Ventilation

The mechanics of ventilation

inspiration (Pressure changes during quiet breathing)

• Diaphragm and intercostal muscle contraction

→ Diaphragm flattens and rib cage expands

→ increases volume in the thoracic cavity

These events stretch the pleural membranes

• That increase the volume of lungs (the alveoli)

• decreases alveolar pressure

P alveolar < P atmospheric

Air flows into lungs from high to low pressure

• This is called inspiration

Expiration (pressure changes during quiet breathing)

• Diaphragm and intercostal muscle relaxation

→ Diaphragm becomes dome shaped again and rib cage returns to original position

→ decreases thoracic cavity volume

Elastic recoil of pleural membranes

• decreases volume of the lungs (the alveoli)

• Increases alveolar pressure

P alveolar > P atmospheric

Air flows out of lungs from high to low pressure

• this is called expiration

• this is passive

Compliance

The ability to stretch

High compliance

• Stretches easily

• A feature of healthy lungs

Low compliance

• Requires more force to stretch

• Caused by restrictive or fibrotic lung diseases that thicken/scar alveolar membranes and inadequate surfactant production (lubricate and minimize fluid surface tension in lungs)

→ inhalation becomes more effortful

Elasticity

The ability of lung tissue to return to its resting volume when stretching force is released

High elasticity

• Returns to baseline volume easily

• A feature of healthy lungs

Low elasticity

• does not return to baseline volume

• Emphysema: lungs have lost elasticity due to long term smoking

→ exhalation becomes extremely effortful

1. Inspiration

2. Expiration

3. Expiration

4. Inspiration

5. Expiration

6. Inspiration