Question 24

Outline the physiological responses to anaemia. (The specific physiological responses to 
hypovolaemia are NOT required.) 

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College Answer

It was expected candidates would expand on the central role haemoglobin has in oxygen delivery and that in the presence of reduced haemoglobin there are various efforts aimed at maintaining oxygen delivery. Cardiac output is increased, systemic vascular resistance is reduced, modifications are seen in regional circulations and as tissue oxygenation begins to falter then the end products of anaerobic metabolism provide a further stimulus to enhance cardiac out and tissue oxygen delivery. Better answers also included a mention of additional factors that enhance tolerance of chronic anaemia (e.g.angiogenesis).

Discussion

Total blood oxygen delivery (DO2)  = CO × CaO2,

and CaO2 =  (sO2 × ceHb × BO) + (PaO2 × 0.03)

where:

Where the ceHb is in decline, the other variables which describe CaO2 are usually not susceptible to large-scale change, and therefor the cardiac output needs increase.

  • Cardiovascular effects of acute isovolaemic anaemia are:
    • Tachycardia
    • Increased stroke volume
    • Increased cardiac output
    • Decreased peripheral vascular resistance
  • Mechanisms of these cardiovascular effects:
    • Vagally mediated tachycardia is partly due to direct aortic arch chemoreceptor activity and partly due to baroreflex activation 
      • Baroreflex activation is due to systemic vasodilation
    • Decreased peripheral vascular resistance is due to:
      • Systemic vasodilation which is mediated by nitric oxide, as the result of decreased oxygen delivery to the tissues (a part of the normal metabolic autoregulation of regional blood flow)
      • Decreased blood viscosity, as viscosity is an important determinant of peripheral vascular resistance
  • Long term effects are related to chronic vasodilation, and include:
    • Salt retention (mediated by aldosterone)
    • Body water volume expansion (mediated by vasopressin and aldosterone)
    • Angiogenesis (to increase the number of capillaries and therefore decrease the diffusion distance between capillaries and cells)

References

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