Which type of hemoglobin is elevated in patients with chronic hypoxia?

In patients who are experiencing chronic hypoxia, there is often an increase in the production of hemoglobin, specifically hemoglobin S (HbS). Chronic low oxygen levels stimulate the production of erythropoietin, which leads to increased red blood cell production. In certain populations, such as those with sickle cell disease, HbS also arises as a result of a genetic mutation affecting the beta-globin chain of hemoglobin. This variant hemoglobin has a higher affinity for oxygen under specific conditions, but it's less soluble than normal hemoglobin A (HbA) and can precipitate under low-oxygen environments, leading to sickling of red blood cells. While elevated levels of HbF (fetal hemoglobin) can occur in some forms of hemoglobinopathies or thalassemias, and HbA2 might rise in beta-thalassemia, it is specifically HbS that is associated with the chronic adaptation to hypoxic conditions, particularly in the context of sickle cell disease. Thus, HbS is the hemoglobin that is notably elevated in chronic hypoxia scenarios related to hemoglobinopathies.