Formed elements in human peripheral blood include white blood cells, platelets (thrombocytes ), and red blood cells (RBC's, erythrocytes ). These images show RBC's and platelets stained with Wright's stain at 1000X magnification.

Red Blood Cells

RBC's are the most numerous cells seen in the peripheral blood and contain the hemoglobin that carries oxygen to the body's cells. A normal adult will have 5 to 6 million RBC's per cubic millimeter of blood. Mature RBC's lack a nucleus, stain pinkish-red and are 5 to 9 microns in diameter. No granules are visible in these doughnut-shaped cells. A pale central area results from the biconcave shape of the cell.

Platelets

Platelets are small (2 to 5 microns in diameter) structures that appear as a clump of reddish-purple granules in a nearly transparent cytoplasm. Fragments formed from megakaryocytes, platelets are essential for the blood clotting. A normal adult has 250,000 to 400,000 platelets per cubic millimeter of blood.