Ammonia is synthesised in tubular cells in several regions of the nephron but especially in the proximal tubule, from the metabolism of amino acids. Ammonia readily diffuses out of the peritubular cells and into the tubular lumen. The ammonia reacts with the hydrogen ion to produce ammonium ion (NH₄⁺). The ammonium ion is one tenth as diffusible as ammonia and hence the ammonium ion is trapped in the lumen whilst the concentration gradient favours further ammonia diffusion into the luminal fluid. In the same manner as for the titratable acid, the excretion of ammonium salt induces the formation of 'new' bicarbonate ions, that are added to the bicarbonate pool.