In the early twentieth century, researchers began to examine hemoglobin – the protein responsible for carrying oxygen from the respiratory organs to the rest of the body – in red blood cells. They found that, when isolated from aging cells – whether from human or cow blood, or from genetically engineered sources – free hemoglobin can be rejuvenated, chemically stabilized, and re-infused as a blood “substitute” that can carry oxygen as effectively as red blood cells, but for a much shorter time.