Buffer Systems and Blood pH

Key points

• • A buffer is a mixture of a weak acid and its conjugate base that resists a change in pH when acid or alkali is added.
• • Blood pH is normally held between about 7.35 and 7.45, and the bicarbonate system is the most important buffer in the plasma.
• • The Henderson-Hasselbalch equation links pH to the ratio of bicarbonate to dissolved carbon dioxide, which under normal conditions is kept at about twenty to one in the blood.
• • The bicarbonate buffer is especially effective because it is an open system whose two parts are separately regulated: the lungs blow off carbon dioxide (the acid side) and the kidneys adjust bicarbonate (the base side).
• • Other buffers include haemoglobin and the plasma proteins, and the phosphate system, which matters most inside cells and in the urine.
• • A buffer works best near its pKa; bicarbonate has a pKa of about 6.1 and works well in blood despite this because it is an open system whose two components are constantly regulated and replenished.