Erythrocytes, or red blood cells function in the transportation of the respiratory gases O₂ and CO₂. They are about 700 times more numerous than leukocytes and 17 times more numerous than platelets. Males have about 5.4 million erythrocytes per cubic millimeter of blood (range: 4.6 - 6.2 million), whereas females have about 4.8 million erythrocytes per cubic millimeter of blood (range: 4.2 - 5.4 million). Therefore, males have approximately 10% more erythrocytes then females.

Normal erythrocytes are biconcave disks about 7.5 mm in diameter with edges that are thicker than the center of the cell. Compared with a flat disk of the same size, the biconcave shape increases the surface area of the erythrocyte. The greater surface area makes the movement of gases into and out of the erythrocyte more rapid. In addition, the erythrocyte can bend or fold around its thin center, decreasing its size and enabling it to pass more easily through small blood vessels.