Question 1

Explain how oxygen supply is maintained to the tissues in chronic anaemia.

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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 × 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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Hatcher, J. D., L. K. Chiu, and D. B. Jennings. "Anemia as a stimulus to aortic and carotid chemoreceptors in the cat." Journal of Applied Physiology 44.5 (1978): 696-702.

Daly, M. de Burgh, Julie L. Hazzledine, and A. Howe. "Reflex respiratory and peripheral vascular responses to stimulation of the isolated perfused aortic arch chemoreceptors of the dog." The Journal of physiology 177.2 (1965): 300-322.

Lahiri, S., et al. "Relative responses of aortic body and carotid body chemoreceptors to hypotension." Journal of Applied Physiology 48.5 (1980): 781-788.

Biro, G. P., J. D. Hatcher, and D. B. Jennings. "The role of the aortic body chemoreceptors in the cardiac and respiratory responses to acute hypoxia in the anesthetized dog." Canadian journal of physiology and pharmacology 51.4 (1973): 249-259.

Whittaker, S. R. F., and F. R. Winton. "The apparent viscosity of blood flowing in the isolated hindlimb of the dog, and its variation with corpuscular concentration." The Journal of physiology 78.4 (1933): 339.

Anand, Inder S., et al. "Pathogenesis of oedema in chronic severe anaemia: studies of body water and sodium, renal function, haemodynamic variables, and plasma hormones." Heart 70.4 (1993): 357-362.