Scanning electron micrograph (SEM) of human red blood cells in physiological isotonic saline solution (0.9% NaCl). Human red blood cells (RBCs), also called erythrocytes, are the most common type of blood cell in vertebrates. They are involved in delivering oxygen to the body tissue. RBCs take up oxygen in the lungs and release it into tissues. The cytoplasm of erythrocytes is rich in hemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the red color of the cells. In humans, mature red blood cells are flexible and oval biconcave disks. RBCs are the most abundant type of cell in human blood, accounting for 40% of the blood volume. Each cubic millimeter of blood contains around five million of them. Because the cells have no nucleus and are subjected to constant physical action, they last only four months before being destroyed and broken down. Magnification: x865 when shortest axis printed at 25 millimetres.