While some texts use the terms acidosis and acidemia interchangeably, there are subtle differences. The low pH in the arterial blood can be called acidemia; it is caused by a metabolic acidosis that is a direct result of poor perfusion and lactic acid produced by anaerobic metabolism in tissues with inadequate oxygen delivery. Inadequate oxygenation is caused by the low PO2 , the low oxygen-carrying capacity of the blood (Hgb 5 g/dL), and circulatory inadequacy due to the sickling itself and to the vascular disease it produces. The low PCO2 reflects the hyperventilation, which is secondary to the respiratory difficulty, and to the anemia, and is also respiratory compensation for the metabolic acidosis. Note that some texts distinguish the two terms by using acidemia to refer specifically to inborn errors of metabolism that lead to low serum pH.