Acid–Base Regulation by the Kidney - Anatomy and Physiology
- What are the two main urinary buffers that let the kidney excrete H+?
- Phosphate (measured as titratable acid) and ammonia, which traps H+ as ammonium (NH4+).
- Which nephron segment reabsorbs most of the filtered bicarbonate?
- The proximal convoluted tubule, which reclaims roughly 80 to 90 percent of filtered bicarbonate.
- Which apical transporter secretes most H+ into the lumen of the proximal tubule?
- The Na+/H+ exchanger (NHE3), which uses the Na+ gradient to move H+ into the tubular fluid.
- Which adrenal hormone, besides promoting Na+ reabsorption, stimulates H+ secretion by type A intercalated cells?
- Aldosterone. This is why excess aldosterone can cause metabolic alkalosis.
- Which collecting duct cells secrete bicarbonate into the urine during alkalosis?
- Type B (beta) intercalated cells, through an apical Cl-/HCO3- exchanger (pendrin).
- Which urinary buffer system increases the most during chronic metabolic acidosis?
- Ammonium excretion. Ammoniagenesis can rise severalfold, while phosphate buffering is limited by the filtered phosphate load.
- A patient takes acetazolamide, a carbonic anhydrase inhibitor. Which acid-base change results?
- Metabolic acidosis. Less filtered bicarbonate is reclaimed in the proximal tubule, so bicarbonate is lost in the urine.
- How is filtered bicarbonate reabsorbed in the proximal tubule?
- Indirectly: secreted H+ combines with it to form CO2 and water, CO2 enters the cell, and bicarbonate is regenerated and sent to the blood.
- When the kidney excretes one H+ bound to a urinary buffer, what does it add to the blood?
- One new bicarbonate ion, which replaces bicarbonate used up buffering fixed acids.
- What is the approximate minimum pH the kidney can produce in urine?
- About 4.5. Because free H+ is limited this way, most acid must be excreted bound to buffers.
- From which amino acid do proximal tubule cells generate ammonium?
- Glutamine. Its metabolism yields ammonium for excretion and new bicarbonate for the blood.
- Which collecting duct cells secrete H+ into the urine during acidosis?
- Type A (alpha) intercalated cells, using apical H+-ATPase and H+/K+-ATPase pumps.