Lecture 18 - Mechanics of breathing Flashcards
The three steps of respiration
1) Pulmonary ventilation
2) External pulmonary respiration
3) Internal tissue respiration
Pulmonary ventilation
The physical movement of air in and out of the lungs
It relies on two principles: Boyle’s law, air moving from high pressure to low pressure
Boyle’s law
When the temperature is constant, the pressure is inversely proportionate to the volume
Intrapulmonary pressure
The force exerted by gases in the alveoli
Will be higher than atmospheric during expiration
Will be lower than atmospheric during inspiration
Intrapleural pressure
Refers to the pressure within the pleural cavity
Its role is to keep the lungs stable so it’s lower than atmospheric and intrapulmonary pressures
Created by elastic recoil of the lungs
Resistance to breathing
Pulmonary compliance and airway tension
Pulmonary compliance: what is it and what is it affected by?
The ease with which the lungs can be expanded
The elasticity of connective tissue
Mobility of surface wall
Surface tension
The elasticity of lung tissue
The measure of elastic recoil
A measure of (lung) volume changes resulting from a given change in pressure
Mobility of surface wall
…
Surface tension
Caused by intermolecular forces between molecules in a liquid
Air-fluid interface surface of the fluid is under tension like a thin membrane being stretched
Like the thin fluid layer between the alveolar cells and the air
Laplace’s law
Describes the relationship between Pressure (P), surface tension (T) and the radius (r) of an alveolus (bubble)
At equilibrium, the tendency of increased pressure to expand the alveolus balances the tendency of surface tension to collapse it
The use of surfactant
Pulmonary surfactant greatly reduces surface tension, increasing compliance
This equalizes the pressure between small and large alveoli
Surfactant helps keep the uniform alveolar size
More concentrated in smaller alveoli (per mm s. area)
Lower surface tension helps equalise pressure among alveoli of different sizes
Easier to inflate smaller alveoli
Work needed to expand alveoli with each breath greatly reduced
T decreases as alveoli get smaller
- allows alveoli dynamically adjust their rates of inflation and deflation
NRDS
Neonatal respiratory distress syndrome - lack of surfactant secretion in premature babies (28-32 weeks gestation)
Reduced compliance
Alveoli collapse on exhalation
Difficult to inflate lungs
50% die without rapid treatment
Airway resistance
Major ‘non-elastic’ source of resistance to gas flow
Resistance high, gas flow low
Resistance determined by the radius and affected by Lung volume and Bronchial smooth muscle
