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Adult hemoglobin (HbA) is a tetramer of two α and two β chains. Each chain holds a heme group with ferrous iron, which is required for oxygen binding. As oxygen binds, hemoglobin shifts from the low-affinity T state toward the higher-affinity R state, producing cooperative binding. Conditions such as lower pH, higher carbon dioxide, increased temperature, and 2,3-BPG reduce oxygen affinity and support oxygen release in tissues. Fetal hemoglobin has higher oxygen affinity than HbA because it binds 2,3-BPG less strongly. Hemoglobin also carries some carbon dioxide, buffers blood through histidine groups, and illustrates how structural changes can alter function, as in sickle hemoglobin.