The acid form of lactate (lactic acid) is produced by various cells of the body. For instance during intense exercise, the muscle cells produce lactic acid, which diffuses out from the muscle cells into the blood where it can be measured. Accumulation of lactic acid occurs when there is a high demand for energy but the supply of oxygen to the cells is limited, such as what happens during intense exercise. It triggers anaerobic metabolism causing lactic acid build up in tissues. Other conditions that trigger rise in lactic acid level are shock, heart failure, and lung disease.