KEY POINTS

1. Buffers, lungs, and kidneys together maintain acid-base balance.

2. Acid excretion is achieved by proton secretion by tubule epithelial cells and buffering in the tubule fluid.

3. The kidneys generate new bicarbonate and excrete ammonium ion.

4. The proximal tubule has a high capacity for H⁺ secretion and bicarbonate reabsorption.

5. The collecting duct determines the final urine pH.

6. The collecting duct can secrete protons and generate acidic urine.

7. The collecting duct can secrete bicarbonate and generate alkaline urine.