Objective. Determination of the dynamics of hemoglobin and hematocrit concentrations in blood, assessment of interrelation between these indicators and blood loss in patients undergoing open-heart surgery using artificial circulation.Materials and methods. A group of 67 patients requiring open-heart surgery using artificial circulation was formed. Each patient was assessed for the dynamics of hemoglobin and hematocrit concentrations in the analysis of the acid-base state of arterial blood at 7 stages of hospital treatment.Results. The dynamics of changes in the content of hemoglobin and erythrocytes were assessed, the volume of blood loss and the presence of correlation of the studied parameters were determined.Conclusion. During open-heart surgery with cardiopulmonary bypass, a decrease in hemoglobin and hematocrit occurs. A correlation was found between hemoglobin concentration during the operation of infrared cardiopulmonary bypass apparatus and the volume of perioperative blood loss.