Distal Tubule H⁺ Pump & Renal Tubular Acidosis (2D)

Distal Nephron Controls

H⁺-ATPase pump activity 90%

Blood acid load 30%

Live Readouts

Steady state

Urine pH (lumen)7.10

Achievable minimum4.40

Blood pH (systemic)7.40

How it works

The α-intercalated cell's apical H⁺-ATPase actively pumps protons out of the cell (near-neutral, tightly regulated intracellular pH ≈ 7.10) into the tubule lumen, building a luminal H⁺ concentration far above the cell's own — active transport, since it moves H⁺ against its concentration gradient using ATP, not down it like passive diffusion:

d[H+]lumen/dt = Vmax(pump, acid load) × (1 − [H+]lumen / [H+]ceiling)
[H+]ceiling = [H+]cell × 10^(3.0 × pump activity)

As the gradient steepens, the pump's net transport rate genuinely declines toward zero — every additional proton costs more work to push against a taller wall of concentration difference, exactly like a real ATPase's kinetics saturating near its thermodynamic limit. The ceiling that rate asymptotes toward is set entirely by pump activity (each 10% of full pump strength buys about 0.3 pH units of achievable gradient); blood acid load only changes how fast the pump approaches that ceiling, never how low it can go.

In distal (Type 1) renal tubular acidosis, the H⁺-ATPase itself is defective, so its ceiling sits high — urine pH plateaus around 6–7 no matter how much acid the blood carries, because the pump simply cannot generate a steeper gradient. That is the classic diagnostic mismatch: an inappropriately alkaline urine during systemic acidemia. A healthy pump reaches urine pH near 4.4–5.0 under the same acid load.

Pump activity — fraction of functional apical H⁺-ATPase; sets the ceiling (minimum achievable urine pH). This is the primary distal RTA defect.
Blood acid load — systemic acid production driving H⁺ delivery to the pump; sets how fast urine pH falls toward whatever ceiling the pump can reach, not how far.

Free H⁺ (lumen) H⁺-ATPase pump (active transport) Cell interior (pH ≈ 7.10)