RESPIRATION PHYSIOLOGY: VENTILATION
Compliance

COMPLIANCE (ELASTIC FORCE)

A. Elastic Forces

1. Due to tendency of tissue to resume its original position after an applied force has been removed

Note:  the Chest Wall is naturally larger than the Lungs, but the Chest Wall and Lungs cannot assume their natural positions because (normally) no air can enter the intrapleural space

2. At the resting (muscles relaxed) volume (FRC), the tendency of the isolated lungs is to collapse and the tendency of the isolated chest wall is to expand

Relaxed intrapleural pressure (Ppl) is negative, about -5 cmH2O (Why negative?)

Note:  Negative intrapleural pressure helps keep airways open

3. Rest or relaxed position: position in which the lung and chest wall elastic forces just balance with no active muscle contraction (that is, the forces are balanced because the force developed by the tendency of the lungs to collapse is equal in magnitude and opposite in direction the force developed by the tendency of the chest wall to expand).   (Question:  volume in the lungs at the rest/relaxed position?) If expanded from this position, the system will tend to collapse; if compressed, the lung-chest wall system will tend to expand when the force if removed. Thus, force is necessary to maintain any position other than the relaxed position. The volume change per unit force (pressure) is termed the compliance (C). The inverse of compliance is stiffness

Pressure-Volume Curves of Lungs and Chest Wall
(slope = compliance; note low compliance at extreme volumes)

B. Compliance (elastance)

1. Definition: C = Δ V / Δ P

2. Typical value: normal young adult of average size:

C approx. 0.1 L/cmH2O

Question:  How much will the lungs change in volume for a pressure change of 5 cmH2O?  Note: ΔV = C x ΔP

Note: normal lung compliance is roughly proportional to lung size, so it is frequently scaled to lung size (specific compliance) or to predicted lung size

Note: Compliance is the inverse of stiffness (S): S = 1/C

S = Δ P / Δ V

Question:  Which is better, a high compliance or a low compliance?

3. Hysteresis

Note: Compliance is the slope of the pressure-volume curve. But when plotting lung-chest wall volume vs. pressure, the curve is not the same during inflation and deflation. The dependence of a property on past history is termed hysteresis.

C. Basis of Compliance (or Stiffness)

1. Total system stiffness (St) is the sum of the lung stiffness (SL) and chest wall stiffness (Scw)

St = SL + Scw

Note:  If either the lung tissue or the chest wall is more stiff than normal, total compliance will be less than normal

2. Chest Wall Stiffness: due to elasticity of tissue; normally responsible for one-half of the total system stiffness

3. Lung Stiffness: contributing factors

a. elasticity of lung tissue

b. surface tension

1) due to air-liquid interface in the alveoli (alveoli are lined with a monomolecular layer of liquid because of high permeability)
2) important because of large alveolar surface area
3) leads to

a) high stiffness (low compliance)
b) alveolar instability, with small alveoli emptying into large alveoli; can cause atelectasis -- alveolar collapse
c) movement of interstitial fluid into the alveoli (life threatening!)

4) effects greatly reduced by the pulmonary surfactant dipalmitoyl phosphatidylcholine (DPPC) secreted by lung type 2 alveolar cells

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c. role of surfactant

1) lower surface tension to about 1/3 of plasma (increase lung compliance)
2) lower surface tension more in small alveoli (increase alveolar stability)
3) helps keep alveoli "dry"

Note:  structures surrounding the alveoli tend to support them and so help counteract the action of surface tension in causing alveolar collapse<

Note:  even though pulmonary surfactant lowers surface tension, some alveoli will still collapse unless the lungs are periodically inflated to a larger volume