Blood contains erythrocytes (red blood cells) and leukocytes (white blood cells). Hematocrit is the volume percent of red blood cells in whole blood (about 40%).
Red blood cells (erythrocytes) are biconcave discs with a thin central region and thicker outer margin. The main function of red blood cells is to transport oxygen bound to hemoglobin. The red color of hemoglobin is due to its iron-containing pigment that binds oxygen. Red blood cells have no nucleus (anucleate) and cannot divide or synthesize proteins. The lifetime of a red blood cell is about 120 days; they are broken down in spleen and liver. Erythrocytes have no mitochondria, so energy is supplied by anaerobic (no oxygen) metabolism of glucose.
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