Hemoglobin is a protein in red blood cells that transports oxygen and carbon dioxide throughout the body. There are three main types of hemoglobin in humans: hemoglobin A, hemoglobin F, and hemoglobin A2. Hemoglobin has a structure of four subunits, each containing a globular protein chain and a heme group with iron. The structure allows hemoglobin to reversibly bind oxygen in the lungs and release it in tissues through a conformational change between tense and relaxed states. Diseases can arise from structural variants that decrease stability or from disorders in globin chain synthesis.