Red blood cells, also called erythrocytes, move through the human (and animal) circulatory system and carry oxygen to the cells in the body. All blood cells are manufactured by stem cells, which live mainly in the bone marrow, via a process called hematopoiesis. The stem cells produce hemocytoblasts that differentiate into the precursors for all the different types of blood cells. Hemocytoblasts mature into three types of blood cells: erythrocytes (red blood cells or RBCs), leukocytes (white blood cells or WBCs - 2 WBCs are shown in yellow color), and thrombocytes (platelets). Enhanced SEM. Image width: 85.3 micrometers. Magnification: 1172x if the image is printed 10 cm wide.