Blood maintains a pH between 7.35 and 7.45. Outside that range, enzymes stop working and cells die. The body keeps pH stable with buffers. A buffer is a solution that resists changes in pH when acids or bases are added. It usually consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. The pair absorbs added hydrogen or hydroxide ions, preventing large swings in pH.

The most important buffer in the blood is the carbonic acid-bicarbonate system. Carbon dioxide dissolves in water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. When acid is added, bicarbonate neutralizes it. When base is added, carbonic acid neutralizes it. The lungs and kidneys regulate the components, keeping the ratio constant. The Henderson-Hasselbalch equation describes the relationship between pH, the acid dissociation constant, and the concentrations of acid and base.

Buffers are used in laboratories, medicine, and industry. They keep enzymes active in biochemical assays. They stabilize drugs in intravenous solutions. They control pH in fermentation and food processing. They are used in cosmetics, cleaning products, and water treatment. A good buffer has a pKa close to the desired pH and a high concentration to resist changes.

Buffer capacity is the amount of acid or base a buffer can absorb before the pH changes significantly. It depends on the concentration of the buffer components and their ratio. A buffer works best when the pH is within one unit of its pKa.