Respiratory System: Pressure-Volume Relationships and Lung Compliance

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What is the forced vital capacity?

The amount of air expired after maximal inspiratory effort

Where is surfactant produced?

In alveolar type I cells and secreted into the alveolus

How does airway resistance behave?

Is increased following bronchial smooth muscle contraction

What happens to dynamic compliance in obstructive disease?

Decreases as respiratory rate and expiratory flow increase

Where is the pressure outside the airway downstream from the equal pressure point?

Less than the driving pressure inside the airway

What does static compliance represent?

The ease with which lungs can be inflated when no airflow is present

What does high compliance in the respiratory system indicate?

Easier inflation of the lungs

In the respiratory system, what is the most likely cause of low compliance?

High elastance

Why are alveoli more compliant at low volumes in the respiratory system?

Decreased elastic recoil

How does lung compliance change with increased breathing frequencies in obstructive diseases?

Increases dramatically

Which statement is true about variations in compliance in the respiratory system?

Lungs are more difficult to stretch with decreased compliance.

Which condition results in decreased lung volume due to excess fibrous tissue?


What is the major impact of emphysema on lung compliance?

Increases distensibility

How is static compliance measured in the respiratory system?

$Cstat = Vt/(Pplat - PEEP)$

Which term describes the tendency of tissues to oppose stretch and increase at higher lung volumes?


What is the primary function of pulmonary surfactant in the respiratory system?

Stabilize alveoli and prevent collapse

What is the significance of the functional residual capacity (FRC)?

It is the volume where outward recoil of the chest wall equals the inward recoil of the lungs

What happens during dynamic compression of small airways?

Collapsing of small airways after the equal pressure point

In emphysema, why is there great difficulty in achieving high airflow rates?

Less alveolar elastic recoil and less traction to oppose dynamic compression

What does the term 'equal pressure point' refer to during forced expiration?

When driving pressure equals surrounding peribronchial pressure

How does airway resistance vary with lung volume?

Airway resistance increases at higher lung volumes

What happens to small airways during forced expiration with an open glottis?

They undergo dynamic compression and collapse

Why is negative pressure breathing a concern regarding upper airway obstruction?

It can obstruct airflow by pulling upper airway inward

How does distribution of airway resistance differ in upper airways compared to lower airways?

(35-50%) in both regions, but lower resistance in parallel small airways

Study Notes

Pressure-Volume Relationships in the Respiratory System

  • Compliance (C) measures the ease of lung distension: C = ΔV/ΔP
    • High compliance indicates loss of elastance; low compliance indicates high elastance
    • Compliance is affected by elastic tissue recoil and surface tension recoil
  • Non-linear aspects of compliance:
    • Compliance decreases at high lung volumes (alveoli become less compliant)
    • Compliance increases at low lung volumes (alveoli become more compliant)
    • At FRC (functional residual capacity), the compliance curve becomes linear

Types of Compliance

  • Static compliance (measured during no airflow)
  • Dynamic compliance (measured during airflow, during inspiration and expiration)
  • Hysteresis: difference between inspiration and expiration curves

Variations in Compliance

  • Increased compliance: lungs are easily stretched (e.g., obstructive disease)
  • Decreased compliance: lungs are difficult to stretch (e.g., restrictive disease)
  • Units of compliance: L/cm H2O or ml/cm H2O; normal value: 0.2 L/cm H2O

Histology of Abnormal Compliance

  • Conditions affecting compliance:
    • Fibrosis: decreased compliance due to excess fibrous tissue
    • Emphysema: increased compliance due to destruction of elastic septa

Pressures Related to Compliance

  • Static compliance: measured in the absence of gas flow (Cstat = Vt/(Pplat - PEEP))
  • Dynamic compliance: measured in the presence of gas flow (Cdyn = Vt/(Ppeak - PEEP))

Clinical Evaluation of Lung Compliance - Specific Compliance

  • Specific compliance: compliance relative to lung volume (C/FRC)
  • Used to standardize compliance for lung size
  • Example: calculating specific compliance for two lungs and one lung


  • Tendency to oppose stretch
  • Elastic recoil of alveolar walls increases at higher lung volumes
  • Increases elastance compresses alveolar gas, raising pressure above atmospheric pressure (during exhalation)

Elastic Recoil of Lungs - Inward

  • Recoil due to elastic tissue follows Hooke's law (F = k * x)
  • Elastic tissue in lungs: F = ΔIPP, x = Δlung volume

Surface Tension

  • Accounts for 2/3 of total elastic recoil forces in normal lungs
  • Surface tension forces attempt to collapse lungs
  • La Place's law: P = 4T/r (for spherical bubbles) or P = 2T/r (for alveoli with one air-liquid interface)
  • Surface tension affects alveolar stability


  • Surface active agent at the fluid surface of the inner lining of alveoli
  • Decreases surface tension
  • Derived from type II alveolar epithelial cells
  • Composition: phospholipids (80%), cholesterol (10%), and surfactant proteins (10%)
  • Surfactant:
    • Increases alveolar compliance
    • Prevents atelectasis
    • Aids in keeping alveoli dry
  • Surfactant deficiency:
    • Conditions: atelectasis, failure of normal lung expansion in premature neonates, pulmonary edema, respiratory distress syndrome
    • Causes: immature lung, hypoxia, interrupted blood supply

Alveolar Interdependence

  • Mechanical interdependence stabilizes alveoli and opposes collapse
  • Elastic septa and capillaries


  • Caused by respiratory changes during anesthesia
  • Decreased FRC, compliance, and increased resistance
  • Prevention methods: positive end-expiratory pressure, recruitment maneuvers, minimizing gas resorption, maintaining muscle tone

Learn about the pressure-volume relationships in the respiratory system, focusing on lung compliance. Understand how compliance affects the distensibility of the lungs, and the impact of elastance on high and low compliance levels.

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