Step by Step
1
Bicarbonate reabsorption
The PCT reabsorbs about 90% of filtered bicarbonate (HCO₃⁻) — preventing this important base from being lost in the urine.
2
H+ secretion
α-intercalated cells in the collecting duct actively secrete H+ against a concentration gradient, creating urine that can be as acidic as pH 4.5. This secreted H+ is buffered by phosphate (forming titratable acid) and by ammonia.
3
Generating brand-new bicarbonate
Glutamine metabolism in the PCT produces ammonium (NH₄⁺, which is excreted) and new bicarbonate (which is added to the blood) — this is a unique capability of the kidneys: they don't just conserve existing bicarbonate, they can generate entirely new bicarbonate to replenish what's been used buffering acid elsewhere in the body.
4
Slow but powerful — and adjustable based on the disorder
Renal acid-base compensation takes days to fully develop, in contrast to the much faster respiratory compensation (which acts within minutes to hours) — but it's ultimately more powerful and complete. In metabolic acidosis, the kidneys increase H+ secretion and NH₄⁺ excretion; in metabolic alkalosis, they decrease H+ secretion and instead excrete HCO₃⁻.
Applied Walkthrough
1
A patient develops chronic metabolic acidosis. Over several days, their kidneys respond by increasing H+ secretion in the collecting duct and increasing ammonium excretion — helping to eliminate excess acid from the body.
2
Simultaneously, glutamine metabolism in the PCT ramps up, generating new bicarbonate that gets added to the blood — directly counteracting the acidosis by replenishing the body's base supply.
3
This renal response takes days to fully develop, much slower than the body's respiratory compensation (which can begin adjusting breathing within minutes) — but the renal mechanism can ultimately correct the imbalance far more completely.
4
If instead this patient had metabolic alkalosis, the kidneys would respond in the opposite direction: reducing H+ secretion and instead excreting excess bicarbonate to help bring pH back down toward normal.
Exam Application
Exams test whether you know the three renal mechanisms for acid-base regulation (bicarbonate reabsorption, H+ secretion, new bicarbonate generation), and whether you understand that renal compensation, while slower than respiratory compensation, is ultimately more powerful.
⚠ Common Trap
The most common trap is assuming the kidneys only reabsorb existing bicarbonate — they can also generate entirely new bicarbonate through glutamine metabolism in the PCT, a distinct and additional mechanism beyond simple reabsorption.
✓ Quick Self-Check
1. What percentage of filtered bicarbonate does the PCT typically reabsorb?
About 90%.
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2. What cells secrete H+ in the collecting duct, and what is the minimum urine pH this can achieve?
α-intercalated cells; minimum urine pH of about 4.5.
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3. How do the kidneys generate brand-new bicarbonate?
Through glutamine metabolism in the PCT, which produces ammonium (excreted) and new bicarbonate (added to blood).
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4. How does renal acid-base compensation compare in speed and power to respiratory compensation?
It's much slower (days versus minutes to hours) but ultimately more powerful and complete.
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5. How do the kidneys respond differently to metabolic acidosis versus metabolic alkalosis?
In acidosis, they increase H+ secretion and NH₄⁺ excretion; in alkalosis, they decrease H+ secretion and excrete HCO₃⁻ instead.
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