Question 24

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

[Click here to toggle visibility of the answers]

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).


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

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


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)


Essex, David W., David K. Jin, and Thomas P. Bradley. "Lactic acidosis secondary to severe anemia in a patient with paroxysmal nocturnal hemoglobinuria." American journal of hematology 55.2 (1997): 110-111

Levy, PAUL S., et al. "Limit to cardiac compensation during acute isovolemic hemodilution: influence of coronary stenosis." American Journal of Physiology-Heart and Circulatory Physiology 265.1 (1993): H340-H349.

VAN DER LINDEN, Ph. "The physiology of acute isovolaemic anaemia." Acta anaesthesiologica Belgica 53.2 (2002): 97-103.

Weisskopff, R. B., et al. "Human cardiovascular and metabolic response to severe, isovolaemic anaemia." JAMA 279 (1988): 217-21.

Doss, Doss N., et al. "Mechanism of systemic vasodilation during normovolemic hemodilution." Anesthesia & Analgesia 81.1 (1995): 30-34.

Rosen, ARTHUR L., et al. "Cardiac output response to extreme hemodilution with hemoglobin solutions of various P50 values." Critical care medicine 7.9 (1979): 380-384.

Fahim, Mohammad, and Maman Singh. "Hemodynamic Responses during Acute Normovolemic Hemodilution in Anesthetized Dogs." The Japanese Journal of Physiology 42.5 (1992): 753-763.

Crystal, George J., and M. Ramez Salem. "Hemodynamic compensation during acute normovolemic hemodilution." Anesthesiology: The Journal of the American Society of Anesthesiologists 100.4 (2004): 1034-1034.

Ickx, Brigitte E., Michel Rigolet, and Philippe J. Van der Linden. "Cardiovascular and Metabolic Response to Acute Normovolemic AnemiaEffects of Anesthesia." Anesthesiology: The Journal of the American Society of Anesthesiologists 93.4 (2000): 1011-1016.

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.