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Coloured scanning electron micrograph (SEM) of human red blood cells in hypotonic solution (0.65% NaCl). Some red blood cells are slightly swollen due to the low concentration of saline. 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 while squeezing through the body's capillaries. 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. The cell membrane is composed of proteins and lipids and this structure provides properties essential for physiological cell function (deformability and stability) while traversing the circulatory system, especially the capillary network. In humans, mature red blood cells are flexible and oval biconcave disks. Each cubic millimeter of blood contains around five million of these tiny, flexible disc-shaped cells. 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: x1,000 when shortest axis printed at 25 millimetres. (KEYSTONE/SCIENCE PHOTO LIBRARY/DENNIS KUNKEL MICROSCOPY/SCIENCE PHOTO LIBRARY)