Acute renal failure is one of the most frequent and life-threatening complications after cardiac surgery. There is a recent growing deal of information suggesting that during the time of cardiopulmonary bypass kidneys may suffer from an imbalance between oxygen supply and oxygen needs. A low hematocrit during cardiopulmonary bypass is associated with an increase risk of acute renal failure, mainly due to a low oxygen delivery. An inadequate oxygen delivery during cardiopulmonary bypass is associated with lactate production, and under normothermic conditions, hyperlactatemia appears for an oxygen delivery below 260 mL min^{− 1} m⁻². The risk of acute renal failure significantly increases for an oxygen delivery approximately at the same value (272 mL min⁻¹ m^{− 2}). During cardiopulmonary bypass, the pump flow should be coupled with the hematocrit to avoid falling below this critical oxygen delivery.