As with all tissues dependent on a vascular supply, muscles depend on red blood cells to transport oxygen to them. Red blood cells, in turn, rely on an oxygen transport protein called hemoglobin to carry oxygen to all parts of the body.
Hemoglobin is a tetramer, meaning a protein composed of four subunits. Hemoglobin also contains four cofactors called heme, a ring-shaped structure with an iron atom at its center. Iron binds oxygen reversibly, allowing hemoglobin to pick up oxygen in the lungs and drop it off in the tissues where it is needed.
Cardiac and skeletal muscles also contain a protein called myoglobin, which binds oxygen more tightly than hemoglobin. Myoglobin provides the heart and skeletal muscles with an oxygen reserve; this comes in handy during short bursts of physical activity.