In those circumstances, addition or removal of hydrogen ions results in minimal change to pH, the purpose of the buffer. The buffer sodium bicarbonate, is filtered by the glomerulus and then reabsorbed in the proximal tubule. The CO 2 diffuses into the cell where it again forms carbonic acid in the presence of carbonic anhydrase.
Water is absorbed passively.
Ammonia is produced enzymatically from glutamine and other amino acids, and is secreted in the tubules. Ammonia production is increased by a severe metabolic acidosis to as much as 700 mmol.day -1 . Excretion of waste products Filtration occurs as blood flows through the glomerulus. Such substances are cleared by secretion into the tubule and excreted from the body in the urine. The opposite occurs when fluid overload occurs.