These are commonly called the red blood corpuscles (R.B.C). As the name indicates they are scarlet red in colour due to the presence of an oxygen carying pigment called Hemoglobin. The shape of the RBCs varies in different groups of animals. In frog the RBC is biconvex and nucleated; the same is the condition in fishes, rep­tiles and birds. The RBCs of mammals are circular and biconcave and do not have any nucleus. The central part of the RBCs is thinner and helps in flexibility.

The sizes of the human RBCs are 7 to 8 mm in diameter. In human beings on an average there are about 4 to 5 million RBCs per cubic mm of blood. The RBC’s are formed in bone marrow by cells called megaloblasts. Re­duction in the number of RBCs causes anemia.

As has already been said the red colour in RBCs is due to hemoglobin which is a conjugated protein. This is basically made up of protein joined together with a heme group hence the name hemoglobin. Structurally each RBC is bound by a differentially permeable membrane. The cytoplasm gradually loses all the usual components of the cell during the course of maturation. As a result it has lot of space to hold hemoglo­bin.

The process of the formation of RBCs is called Erythropoiesis. Pro­teins and other iron components stimulate the production of RBCs. Usually human RBCs have a life span of about 120 days and worn out RBC’s are destroyed by phagocytosis in the blood itself and also in the spleen and liver.