BARSTOW, THOMAS J., DAN M. COOPER, ERIC M. SOBEL,
ELLIOT M. LANDAW, AND SAM EPSTEIN.

Am. J. Phvsiol. 259 (Regulatory Integrative Comp. Physiol. 28): Rl63-
Rl?l, 1990.-

The effect of changes in metabolic rate on the
dynamics of CO, exchange among its various compartments in
the human body is not well understood. We examined COz
dynamics in six healthy male subjects using an intravenous
bolus of [ “‘Clbicarbonate. Subjects were studied while resting,
during light exercise [50% of the lactate threshold (LT), 3-4
times resting 0, uptake (VOW)], and during moderate exercise
(95% of the LT, 6 times resting VO,). The sum of three
exponential terms well described the washout of 1’3C02 in exhaled
breath both at rest and during each exercise level despite
substantial increases in metabolic rate accompanying the exercise
studies. Average recovery of “‘C label rose from 67%
during rest to 80% during light and moderate exercise (P <
0.01). The estimate of CO, elimination (ho2) calculated from
the washout parameters and corrected for recovery was in very
good agreement with the VCO~ directly measured simultaneously
breath by breath (r = 0.993, SE for VCO~ = 0.079 l/min).
By use of a three-compartment mammillary model, the quantity
of CO, in the central pool (Q1) doubled from rest to light
exercise (233 & 60 to 479 t 76 mmol, P < 0.01) but did not
change further with moderate exercise (458 & 74 mmol). Rate
constants for exchange between pools and for irreversible loss
from the system tended to increase with metabolic rate, but
there was large variation in the responses. We conclude that
the compartmental dynamics of CO, transport and storage are
very sensitive to changes in metabolic rate induced by exercise.