AIRWAY RESISTANCE

AIRWAY RESISTANCE

• Airway resistance not seen with small airways – Due to its very large total cross-sectional area.
• Large-sized blood vessels too wider to offer airway resistance.
• Airway resistance seen mainly in medium-sized (2-4-mm  diameter) – Major site of airway resistance.

Determinants of airway resistance:

• Major physical determinants:

1. Radius of airway:

• Radius has very marked effect on resistance.
• If radius is halved, resistance increases sixteen-fold.
• This is according to Poiseuille’s equation.

2. Nature & velocity of flow:

• Two types:
  • Turbulence flow.
  • Laminar flow.

Factors determining laminar or turbulent flow is:

• Reynolds number – Quantifies ratio of inertial to viscous forces.
• Defined by,
  • Radius (r)
  • Density (d)
  • Viscosity (η)
  • Gas velocity (V).

Interpretation:

• Reynolds number >2000 – Turbulence occurs.

• Reynolds number < 2000 – Laminar flow occurs.

• For laminar flow, airway resistance is directly proportional pressure gradient & flow.
• For turbulence flow, airway resistance is approximately proportional to pressure gradient & flow & requires larger pressure gradient.

• Laminar airflow in small airways – Due to low velocity & smaller radius.
• During resting breathing, flow is turbulent in trachea & laminar in smaller peripheral airways.

2. Density of air:

• Increased airway resistance, if inspired air is denser.

3. During respiratory mechanisms:

• Airway resistance is lower during inspiration.
• Airway resistance is higher during expiration (forced expiration) – Due to dynamic compression of airway.
• In forced expiration, expiration stops in later stages due to complete collapse of smaller airways.

4. Lung volume:

• Higher lung volumes – Lower airway resistance.
• Lower lung volumes – Higher airway resistance.

Don’t Forget to Solve all the previous Year Question asked on AIRWAY RESISTANCE
Click Here to Start Quiz