Pressure Difference Generation and Boyle's Law Quiz

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

Elastance

• 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

Surfactant

• 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

Atelectasis

• 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