Hemoglobin failure, or abnormal hemoglobin (such as that of sickle-cell disease). The ability of each hemoglobin molecule to carry oxygen is normally modified by altered blood pH or CO, causing an altered oxygen–hemoglobin dissociation curve. However, it can also be pathologically altered in, e.g., carbon monoxide poisoning. Decrease of hemoglobin, with or without an absolute decrease of red blood cells, leads to symptoms of anemia. Anemia has many different causes, although iron deficiency and its resultant iron deficiency anemia are the most common causes in the Western world. As absence of iron decreases heme synthesis, red blood cells in iron deficiency
Oxygen–hemoglobin dissociation curve remaining three oxygen molecules. How much of that capacity is filled by oxygen at any time is called the oxygen saturation. Expressed as a percentage, the oxygen saturation is the ratio of the amount of oxygen bound to the hemoglobin, to the oxygen-carrying capacity of the hemoglobin. The oxygen-carrying capacity of hemoglobin is determined by the type of hemoglobin present in the blood. The amount of oxygen bound to the hemoglobin at any time is related, in large part, to the partial pressure of oxygen to which the hemoglobin is exposed. In the lungs, at the alveolar–capillary interface, the partial