Our muscles need to be constantly supplied with energy during physical exertion, and this energy comes from two main components: sugar and oxygen. Oxygen is carried throughout the body by red blood cells by binding to a protein in the cell called hemoglobin. The circulation of oxygenated blood throughout the body is a cyclic process - when we take a breath of air in, the lungs help transfer oxygen from the air so that every hemoglobin is full of oxygen. This is oxygenated hemoglobin, or oxyhemoglobin. When the red blood cell circulators to a muscle during physical exertion, it becomes deoxygenated hemoglobin, or deoxy-hemoglobin.