The Henderson-Hasselbalch equation is useful for calculating blood pH, because blood is a buffer solution. In the clinical setting, this equation is usually used to calculate HCO3 from measurements of pH and PaCO2 in arterial blood gases. The amount of metabolic acid accumulating can also be quantitated by using buffer base deviation, a derivative estimate of the metabolic as opposed to the respiratory component. In hypovolemic shock for example, approximately 50% of the metabolic acid accumulation is lactic acid, which disappears as blood flow and oxygen debt are corrected.