Arterial H+ regulation during exercise in humans.

Resting arterial H+ concentration ([H+]a) is in the nanomolar range (40±2 nm/L) while its production is in the millimolar range/min, with little variation from subject to subject. To determine the precision with which [H(+)]a is regulated during exercise, [H+]a, PaCO2 and ventilation (V˙(E)) were measured during progressively increasing work rate exercise in 16 normal subjects. (V˙(E)) increased with [H+]a, the latter attributable to PaCO2 increase below the lactic acidosis threshold (LAT) (ΔV˙(E)/Δ[H+]a ≈ 15   L   min(-1)   nanomol(-1)). [H+]a and PaCO2 increased, simultaneously, as work rate was increased below LAT. PaCO2 reversed direction of change between LAT and ventilatory compensation point (VCP). Above LAT, [H+]a increase relative to (V˙(E)) increase was greater than below LAT. PaCO2 decreased above the LAT, while [H+]a continued to increase. Thus the exercise acidosis was converted from respiratory, below, to a metabolic, above the LAT. We conclude that [H+]a is increased and regulated over the full range of exercise, but with less sensitivity above the LAT.