Hemoglobin is a protein found in red blood cells that is responsible for transporting oxygen from the lungs to various tissues in the body. Each hemoglobin molecule can carry up to four oxygen molecules. However, it's important to note that hemoglobin does not carry individual oxygen atoms; it binds to oxygen molecules (O2) and transports them. The iron atoms in the heme groups of hemoglobin are responsible for binding with the oxygen molecules.