Color-enhanced SEM of red blood cells (RBCs). Also called erythrocytes, they are the most common type of blood cell and the vertebrate organism'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 or gills 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 such as deformability and stability while traversing the circulatory system and specifically the capillary network. In humans, mature red blood cells are flexible and oval biconcave disks. Magnification @ 10,000x, 8x10 print. This image has been color enhanced.