Hemoglobin

Hemoglobin is a protein in red blood cells that carries oxygen from the lungs to tissues and returns carbon dioxide for exhalation. It is a tetramer composed of four globin chains bound to four heme groups, each with an iron ion that binds one oxygen molecule reversibly. In the lungs, iron binds O2; in tissues, O2 is released. Hemoglobin also binds CO2, forming carbaminohemoglobin, and helps regulate blood pH.

Oxygen binding is cooperative, yielding a sigmoidal dissociation curve. Affinity decreases with lower pH, higher partial pressure of CO2, higher temperature, and higher levels of 2,3-bisphosphoglycerate (2,3-BPG). This Bohr effect facilitates O2 release in metabolically active tissues. Fetal hemoglobin (HbF, α2γ2) has higher O2 affinity than adult HbA, aiding fetal oxygen uptake; adult HbA predominates, with HbA2 as a minor component.

Hemoglobin synthesis occurs during red cell formation in the bone marrow. It comprises the globin genes HBA1/HBA2 (α chains) and HBB (β chains), with γ chains expressed in fetal life (HbF). After birth, HbF declines as HbA and HbA2 predominate.

Clinical measurements include hemoglobin concentration and oxygen-carrying capacity. Abnormal forms cause disorders such as sickle cell disease (HbS), alpha- or beta-thalassemias, and methemoglobinemia. Hemoglobin abnormalities underpin anemia and polycythemia, while newborn screening and genetic testing assist in diagnosing inherited hemoglobinopathies.