Hemodilution is the practice of intentionally removing red blood cells to lower the haematocrit. This has the obvious result that the red cells can be replaced later if needed. However, the practice was first used to improve circulatory hemodynamics: lowered viscosity can increase the venous return to the heart and thereby raise cardiac output.The question of how much the hematocrit can be reduced without causing detrimental effects has been addressed in animals and human studies. The conclusion seems to be that in the absence of disease the optimal hematocrit may be around 20 per cent, but that levels as low as 5 g/dL can still be tolerated at sea level in healthy volunteers. Since the heart has a high metabolic demand for oxygen, and since the coronary sinus (venous) PO2 is so low that oxygen extraction is almost maximal under normal conditions, the heart is likely to be the organ most at risk from intentional hemodilution. The concept that reduced blood viscosity is beneficial at moderate hemodilution may not apply at extreme hemodilution, where local shear forces are necessary to maintain release of nitric oxide, a vasodilator. Maintenance of these shear forces may be possible at very low hematocrit if the hemodiluent has relatively increased viscosity.