Classes of Exercise There are two classes of exercise: • aerobic producing carbonic acid • anaerobic producing lactic acid The diagram below shows how ventilation and blood gases are related to the exercise work rate.
Respiratory Variables
Aerobic
Anaerobic
Pco2
V
ila ent
tion
Po2 , [H+] & lactate
Exercise Work Rate As you can see, for aerobic exercise ventilation is linearly related to the increase in work rate, but above what some call the anaerobic threshold ventilation increases out of proportion to the increase in work rate. This latter increase means that ventilation exceeds that required for gas exchange and so PCO2 falls and PO2 rises. Aerobic Exercise: Ventilation increases with workload but PCO2 and PO2 remain roughly constant. Most of the drive to breathe is from neural sources, with the chemoreflexes acting to fine-tune the system. Anaerobic Exercise: Ventilation is driven by factors such as: • increased central command and muscle afferent feedback as the muscles tire. • lactic acidosis via the peripheral chemoreflex (the central chemoreflex is behind the bloodbrain barrier to polar solutes). • increased body temperature. • increased catecholamines.
Exercise Page 3