Hemoglobin

Hemoglobin is an iron-containing protein in red blood cells that transports oxygen from the lungs to tissues and facilitates the carriage of carbon dioxide back to the lungs. It is composed of globin protein chains containing heme groups, each of which can reversibly bind oxygen. Hemoglobin also contributes to blood’s acid–base buffering capacity. Its production is stimulated principally by erythropoietin in response to reduced tissue oxygen availability, with iron, folate, vitamin B12, and other nutrients required for effective erythropoiesis.

Hemoglobin concentration is a routinely measured clinical metric, usually reported as part of a complete blood count alongside hematocrit and red-cell indices. A reduced concentration is termed anemia and may reflect blood loss, nutritional deficiency, inflammation, impaired erythropoietin production, hemolysis, or inherited disorders such as sickle-cell disease. Hemoglobin values are interpreted in relation to age, sex, pregnancy status, altitude, hydration, and clinical context. In research and clinical risk models, hemoglobin may be evaluated with inflammatory, nutritional, renal, and hematologic variables including C-reactive protein (CRP), albumin, white blood cell count, and platelet count. It is distinct from glycated hemoglobin (HbA1c), which reflects the proportion of hemoglobin modified by glucose over the preceding several weeks and is used primarily in assessment of glycemic exposure.