To maintain the blood pH at 7.35 to 7.45 there are three primary systems to regulate hydrogen ion concentration in the body: the buffer mechanism first line of defense, respiratory mechanism second line of defense, renal mechanism- third line of defense. The most important buffer systems of the blood include bicarbonate buffer, phosphate buffer, protein buffer and haemoglobin buffer. The lungs function by regulating the concentration of carbonic acid (H2CO3) in the blood. Long term acid base balance is exerted by renal mechanism by maintaining the alkali reserve and by excreting excess acids or bases. Acid base imbalance occurs when the blood pH is shifted out of the normal range 7.35–7.45. An excess of acid in the blood is called acidemia and an excess of base is called alkalemia. Acidosis and alkalosis referred to pathological states that can lead to acidemia or alkalemia. Acidosis and alkalosis are classified in terms of their immediate cause such as, metabolic acidosis, respiratory acidosis, metabolic alkalosis and respiratory alkalosis.