Red blood cells. Computer artwork of red blood cells and platelets (orange/yellow) in a blood vessel. Red blood cells, or erythrocytes, are biconcave disc-shaped cells. They contain the red pigment haemoglobin, which transports oxygen from the lungs to the body's cells where it is used in respiration. The platelets, or thrombocytes, have changed from their unactivated disc-like form to become irregular in shape. In damaged blood vessels this change helps the blood to clot. Both red blood cells and platelets are formed in the bone marrow.