Myoglobin is a protein in muscle tissue that holds oxygen and iron. It is chemically similar to hemoglobin. It is found in most mammals. It is held in particularly high concentrations in animals that spend a great deal of time underwater as it can assist with the storage of oxygen within the body's tissues.
Myoglobin is only released when muscle tissue is damaged. Small amounts are routinely filtered by the kidneys, so in a normal patient, myoglobin should not be detectable in the bloodstream.