EXAMPLE Lung recoil inwards and chest wall recoil outwards causes - - PDF document

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Oct 30, 2022 10 likes •40 views

Lung compliance and effect of pneumothorax EXAMPLE Lung recoil inwards and chest wall recoil outwards causes negative - intrapleural pressure Reduced pleural pressure positive transmural pressure increase - lung volume reduced

SLIDE 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 Ⓒ One 2 One Medicine: Pre-clinical revision course 2014 Page !

10 22 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

SLIDE 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

11 22 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

SLIDE 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

12 22 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