During the exercise period (10-15min) the blood lactic acid concentration increases to about 13.2 mmol/dL (same units as on graph) as the individual is having problems keeping up their aerobic respiration. After 15min, they stop exercising and the lactic acid concentration starts to return to normal as their body is able to take in enough oxygen and catches up with the excess lactic acid, metabolizing it into CO2 and H2O. The period between 15-20 min shows the fastest reduction in concentration.
