This chapter discusses regulation of acid-base balance, which is a remarkable achievement because normal metabolism poses a constant threat to it. Kidneys eliminate acid from the body by secreting hydrogen ions into the urinay tubules. A buffer usually consists of a mixture of a weak acid and its salt. Plasma has a significant quantity of proteins which contribute to the buffering capacity of blood. Deoxygenated haemoglobin is a better buffer than oxygenated haemoglobin. Metabolism produces acidic by products. In some abnormal situations respiratory depression or pathological hyperventilation may lead to an acid-base disturbance called respiratory acidosis or respiratory alkalosis respectively. Distal nephron includes the late distal tubule, collecting tubule and collecting duct. Renal ammonia synthesis is also increased possibly through an increase in glutaminase activity. Metabolism produces carbon dioxide, which is freely diffusible across cell membranes. Disturbances of acid-base balance are conventionally divided into acidosis and alkalosis.