A human red blood cell’s hemoglobin has four parts, called subunits. In normal adult hemoglobin, two alpha-globin chains and two beta-globin chains join to form one hemoglobin molecule.
Each subunit contains a heme group, an iron-containing structure that can bind one oxygen molecule. Because hemoglobin has four subunits, a single molecule can carry up to four oxygen molecules from the lungs to body tissues. It also helps transport some carbon dioxide back toward the lungs.
Red blood cells are specialized for this job: they lack a nucleus and most other organelles, leaving more room for hemoglobin. A typical red blood cell contains hundreds of millions of hemoglobin molecules and circulates for roughly 120 days before being removed by the spleen and liver.
The four-part structure is sometimes confused with the four basic components of blood—red cells, white cells, platelets, and plasma. Those are parts of blood, whereas the four subunits belong to each hemoglobin molecule.