An experiment was done where a group of subjects exhaled through narrow

tubes into a solution of Bromothymol Blue (BTB), after sustained amounts of

exercise (running 2 flights of stairs). BTB is an acid base indicator that turns

from blue-green to yellow in the presence of an acid.

The subjects had the time it took for them to turn the BTB yellow under three

conditions: Initial (at rest), after each of their 3 runs (done one after

another). The graph that results is below.