Mechanics of Ventilation

Question 1

General Pressure Concept

Answer 1

• For air to move in and out of the lungs there must be a pressure difference between the alveolar pressure and atmospheric pressure (0 cm H20).
  
  • Alveolar P < Atmospheric P → air goes inside the lungs = negative P breathing
  • Alveolad P > Atmospheric P → air leaves the lungs
  • Positive P ventilation raises P at the nose and mouth so that air can enter the lungs

Question 2

Negative-Pressure Breathing: how alveolar P is made less than atmospheric P

Answer 2

• Muscles of inspiration contract → ⇡volume of alveoli → ⇣alveolar pressure
  
  • Alveoli cannot expand themselves; only expand passively due to an ⇡ transmural P difference (intrapleural P - alveolar P)

Question 3

Negative Intrapleural P

Answer 3

• Pressure in the pleural space is usually subatmospheric because of the relationship between the lungs and the chest wall
• End of expiration: respiratory mm. are relaxed, lungs and chest wall act in opposite directions
  
  • Lungs: inward recoil of alveoli → ⇣volume
  • Chest wall: outward recoil → ⇡volume
  • Intrapleural P = -5 cm H20
• Inspiration: respiratory mm. contract → intrapleural P gets more negative
  
  • Alveoli distend → alveolar P ⇣ to <atm></atm>
  </atm>

Question 4

Breathing Cycle at Rest

Answer 4

• Rest = period when diaphragm is at its equilibrium position (between breaths)
  
  • No air is moving into or out of the lungs
  • Alveolar P = Atmospheric P = 0 cm H2O
  • Intrapleural P is negative = -5 cm H2O
    
    • this is because the opposing forces of the lungs trying to collapse and the chest wall trying to expand creates a negative pressure in the space between them
  • Transmural P = Alveolar P - Intrapleural P = 0 - (-5) = +5 cm H2O

Question 5

Breathing Cycle during Inspiration

Answer 5

• Diaphragm contracts → thoracic volume ⇡ → ⇣P in the lungs → alveolar P< atm P → pressure gradient that drives air into the lungs
• During inspiration intrapleural P becomes even more negative (-8 cm H2O at end) because:
  
  • Thoracic expansion → ⇡elastic recoil of lung pulling more against intrapleural space
  • Airway and alveolar pressures are negative
• At the end of inspiration, alveolar P ≈ atm P → dissipated pressure gradient → cessation of airflow into the lungs

Question 6

Breathing Cycle during Expiration

Answer 6

• Normally a PASSIVE process
• Elastic forces of lungs compress alveoli → alveolor P > atm P → airflow out of the lungs
  
  • Volume expired = V_T
  • Volume remaining in the lungs = FRC
• At the end of expiration, all volumes and pressures return to what they were at rest to restart the cycle

Question 7

Breathing Cycle during Forced Expiration

Answer 7

• Forced expiration makes the pressures in the lungs and airways more positive than in normal expiration
• Contraction of expiratory muscles → ⇡intrapleural P
  
  • as long as transmural pressures across airways and alveoli are still positive, this ⇡intrapleural P will not cause the airways or alveoli to close (collapse)

Question 8

Muscles of Inspiration

Answer 8

• The diaphram***
  
  • Contracts → abdominal contents pushed downward → thorax expands (⇡volume) → ⇣intrathoracic P → air flows into the lungs
• During exercise other muscles can be used since breathing frequency and V_T ⇡
  
  • External intercostal mm.
  • Accessory mm.
    
    • Sternocleidomastoid (SCM) mm.

Question 9

Muscles of Expiration

Answer 9

• Expiration is normally a passive process that occurs due to the elastic recoil of the lungs
• During exercise or on diseases with ⇡airway resistance expiratory mm. may aid the expiratory process:
  
  • internal intercostal mm.
    
    • help pull the ribs downward and inward
  • abdominal mm.
    
    • compress the abdominal cavity
    • push the diaphragm back up

Question 10

Lung Compliance

Answer 10

• Compliance measures the distensibility of a system; how volume changes if pressure changes
• Lung compliance = the change in lung volume for a given change in pressure
  
  • ​It is inversely correlated with the elastic properties of the lungs → greater elastance, lower compliance