Hemoglobin is a protein found in red blood cells that carries oxygen from the lungs to the rest of the body. It is made up of four protein subunits, each containing an iron atom that binds to oxygen molecules. Hemoglobin also helps transport carbon dioxide from the body's tissues back to the lungs, where it can be exhaled. Hemoglobin is essential for life and without it, cells would not be able to get enough oxygen to function properly.