Red blood cells (erythrocytes). False-color scanning electron micrograph of a group of red blood cells on the endocardium, the innermost layer lining the heart. During its short life (about four months) a red blood cell covers about 15km every day for a total of 1500km. In a liter of blood there are normally 5000 billion red blood cells. They transport oxygen from the lungs to the tissues where it is necessary for all metabolic processes. The endocardium is composed of a thin layer of flattened epithelial cells supported by an underlying basement membrane of connective tissue. Magnification: x2220 at 6x7cm size. x3750 at 4x5