<P> One microliter of blood contains: </P> <Ul> <Li> 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes: Red blood cells contain the blood's hemoglobin and distribute oxygen . Mature red blood cells lack a nucleus and organelles in mammals . The red blood cells (together with endothelial vessel cells and other cells) are also marked by glycoproteins that define the different blood types . The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45% . The combined surface area of all red blood cells of the human body would be roughly 2,000 times as great as the body's exterior surface . </Li> <Li> 4,000--11,000 leukocytes: White blood cells are part of the body's immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances . The cancer of leukocytes is called leukemia . </Li> <Li> 200,000--500,000 thrombocytes: Also called platelets, they take part in blood clotting (coagulation). Fibrin from the coagulation cascade creates a mesh over the platelet plug . </Li> </Ul> <Li> 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes: Red blood cells contain the blood's hemoglobin and distribute oxygen . Mature red blood cells lack a nucleus and organelles in mammals . The red blood cells (together with endothelial vessel cells and other cells) are also marked by glycoproteins that define the different blood types . The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45% . The combined surface area of all red blood cells of the human body would be roughly 2,000 times as great as the body's exterior surface . </Li> <Li> 4,000--11,000 leukocytes: White blood cells are part of the body's immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances . The cancer of leukocytes is called leukemia . </Li>

The percentage of red blood cells compared to blood volume is known as