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Questions and Answers
What happens to the chest wall recoil at lung volumes above 70% TLC?
How does the integrity disturbance like pneumothorax affect alveoli?
What happens to airway resistance with increasing lung volume?
What is the Equal Pressure Point during forced expiration?
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Why does emphysema lead to difficulty in achieving high airflow rates?
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What opposes dynamic compression of airways during forced expiration?
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What is the main role of pulmonary surfactant in the lungs?
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According to La Place's law, what is the relationship between pressure (P), surface tension (T), and radius (r) in a spherical bubble?
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How does pulmonary surfactant affect alveolar stability?
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What happens to surface tension during inspiration in the lungs?
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How does surfactant deficiency contribute to respiratory distress syndrome?
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What is the effect of surfactant on alveolar compliance?
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In the context of alveolar interdependence, what stabilizes alveoli and prevents collapse?
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What would happen if two nearby alveoli with different radii have the same surface tension according to La Place's law?
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What is the primary function of pulmonary surfactant on alveolar walls?
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How does surfactant contribute to maintaining lung compliance?
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What is the main function of pulmonary surfactant?
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In the context of alveolar stability, what happens to alveoli during inspiration?
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What is the relationship between lung volume and airway pressure during quiet expiration?
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How does a negative intrapleural pressure affect alveolar stability at functional residual capacity (FRC)?
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What is the impact of surface tension on alveolar stability?
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According to Laplace's law, how does surface tension affect small alveoli compared to large alveoli?
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What happens to airway pressure when lung volume decreases during expiration?
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How does negative pressure breathing differ from positive pressure breathing?
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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
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
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Description
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.