Question 1

College Answer

Candidates were expected to base their answer around the variables involved in the equations that describe oxygen content in blood and oxygen delivery. Although most candidates      mentioned changes in haemoglobin that increase oxygen carriage, a more complete     discussion of the changes that influence cardiac output and the peripheral circulation was often omitted

Discussion

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

and CaO₂ =  (sO₂ × ceHb × BO₂ ) + (PaO₂ × 0.03)

where:

• ceHb = the effective haemoglobin concentration
• CO = cardiac output
• PaO₂ = the partial pressure of oxygen in arterial gas
• 0.03 = the content, in ml/L/mmHg, of dissolved oxygen in blood
• BO₂ =  the maximum amount of Hb-bound O₂ per unit volume of blood (normally 1.39)
• sO₂ = oxygen saturation

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)

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