Human urine is usually acidic because Hydrogen ions are actively secreted into the filtrate. Potassium and sodium exchange generates acidity. Excreted plasma proteins are acidic. The sodium transporter exchanges one hydrogen ion for each sodium ion in peritubular capillaries.

Fresh urine is usually acidic. The pH normally is within the range of 5.5 to 7 with an average of 6.2. The kidneys excrete a variable amount of hydrogen ions into the urine and conserve bicarbonate ions, which are an important buffer of H⁻in the blood. Both of these activities help regulate blood pH. The tubule cell secretes the H ion and the ammonia into the renal filtrate, and two Na ions are reabsorbed in exchange. In the filtrate, the H ion and ammonia form NH₄, which reacts with a chloride ion to form NH₄Cl (ammonium chloride) that is excreted in urine. As the Na ions are returned to the blood in the peritubular capillaries, HCO₃ions follow. Two H ions are excreted in urine, and two Na ions and two HCO₃ions are returned to the blood. As reactions like these take place, the body fluids are prevented from becoming too acidic.