Mechanics of breathing Flashcards
Define airway resistance
Airway resistance is the resistance of the respiratory tract to airflow during inspiration and expiration.
For air to reach gas exchange surfaces such as the alveoli, the air must pass through a series of increasingly narrow and numerous airways.
What does Hagen-Poiseuille equation show?
As an airway’s radius decreases, the resistance increases and the airflow decreases dramatically.
How does air travel in our airways?
Air travels in a laminar flow (in one direction)
How is airway resistance affected by respiratory diseases?
Respiratory diseases such as COPD can cause a loss of airway patency (patency means a state of being open). This is due to the degradation of structures such as elastin which provide radial traction to splint bronchioles open against +ve Palv. Without radial traction, bronchioles collapse thus causing an obstruction.
radial traction = the force exerted by the lung parenchyma to keep the airways open
Define lung compliance (CL)
How easily the lungs can be expanded.
Lung compliance quantifies the relationship between the level of expansive force applied to the lung and the resulting change in lung volume.
It describes the relationship between changes in lung volume and transpulmonary pressure.
How is lung compliance determined?
It is determined by the structure of the lung tissue e.g. the density of elastic fibres
What does a high lung compliance mean?
higher lung compliance =
less elastic recoil =
less force required to inflate =
greater volume change per pressure change = greater gradient on the volume pressure curve
What does a low lung compliance mean?
lower compliance =
more elastic recoil =
more force required to inflate =
less volume change per pressure change = lower gradient on volume pressure curve
which factors affect lung compliance?
Chest wall mechanics:
-scoliosis, muscular dystrophy and obesity all affect the chest wall mechanics and decrease lung compliance
Alveolar surface tension:
-neonatal respiratory distress syndrome NRDS decreases lung compliance
Elastin fibres:
- fibrosis decreases lung compliance
-COPD and emphysema increases lung compliance as it degrades the structural fibres
How does emphysema increase lung compliance?
emphysema involves degradation of elastin fibres making the lung less stiff and more complaint (but reducing recoil).
How does pulmonary fibrosis decrease lung compliance?
pulmonary fibrosis involves scarring and deposition of structural fibres such as collagen making the lung stiff and less compliant.
How is lung compliance calculated?
Lung compliance (CL) = Change in volume/Change in pressure Compliance is expressed as the gradient of the curve. The steeper the curve, the greater the level of lung compliance
What is pulmonary surfactant?
Pulmonary surfactant is a complex of phospholipids and proteins formed by type II alveolar cells. The proteins and lipids that make up the surfactant have both hydrophilic and hydrophobic regions.
What is the role of pulmonary surfactant?
it reduces surface tension at the air-liquid interface, increasing compliance and preventing alveolar collapse and oedema
What does the law of Laplace describe and how does pulmonary surfactant resolve any issues which can arise?
- describes the relationship between collapsing pressure, the radius of the bubble, the surface tension (which depends on the nature of the fluid)
- pressure and bubble radius are inversely proportional which means that smaller bubbles generate greater pressure than larger ones
- as gases naturally move from areas of high to low pressure, if bubbles of varying size are connected (such as different sized alveoli connected by airways), the smaller bubble will empty into larger ones due to the pressure gradient
- so, within the lungs, the pressure gradients that would be created between different sized alveoli would result in smaller alveoli collapsing into larger ones
- this would make inflation of the lung very difficult
- the potential problem of smaller alveoli collapsing is resolved by the presence of pulmonary surfactant, a phospholipoprotein secreted by type II pneumocytes (alveolar cells)
