Red blood cells (RBCs), also called erythrocytes, make up the biggest part of blood cells. They bind and deliver oxygen from the lungs to all areas of the body. Oxygen binding is controlled by the protein hemoglobin, which is found in high concentration in the cytoplasm of RBCs.
Human erythrocytes have a diameter of approximately 6-8 µm and a discoid shape (Figure 1). Typically, adults have around 20-30 x 1012 RBCs.
Figure 1: Red blood cells (RBCs)