The primary function of red blood cells is to transport oxygen from the lungs to the cells of the body. RBCs contain a protein called hemoglobin that actually carries the oxygen. In the capillaries, the oxygen is released to be used by the cells of the body.he ratio of cells in normal blood is 600 RBCs for each white blood cell and 40 platelets. There are several things about RBCs that make them unusual: 1  An RBC has a strange shape -- a biconcave disc that is round and flat, sort of like a shallow bowl. 2  An RBC has no nucleus.