Components of blood.

(a) When a blood sample is centrifuged, it forms three visible layers. Plasma is composed of water and dissolved solutes, leukocytes are the white blood cells that make up part of the body's immune system, and erythrocytes are red blood cells, which carry oxygen. An additional component of blood, thrombocytes (platelets in mammals), which participate in blood clotting reactions, is not visible under these conditions because of small numbers and size. Note: The leukocyte layer is enlarged and not to scale, for illustrative purposes. (b) The characteristic biconcave shape of mammalian erythrocytes, seen here in a SEM magnified more than 1,600 times, increases their surface area and aids in the diffusion of oxygen from the cell walls to the extracellular fluid.