The cardiopulmonary bypass (CPB) circuit must be primed with a fluid solution. The volume of prime required is either based on a standard empirically derived volume greater than a minimum safe priming volume, or may be guided by the patient's weight or body surface area. The initial hematocrit (HCT) achieved after initiation of CPB is determined by the volume of the prime in relation to the patient's pre-CPB HCT. There are many different recipes for priming solutions using crystalloid, colloid or blood as primary constituents. Blood was used to prime the CPB circuit in an attempt to preserve a high hematocrit; early in the evolution of CPB this was thought to be an important determinant for successful outcome. The idea of using oxygen-carrying solutions as blood substitutes may be an attractive means of maintaining oxygen delivery. They would address the expense, limited supply and disease transmission associated with blood transfusion.