Red blood cells. Scanning Electron Micrograph (SEM) of six human red blood cells showing the cells' characteristic biconcave-disc shape. Red blood cells carry oxygen. The red pigment inside the cell, hemoglobin, combines with oxygen as the blood passes through the lungs and then releases it in other parts of the body. The cells are made in massive numbers by bone marrow: a single pinhead-sized drop of blood contains about five million. Because they are subjected to constant bombardment as they pass through the bloodstream, red blood cells last only around 4 months after which they are destroyed by the liver. Magnification unknown.