Distribution of Ventilation in Normal lung

• • alveoli at the bottom are smaller so during ventilation they have the capacity to expand more compared to the bigger alveoli at the top
• V/Q Ventilation Summary:

  • Pleural pressure becomes less negative down upright lung
  • In the self-ventilating adult
  • Apices (non-dependent regions / upper lung) have greatest initial volume
  • Bases can exhibit greatest volume change - as they have the capacity to open more
  • Dependent (lower lung) regions more compliant
  • Optimal ventilation is in the lower third of the lung
  • Ventilation influenced by differences in regional mechanics & airflow resistance
• • In the self-ventilating adult the dependent part of the lung will remain preferentially (better) ventilated in ANY position
  • Dependent part of lung just means the lowest part in relation to gravity
• look at photo:

  • In upright position the dependent lung is preferentially ventilated
  • When upside down the dependent lung is preferentially ventilated
  • In right side lying the dependent lung is preferentially ventilated
  • In left side lying the dependent lung is preferentially ventilated

  
• • if in side lying, the lung on the bottom is the dependent lung
  • this means the lung on top has more negative pleural pressure - alveoli wide open
  • so the bottom lung has more positive pleural pressure, so have a greater capacity to expand, improving ventilation
  • (dependent aspect of lung always has greater capacity to expand)
• • Distribution of ventilation is affected by the pleural pressure gradient down the upright lung
  • chest wall wants to expand, elastic nature of lungs want to contract, so the opposition between the 2 creates pleural pressure
  • pleural pressure is more negative at the top of lung (bigger alveoli) and more positive at the bottom of the lung (smaller alveoli)
  • as its more negative at the top, it pulls the alveoli open as theres more opposing forces between the lung and chest wall
  • as its more positive at the bottom, the alveoli are smaller

  
• • pleura coat the lungs and the inside of the chest wall
  • the space in between the 2 pleura is the pleura space which has pleural fluid