Pressure Difference Generation and Boyle's Law Quiz

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24 Questions

What happens to the chest wall recoil at lung volumes above 70% TLC?

It becomes inward.

How does the integrity disturbance like pneumothorax affect alveoli?

Alveoli tend to collapse.

What happens to airway resistance with increasing lung volume?

Airway resistance decreases.

What is the Equal Pressure Point during forced expiration?

The point where airway pressure equals surrounding peribronchial pressure.

Why does emphysema lead to difficulty in achieving high airflow rates?

Decreased alveolar elastic recoil.

What opposes dynamic compression of airways during forced expiration?

Alveolar elastic recoil pressure.

What is the main role of pulmonary surfactant in the lungs?

Decrease surface tension

According to La Place's law, what is the relationship between pressure (P), surface tension (T), and radius (r) in a spherical bubble?

P is directly proportional to T and inversely proportional to r

How does pulmonary surfactant affect alveolar stability?

Decreases surface tension in smaller alveoli

What happens to surface tension during inspiration in the lungs?

Increases due to higher surfactant concentration

How does surfactant deficiency contribute to respiratory distress syndrome?

Results in atelectasis and hypoxemia

What is the effect of surfactant on alveolar compliance?

Increases compliance by reducing surface tension

In the context of alveolar interdependence, what stabilizes alveoli and prevents collapse?

Mechanical interdependence and elastic septa

What would happen if two nearby alveoli with different radii have the same surface tension according to La Place's law?

The alveolus with the smaller radius would generate higher internal pressure

What is the primary function of pulmonary surfactant on alveolar walls?

Decrease surface tension.

How does surfactant contribute to maintaining lung compliance?

By decreasing surface tension.

What is the main function of pulmonary surfactant?

Decrease surface tension in the alveoli

In the context of alveolar stability, what happens to alveoli during inspiration?

Alveoli expand passively in response to increased transmural pressure

What is the relationship between lung volume and airway pressure during quiet expiration?

Increased lung volume increases airway pressure

How does a negative intrapleural pressure affect alveolar stability at functional residual capacity (FRC)?

Assists in keeping the lungs expanded

What is the impact of surface tension on alveolar stability?

Surface tension prevents alveolar collapse

According to Laplace's law, how does surface tension affect small alveoli compared to large alveoli?

Surface tension increases in small alveoli

What happens to airway pressure when lung volume decreases during expiration?

Airway pressure increases above atmospheric pressure

How does negative pressure breathing differ from positive pressure breathing?

Negative pressure breathing involves air moving into the lungs due to atmospheric pressure exceeding airway pressure

Study Notes

Boyle's Law

  • States that pressure is inversely proportional to volume at a constant temperature
  • Mathematical expression: P1V1 = P2V2
  • Examples:
    • Pre-inspiration: V = 3L, P = 4 mmHg
    • Inspiration: V = 4L, P = 3 mmHg
    • Active Expiration: V = 2L, P = 6 mmHg

Inspiration and Expiration

  • Inspiration is an active process
  • Lungs passively expand
  • Air will move from a high-pressure area to a low-pressure area (pressure gradient)
  • No pressure difference = no movement
  • Pressure difference must be sufficient to overcome the resistance to airflow offered by the conducting airways

Pressure Gradient and Air Movement

  • Airway pressure equals ambient air pressure (barometric pressure) down to alveoli at end-expiration
  • Alveolar pressure is considered 0 cm H2O pressure at end-expiration
  • Atmospheric pressure (760 mmHg) is referred to as 0 cm H2O
  • Negative pressure breathing: air moves into lungs as a result of airway pressure decreasing below atmospheric pressure
  • Positive pressure breathing: air moves into lungs as a result of atmospheric pressure exceeding airway pressure

Passive Expansion of Alveoli

  • A pressure difference between the atmosphere and alveoli must be established to move air in/out of alveoli
  • During inspiration, alveoli expand passively in response to an increased transmural pressure difference
  • Increased volume, decreased pressure
  • During normal quiet expiration, the elastic recoil of the alveoli returns them to their original volume
  • Decreased volume, increased pressure

Air Movement Into and Out of Lungs

  • Air moves into lungs when alveolar pressure is lower than atmospheric pressure
  • Increasing lung volume lowers airway pressure below atmospheric pressure, creating a pressure gradient that allows airflow into the lungs
  • Air moves out of lungs when alveolar pressure is sufficiently higher than atmospheric pressure
  • Decreasing lung volume increases airway pressure above atmospheric pressure, creating a pressure gradient that allows airflow out of the lungs

Functional Residual Capacity (FRC)

  • Volume of gas in the lungs at the end of a normal tidal expiration, when no respiratory muscles are actively contracting
  • Occurs at the end of passive expiration
  • Determined by the balance of two forces: inward recoil of the lungs and outward recoil of the chest wall

Intrapleural Pressure

  • Pressure within the pleural cavity
  • Normally negative (subatmospheric) at FRC (relaxation volume)
  • Lungs have a tendency to collapse due to inward elastic recoil of distended alveoli
  • Chest wall has a tendency to expand outward due to elastic recoil of flexible thorax
  • Negative IPP (-3 to -5 cm H2O) is caused by the mechanical interaction between lung and chest wall
  • At end-expiration, lung and chest are at an equilibrium of rest
  • At end-inspiration of a normal tidal volume, IPP becomes more negative (-8 cm H2O)

Compliance of the Lung

  • Measure of the lung's distensibility
  • The ease with which lungs can be inflated
  • Compliance is volume-dependent: alveoli are more compliant at low volumes and less compliant at high volumes
  • Measured by the slope of the pressure-volume curve
  • It can be affected by diseases such as fibrosis and emphysema

Types of Compliance

  • Static compliance: measured in the absence of gas flow
  • Dynamic compliance: measured in the presence of gas flow
  • Specific compliance: a measure of distensibility of lung as it relates to lung volume

Clinical Evaluation of Lung Compliance

  • Specific compliance is used to denote compliance with respect to original lung volume
  • Compliance decreases with decreased lung volume; specific compliance does not
  • Standardizes for overall lung size/volume


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

Elastic Recoil of Lungs

  • Due to elastic tissue and surface tension
  • Elastic tissue recoil: property of matter that causes it to return to its original position or configuration after having been displaced (stretched)
  • Surface tension: accounts for 2/3 of total elastic recoil forces in the normal lung
  • Surface tension forces occur at any gas-liquid interface
  • Generated by cohesive forces between water molecules and hydrogen bonding when unopposed at the surface of a liquid


  • Surface active agent that decreases surface tension
  • Derived from type II alveolar epithelial cells (type II pneumocytes)
  • Major role is stabilizing alveoli by equalizing pressure inside smaller alveoli
  • Reduces surface tension recoil forces and the muscular efforts needed to ventilate
  • Aids in keeping the alveoli dry


  • Caused by respiratory changes during anesthesia
  • FRC decreases
  • Compliance decreases
  • Resistance increases
  • Develops in 90% of anesthetized lungs

Interaction of Lung and Chest Wall

  • Inward elastic recoil of lung normally opposes the outward recoil of the chest wall
  • Chest wall recoil becomes inward at 70% TLC
  • FRC is the volume of gas in the lungs at the end of a normal tidal expiration, when no respiratory muscles are actively contracting

This quiz explores the concept of pressure difference generation and Boyle's Law, which states that pressure is inversely proportional to volume at a constant temperature. It covers the relationship between pressure and volume changes in a gas within a container.

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