Hemoglobin is a protein found in red blood cells that is responsible for transporting oxygen from the lungs to the body's tissues and returning carbon dioxide from the tissues back to the lungs. It consists of four subunits, each containing a heme group that binds to oxygen. This essential protein plays a crucial role in maintaining the body's respiratory and metabolic functions. Abnormalities in hemoglobin can lead to various blood disorders, such as anemia and sickle cell disease.