Hemoglobin is the oxygen-carrying pigment in red blood cells that gives them their red color. It transports oxygen from the lungs to tissues throughout the body, which is essential for human survival. Hemoglobin binds to oxygen in the lungs and releases it to tissues. There can be too much or too little hemoglobin in the body. Blood doping artificially increases red blood cell and hemoglobin levels through injections of hormones like EPO to improve endurance, especially in cycling. It allows more oxygen to be carried throughout the body.