Carbon Dioxide Transport by Hemoglobin-Based Blood Substitutes

Abstract

Carbon dioxide transport by blood is often overlooked when considering the design and clinical potential of cell-free O₂ carriers. However, approximately the same amount of CO₂is removed from respiring tissue as O₂delivered to, and the implications for blood substitutes must be considered, especially in critically ill patients where tissue CO₂ build-up could be very high. Approximately 23% of total CO₂ is transported as carbamate (i.e. bound to hemoglobin) and is “oxylabile” (the affinity of deoxy hemoglobin for CO₂ is higher than that of oxyhemoglobin). In addition to this important role of hemoglobin, red cells are also critical to overall CO₂ transport because they contain carbonic anhydrase which permits the rapid hydration of CO₂ to bicarbonate and hydrogen ion. Without this enzyme, the hydration reaction would be much slower than the circulation time. Except for αα-hemoglobin, which has reduced CO₂ binding, little is known about the effects of crosslinking on CO₂ binding. It is of interest to consider how CO₂ transport is handled naturally by underwater (crocodiles) and high altitude animals (sheep and goats) who are faced with O₂ shortages in nature.