Hemoglobin by molecules such as carbon monoxide (for example, from tobacco smoking, exhaust gas, and incomplete combustion in furnaces). CO competes with oxygen at the heme binding site. Hemoglobin's binding affinity for CO is 250 times greater than its affinity for oxygen, meaning that small amounts of CO dramatically reduce hemoglobin's ability to transport oxygen. Since carbon monoxide is a colorless, odorless and tasteless gas, and poses a potentially fatal threat, carbon monoxide detectors have become commercially available to warn of dangerous levels in residences. When hemoglobin combines with CO, it forms a very bright red compound called carboxyhemoglobin, which may
Fetal hemoglobin round to sickle-shaped. These defective red blood cells have a greater tendency to stack on top of one another and block blood vessels. These invariably lead to so-called "painful vaso-occlusive episodes", which are a hallmark of the disease. If fetal hemoglobin remains the predominant form of hemoglobin after birth, the number of painful episodes decreases in patients with sickle-cell disease. Hydroxyurea promotes the production of fetal hemoglobin and can thus be used to treat sickle-cell disease. The fetal hemoglobin's reduction in the severity of the disease comes from its ability to inhibit the formation of hemoglobin aggregates within red blood