Respiratory System 2: Ventilation and Airway Resistance

VENTILATION AND AIRWAY RESISTANCE


 Thin layer of pleural sac between
lungs and chest wall.
 Both the lungs and the chest wall both
have a degree of elastic recoil in
opposite directions.
 Volume of air in lungs is at functional
residual capacity (equal balance of
elasticity between lungs and chest
wall, therefore, intra-alveolar pressure
= 0mmHg).
 Intrapleural pressure is slightly
negative due to opposing forces of
lungs and chest wall.
 Changes in alveolar pressure determine direction of air flow.

Boyle’s Law – as the volume in the lungs increases, pressure decreases
(Inversely related).

PV = k
Inspiration

 Diaphragm flattens and moves down.
 Contraction of external intercostal muscles – ribs move up and out.
 Increases volume of thoracic cavity.
 Decrease in intra-alveolar pressure causes air flow into lungs.

Expiration

 During quiet breathing, expiration is passive. Lungs recoil,
decreasing lung volume, causing alveolar pressure to increase
greater than atmospheric pressure.
 Forceful expiration, expiratory (internal intercostal) muscles
contract – rapid decrease in lung volume.

Pneumothorax – pleural cavity is compromised – air in thorax and you
get a medistinal shift towards the opposite side of the collapsed lung.

At rest we use 5% of our O2 consumption (increases to 30% during
exercise/stress).

Airway Resistance
 Q = changeP/R
 Major sites of resistance are upper airways and medium-sized
bronchi.
 Increase in airway resistance – gas flow will decrease.
 Resistance is important as a determinant of flow of gas within
airways.

Poiseuille’s Law  Resistance of airway is very sensitive to
radius of airway.
 Small changes to radius lead to large
changes in airway resistance.
R - Resistance of tube, L - length
η – Viscosity, r - radius