The McGill Physiology Virtual Lab

Exercise Physiology Laboratory

 Respiratory contribution
 

During exercise, ventilation may increase 20 times. The increase does not occur because of changes in PO2, PCO2 and [H+]; the major stimuli to ventilation during exercise remain unclear.
  PACO2 = PaCO2 and PACO2 = VCO2/VA
   

During moderate exercise ventilation increases in the exact proportion to VCO2. During severe exercise, a person hyperventilates.
Systemic venous PO2 decreases during exercise, alveolar and systemic PO2 stay at normal levels because VO2 and VA increases in proportion.
During very strenuous exercise PaCO2 decreases, but hyperventilation does not produce an increase in PaO2, but PAO2 increases. In highly trained athletes, PaO2 may decrease.
There is no accumulation of excess [H+] resulting from accumulation of CO2. During strenuous exercise there is an increase in arterial [H+] because of generation and release of lactic acid into the blood; an increase in [H+] could be a stimulus for increased ventilation during severe exercise.


Respiratory responses to exercise

Other factors

  1. reflex input from mechanoreceptors in joints and muscles

  2. an increase in body temperature

  3. stimulation of respiratory motorneurones by supramedullary inputs

  4. an increase in epinephrine concentration

  5. an increase in the plasma [K+]

  6. a conditioned (learned) response mediated neuronal input to the respiratory neurons.

There is an abrupt increase in VE at the beginning of exercise, and an equally abrupt decrease at the end is due to the learned response to exercise.

Relationship between steady-state ventilation and workload.
The coloured area indicates that ventilation increases out of proportion to work loads (hyperventilation) at moderate to heavy loads.
 
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