12. Flow Volume Loops

• flow = velocity x area
• there is a smaller cross sectional area in large areas, and increased velocity
• bernoullis principle = increasing gas velocity causes decreasing pressure

  • increased gas velocity in large airways and lower pressure
• reynolds number = (2 x radius x density x velocity) / viscosity

  • determines whether flow is laminar or turbulent
• increased turbulence in larger airways

  • pressure falls more quickly due to turbulent flow
• airway resistance is lowest at the greatest lung volume

  • max flow can be generated
• presure falls from alveoli to trachea
• transmural pressure = pressure in - pressure out

  • in alveoli = (elastic recoil pressure + pleural pressure) - pleural pressure
  • bigger elastic recoil = bigger transmural pressure
  • elastic recoil has nothing to do with respiratory muscles - expiratory muscles cannot be trained to increase respiratory flow
  • in airway = airway pressure - pleural pressure
• during a forced expiration a positive pleural pressure will be produced
• at the end of inspiration pleural pressure will be more negative

  • barometric pressure is always 0
  • when inspiratory muscles relax, elastic recoil takes over and generates a small positive alveolar pressure
  • produces gradient of flow
• at every point from the alveoli out to the air there is positive pressure inside the respiratory system

  • outside pleural pressure is always negative
  • creates positive distending pressure so airways are kept open and don't collapse
• by forcefully expiring and creating a positive pleural pressure it allows for a greater alveolar pressure due to elastic recoil
• if pressure outside is positive, there will be points where the pressure inside and outside is equal (EPP)

  • determined by how large elastic recoil is and the pleural pressure
• collapsing force is not reached until high up in the airways - these won't collapse as they are supported by cartilaginous rings
• location of EPP is determined by elastic recoil and fall in pressure along the airways

  • location of EPP will move to smaller airways at forced expiration from below TLC. Airway collapse
• elastic recoil is greatest at total lung capacity
• surfactant is least effective at total lung capacity
• able to generate the largest positive pleural pressure at total lung capacity
• maximum expiratory flow occurs at total lung capacity