Average volume of blood in a healthy adult individual is some 5000 ml (3000 ml plasma + 2000 ml erythrocytes). After an episode of acute bleeding, an organism will compensate the loss of plasma within one to three days, but low red blood cell count will remain. In chronic blood loss that a patient cannot compensate, hemoglobin level is lowered to dangerously low values hence severe anemia exists. Lowered blood viscosity results in lowered resistance to blood flow through peripheral blood vessels that are dilated due to hypoxia. HBOT could temporarily meet the needs of an organism for oxygen even without blood and this is what makes this method a "bloodless transfusion". In cases of profound blood loss, when replacement of lost volume is not possible due to whatever reason, when problems of cross-matching exist, when there are concerns or doubts dealing with blood safety or due to religious beliefs, intermittent application of HBOT will enable enough oxygen for basic metabolic needs and normal functioning of all vital processes.
Links: oxygen in blood plasma anemia hemoglobin oxygen therapy