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Usmle Step 1 Review

Lung and Chest Wall

1

Elastic Recoil of Lung and Chest Wall

• Definition:

  Elastic recoil is the tendency of a structure to return to its original shape after being stretched.

• In the respiratory system:

  • Lungs: Tend to collapse inward because of elastic fibers and surface tension.
  • Chest wall: Tends to spring outward.

• Balance at FRC (Functional Residual Capacity):

  • At the end of a normal exhalation, the inward pull of lungs = outward pull of chest wall.
  • Airway & alveolar pressure = atmospheric pressure → called zero pressure point.
  • Intrapleural pressure is negative → prevents lung collapse (atelectasis).
  • Pulmonary vascular resistance (PVR) is lowest here.

Tip: Think of a balloon (lung) inside a box (chest wall). The balloon wants to shrink, the box wants to expand. At FRC, the forces are balanced.

2

Compliance

Definition: How easily the lungs and chest wall expand when pressure is applied.

Compliance = ΔV / ΔP (Change in volume / change in pressure)

Factors affecting compliance:

↑ Compliance (easier to inflate):

• • Emphysema (loss of elastic tissue)
• • Aging (lungs lose elastic fibers)

↓ Compliance (harder to inflate):

• • Pulmonary fibrosis
• • Pneumonia
• • ARDS (Acute Respiratory Distress Syndrome)
• • Pulmonary edema

3

Hysteresis

• Definition:

  The lung follows a different pressure-volume curve during inflation versus deflation.

• Why:

  During inflation, surface tension in alveoli must be overcome to open them. Deflation is easier because the alveoli are already open.

• Clinical relevance:

  Explains why surfactant is critical—it reduces surface tension, making inflation easier.

Summary

• Lungs = inward pull
• Chest wall = outward pull
• FRC = balance point

• Compliance = elasticity
• Hysteresis = surf. tension effect

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Medical Physiology Review