17 Lung Mechanics

Question 1

Describe the mechanics of ventilation in COPD and restrictive lungs

Answer 1

COPD: function at higher volume with more compliant lungs
Restrictive: function at lower volume with less compliant lungs

Question 2

Define compliance and elastance

Answer 2

Compliance: the tendency to distort under pressure (high V/P)

Elastance: the tendency of the alveoli to recoil to its original volume (high P/V)

Question 3

Describe why is there lower ventilation at the apex of the lungs

Answer 3

Due to gravity, there is:

• larger alveoli
• more negative intrapleural pressure
• requires greater transmural pressure to expand the alveoli

Question 4

Explain why is there more perfusion to the base of the lungs

Answer 4

Due to gravity:

• more recruitment of blood vessels
• less resistance
• increased flow

Question 5

What is the effect of ventilating fluid filled lungs

Answer 5

Inflation and deflation require less pressure in fluid-filled lungs due to elimination of the air-water interface

Question 6

State the function of surfactant

Answer 6

1. Reduces surface tension to make alveoli more compliant
2. Prevents collapse of smaller alveoli
3. Reduces work of breathing

Question 7

Describe the relationship between resistance and airway generations

Answer 7

Resistance initially increases from generation 1 to 4, subsequently resistance decreases exponentially

Question 8

Describe the relationship between lung volume and resistance/conductance

Answer 8

As the lung volume increases, there is (non-linear) reduction in resistance and (linear) increase in conductance

This indicates when you’re at lung capacity, there is least resistance whereas when you’re at residual volume, you have high airway resistance

Question 9

Why is the airway supported by cartilage

Answer 9

During forced expiration, there is an increase in transmural pressure that will narrow the airway. Cartilage is to prevent airway collapsing