∆Hb
  • Readily releases O2
  • KA­ BOHR EFFECT
  • ↓CvO2

∆ Hb Affinity

  • ↑O2 extraction = ↑deoxyHb
  • RBC metabolism completely anaerobic
  • ↑2,3 DPG production via RAPPORT-LEUBERING SHUNT
  • ↑2,3 DPG → R) SHIFT ODC
  • ∴ further offloading O2

Renal Effects

  • ↓Blood vol → ↑Na & H­2O retention
  • Restores circulating volume
  • Hypoxia to peritubular caps → ↑EPO production
  • ↑rate production & turnover RBC to restore [Hb]

Respiratory Effects

  • ↑CO2 & ↓pH → stimulate ChemoR
  • ↑MV
  • ↓pCO2