A scale used to measure the degree of acidity or alkalinity of a liquid ranging from 0 (highly acidic) to 14 (highly alkaline) with a pH of 7.0 being neutral. pH is actually a logarithmic scale of the free hydrogen H and hydroxyl OH ions in the solution. The pH of blood is normally maintained very slightly alkaline at 7.35-7.45 aided by the presence of buffers such as bicarbonate and phosphate. The pH is very tightly regulated by the body as part of the normal mechanisms maintaining homeostasis. Disturbances outside the normal are called acidosis and alkalosis and may be respiratory or metabolic in origin.