Exercise

Time course of changes in ventilation

Exercise

• At, just immediately just before, the start of exercise, ventilation increases instantly (phase I)
• At moderate levels of exercise, ventilation then increases further with time (phase II), and an equilibrium level of ventilation (phase III) is reached within 3 minutes.
• With heavy exercise, ventilation increases further and reaches equilibrium at a higher level of ventilation
• With severe exercise, ventilation continues to increase

##20050306(03) - "Exercise and ventilation"

Recovery

• After exercise, ventilation falls to resting levels in a few minutes.
• More intense exercise
  --> Oxygen debt greater and lactate level higher
  --> Recovery takes longer

Control of ventilation during exercise

Control of ventilation during exercise is still largely unknown

• Phase I can be in part due to a learned response
  * Can also be due to joint/muscle receptor
• Arterial blood gases are probably not the main factor in the increase of ventilation during exercise.
  * PaO2 is normal
  * PaCO2 is often reduced
  * However, 100% O2 does reduce minute volume for a particular level of oxygen consumption.
• Hyperthermia
  * May have small contribution
• Metabolic acidosis
  * Causes excess ventilation during heavy and severe exercise

Role of training

Training affects

• Owles point
• Cardiac fitness
  --> thus oxygen delivery
  --> thus the aerobic exercise capacity
• Ability of muscles to remove lactate
• (In animal study) ability of liver to remove lactate
• Tolerance to lactate
• Fraction of MBC that can be sustained, via stronger and more enduring respiratory muscles

Training does not affect

• Linear relationship between power generated and oxygen consumed
• Maximal expiratory flow
• Slope of ventilation vs oxygen consumption curve (but Owles point is affected)

Summary

Training improves

• Performance of skeletal muscles and cardiovascular system
• Not respiratory function

Additional notes

Levels of exercise

Moderate exercise

• Below the subject's anaerobic threshold
• Arterial blood lactate is not raised
• Considered as steady state

Heavy exercise

• Above the anaerobic threshold
• Arterial blood lactate is elevated but remains constant
• Considered as steady state

Severe exercise

• Well above the anaerobic threshold
• Arterial blood lactate continues to rise
• Unsteady state