2013C16 Explain how oxygen supply to organs is maintained during isovolumic haemodilution.



·      Isovolumic haemodilution

·      Equations and normal values

·      Direct effect

·      Short term compensation

·      Medium term compensation


Isovolumic haemodilution:


Same blood volume but ↓Hct


↓O2 content

↓O2 delivery

↓tissue pO2

↑Anaerobic metabolism, lactic acidosis


Relevant equations:

Ohm’s law

Blood flow = driving pressure / resistance

Poiseuille’s law

Resistance to laminar flow = (8 x length x viscosity) / (π x radius4)

Fick equation

VO2 = CO x (CaO2 – CvO2)

O2 delivery

DO2 = CO x CaO2

O2 content

CaO2 = [Hb] x Hufner x SaO2 + 0.03 x PaO2 (per litre)

O2 extraction

EO2 = VO2/DO2

Normal values

DO2 1L/min

CO 5L/min

CaO2 200mL/L

EO2 0.25 (i.e. significant reserve for most tissues)

Alveolar ventilation equation


Alveolar gas equation

PAO2 = PiO2 – PaCO2/0.8


Direct effects:

↓O2 supply

↓Hb -> ↓CaO2 -> ↓DO2

↑O2 supply

↓Hb -> ↓HCt -> ↓viscosity -> ↓resistance -> CO -> ↑DO2


Short term compensation (seconds-minutes):


(A) Local metabolic autoregulation:

o Anaerobic -> ↑H+/K+/lactate -> vasodilatation -> capillary recruitment and distension

o ↑preload, ↓afterload -> ↑CO -> DO2

o ↑area for gas exchange -> ↑EO2 also

o Important in heart, skeletal muscle, brain

(B) Sympathetic nervous discharge:

o Severe anaemic hypoxia (Hb <50?) -> CNS acidosis -> ↑SNS activation

o ↑ CO -> ↑DO2


*Note gains are offset by ↑VO2*


(A) Local metabolic autoregulation

o Capillary recruitment and distension as above

o ↑Area for gas exchange -> ↑EO2

(B) Oxyhaemoglobin dissociation curve (OHDC) shift

o Anaerobic metabolism -> ↑tissue [H+], ↑RBC [2,3-DPG] -> right shift OHDC

o Note offset by ↑VA -> ↓PaCO2 -> left shift OHDC


·  Anaerobic metabolism -> ↑[H+] -> ↑chemoreceptor activation -> ↑VA

·  ↑VA -> ↓PACO2/PaCO2 -> ↑PAO2/PaO2 -> ↑DO2


*Note gains are offset by ↑VO2*


Medium term (days-weeks)


·  ↓pO2 in renal interstitium -> ↑EPO production

·  Hormonal response within hours

·  RBC mass restored in 3 weeks


·  e.g. ↑VEGF expression

↑Cell oxidative capacity

·  ↑Mitochondrial density

·  ↑Oxidative enzymes (TCA cycle, fatty acid oxidation)




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