1. Surface tension
- van de Waals forces of adjacent water molecules
- Two forces:
- Circumferential
- Tangential (radial) = wall tension
- Opposite force = gas pressure
- The internal pressure required to keep a sphere
inflated.
- Laplace’s Law: P = 2T/r (T = tension of liquid)
- Therefore smaller alveoli require a higher internal
pressure to stay inflated than larger alveoli
!
Pulmonary surfactant
- = detergent that ↓ surface tension in alveoli
- Hydro-philic & -phobic
1. Increases lung compliance
Large hydrophilic heads sit on the alveolar surface with
hydrophobic tails facing inwards. They sit between water
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Lung compliance and
effect of pneumothorax
- Lung recoil inwards and chest wall recoil outwards causes negative
intrapleural pressure
- Reduced pleural pressure → positive transmural pressure → increase
lung volume → reduced airway pressure → inspiration
- FRC is at most efficient part of compliance curve
Effect of surfactant on
alveolar pressure
EXAMPLE

2. molecules and interrupt their van de Waals forces, which causes a reduction in the radial
(collapsing) and circumferential forces. Surface tension is 70% of lung elasticity, which is 1/
compliance. Therefore reducing elasticity causes and increase in compliance. Raising the
lung’s compliance means that less pressure is required to cause a change in lung volume -
therefore the work of breathing is reduced.
2. Stabilises alveoli
- Area-dependent effect → greater reduction in surface tension in smaller alveoli.
Necessary because Laplace’s Law states that smaller have higher internal pressure
therefore prevents smaller emptying into larger.
3. Reduces fluid movement into the alveoli
- More compliant → pleural pressure (& transmural pressure) can be lower →
reduces suction of fluid into alveoli from pulmonary capillaries
!
Resistance in the respiratory system
- Resistance = ∆P/Flow
- Airways (80%) + Tissues (10%)
- Dynamic resistance causes right shift of compliance curve higher pressure needed
to cause volume change
- Poiseuille's law: Resistance ∝ 1/r4
- Large airways give majority due to a lower total X-section area (see above graph)
- Radius = sum of radii for all of one airway division
- Large airways give majority due to:
- ↓ total X-section area
- More turbulent flow
- Two forms of flow:
- Laminar → low resistance
- Turbulent → high resistance
!
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Types of airflow
Reynold's number
- Higher number gives higher
probability of turbulent flow
- Higher:
- Higher velocity
- Larger diameter
- Branching
Bronchoconstriction:
- Vagus (some rest tone) & ↑ mucus
- Low airway CO2
- Leukotrienes, histamine,
bradykinin
!
Bronchodilation:
- β2-adrenoreceptor ligation
- Non-adrenergic, non-cholinergic
innervation
EXAMPLE

3. Flow limitation and obstructive airway disease
- During expiration pressure highest in alveoli and lowest at mouth
- Loss of pressure (energy) due to resistance
- Higher flow generated by greater collapsing transmural pressure - equal along airway
[inside the thoracic cage]
- Airway pressure must be greater than transmural pressure to keep airways open
- Theoretical point where:
- Transmural (collapsing) pressure = airway pressure
- Airway collapses & flow stops
- Point is more proximal with:
- Increased resistance (bronchoconstriction, mucus)
- Lower airway pressure (loss of elastic tissue)
- = Obstructive lung disease: asthma & COPD: chronic bronchitis & emphysema
!
Gas exchange and transport
!
Alveoloar gases
- CO2
- Carbon dioxide from venous blood to alveoli: PACO2 < PvCO2
- PACO2 ∝VCO2/VA
- If VCO2 is constant (rest) then PaCO2 ➙ efficiency of VA
- Oxygen
- Oxygen: from alveoli into venous blood
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Flow limitation during expiration
- Surfactant increases lung compliance and stabilises alveoli
- Turbulent flow increases resistance - predicted by high Reynolds
number
- COPD causes expiratory limitation due to increased resistance
EXAMPLE