Red blood cell thus much more common than the other blood particles: there are about 4,000–11,000 white blood cells and about 150,000–400,000 platelets per microliter. Human red blood cells take on average 60 seconds to complete one cycle of circulation. The blood's red color is due to the spectral properties of the hemic iron ions in hemoglobin. Each human red blood cell contains approximately 270 million of these hemoglobin molecules. Each hemoglobin molecule carries four heme groups; hemoglobin constitutes about a third of the total cell volume. Hemoglobin is responsible for the transport of more than 98% of the oxygen in the body
Packed red blood cells used to restore oxygen-carrying capacity in people with anemia due to trauma or other medical problems, and are by far the most common blood component used in transfusion medicine. Historically they were transfused as part of whole blood, but are now typically used separately as RBCs and plasma components. The process of identifying a compatible blood product for transfusion is complicated. Side effects can include allergic reactions including anaphylaxis, red blood cell breakdown, fluid overload, infection, and lung injury. Giving incompatible RBCs to a person can be fatal. With current testing methods in high-income countries the risk of infection is