HHb

HHb, short for deoxyhemoglobin, is the form of hemoglobin in which the heme iron is not bound to oxygen. It contains iron in the ferrous (Fe2+) state and is the predominant form of hemoglobin in venous blood after oxygen has been released to the tissues. The complementary form is oxyhemoglobin (HbO2), in which iron binds one molecule of O2.

Molecular state and function: In deoxygenated hemoglobin, the protein adopts a conformational state that lowers its affinity for oxygen, enabling efficient release of oxygen in tissues. HHb participates in the transport of carbon dioxide and in acid–base balance as part of the broader function of hemoglobin in gas exchange.

Physiological significance: The deoxygenation of blood affects interactions with other molecules; deoxygenated hemoglobin can bind hydrogen ions and carbon dioxide more readily than oxyhemoglobin, an effect related to the Bohr and Haldane effects. These properties help optimize oxygen delivery to tissues and CO2 removal from the body.

Color and measurement: HHb is a darker red color compared with oxyhemoglobin. Its different optical properties are exploited in spectrophotometric methods and noninvasive devices such as pulse oximeters and near-infrared spectroscopy to estimate tissue oxygenation and hemoglobin states.

Clinical relevance: The balance between HHb and HbO2 reflects tissue oxygen delivery and extraction. Abnormal HHb levels can indicate hypoxia, anemia, or circulatory problems, and are used in clinical assessments of respiratory and metabolic status.

See also: Hemoglobin, Oxyhemoglobin, Deoxyhemoglobin.