<P> Several blood tests involve red blood cells . These include a RBC count (the number of red blood cells per volume of blood), calculation of the hematocrit (percentage of blood volume occupied by red blood cells), and the erythrocyte sedimentation rate . The blood type needs to be determined to prepare for a blood transfusion or an organ transplantation . </P> <P> Many diseases involving red blood cells are diagnosed with a blood film (or peripheral blood smear), where a thin layer of blood is smeared on a microscope slide . This may reveal abnormalities of red blood cell shape and form . When red blood cells sometimes occur as a stack, flat side next to flat side . This is known as rouleaux formation, and it occurs more often if the levels of certain serum proteins are elevated, as for instance during inflammation . </P> <P> Red blood cells can be obtained from whole blood by centrifugation, which separates the cells from the blood plasma in a process known as blood fractionation . Packed red blood cells, which are made in this way from whole blood with the plasma removed, are used in transfusion medicine . During plasma donation, the red blood cells are pumped back into the body right away and only the plasma is collected . </P> <P> Some athletes have tried to improve their performance by blood doping: first about 1 litre of their blood is extracted, then the red blood cells are isolated, frozen and stored, to be reinjected shortly before the competition . (Red blood cells can be conserved for 5 weeks at − 79 ° C or − 110 ° F) This practice is hard to detect but may endanger the human cardiovascular system which is not equipped to deal with blood of the resulting higher viscosity . Another method of blood doping involves injection with erythropoietin in order to stimulate production of red blood cells . Both practices are banned by the World Anti-Doping Agency . </P>

How long does it take for a blood cell to divide