Color enhanced SEM of normal red blood corpuscles. Red blood cells (RBCs), also called erythrocytes, are the most common type of blood cell and the vertebrate's principal means of delivering oxygen (O2) to the body tissues- via blood flow through the circulatory system. RBCs take up oxygen in the lungs and release it into tissues while squeezing through the body's capillaries. In humans, mature red blood cells are flexible and oval biconcave disks. They lack a cell nucleus and most organelles, in order to accommodate maximum space for hemoglobin; they can be viewed as sacks of hemoglobin, with a plasma membrane as the sack. Approximately 2.4 million new erythrocytes are produced per second in human adults. The cells develop in the bone marrow and circulate for about 100-120 days in the body before their components are recycled by macrophages. Magnification: 6750x, 5 x 7 inches.