Respiratory Physiology - Mechanics of Breathing Flashcards
Muscles of inspiration
Diaphragm
External intercostal muscles
Accessory muscles under exertion also used
Form of respiration in health
Negative pressure ventilation
How is negative intrathoracic pressure generated
Diaphragm contracts + flattens and intercostal muscles contract to lift rib cage
Increases lung volume and therefore decreases intrathoracic pressure
Generates pressure gradient from atmosphere in to the lungs
Innervation of diaphragm
2 phrenic nerves
C3/4/5
Method of expiration in normal circumstances
Passive expiration
Method of expiration in exertion
Element of active expiration with contraction of abdominal muscles + contraction of internal intercostal muscles
Pressure-volume curve of lungs
At atmospheric pressure, there is still small volume of gas in the lung
This is called minimal volume - difficult to remove due to closure of small airways
Non-linear relationship
Hysteresis - behaviour in expiration lags behind behaviour in inspiration
Get the same curve if using negative or positive pressures
Compliance definition
Change in volume / Change in pressure
Units = ml/cmH2O
Normal compliance of lung
~ 200 ml/cmH2O
Examples where compliance is reduced
- Pulmonary fibrosis
- Pulmonary oedema
Lung tissue stiffer
Examples where compliance is increased
- Emphysema
- Increased age
Loss of lung architecture and loss of elastic recoil
Proteins contributing to elastic properties / compliance of the lung
Collagen
Elastin
Factors contributing to elastic properties / compliance of the lung
Change in lung protein geometry
Surface tension of alveolar lining fluid
Surface tension definition
Total force across imaginary line 1cm across the surface
Picture A
Molecules of water attract each other more than they attract molecules of air
Units of surface tension
dynes / cm
or
milli-Newtons / m
Features of surface tension on a curved surface
On a curved surface, surface tension produces pressure
Picture B
Laplace’s law
Relates to pressure in a soap bubble
Pressure = 4 x surface tension / radius
P = 4T / r
Implication of Laplace’s law
Pressure is higher in spheres of smaller radius, and therefore they inflate the larger spheres
Picture C
Pressure-volume curve of lung when inflated with fluid vs air
Easier to inflate lung with fluid than with air, with less hysteresis
Due to loss of surface tension when fluid used to inflate lung
Surfactant definition
Material which reduces surface tension of alveolar layer
Effect of surfactant on surface tension
Surface tension does not change with changes in area with water
Detergent reduces the surface tension but again unaffected by area changes
Surfactant results in surface tension altering with changes in area.
Surface tension very low at lower area %
Therefore aids inflation with air and elastic recoil at higher area %
Main constituent of pulmonary surfactants
Dipalmitoylphosphatidylcholine
(DPPC)
Phospholipid
What produces pulmonary surfactants
Type 2 alveolar epithelial cells
Secreted as lamellar inclusion bodies
3 functions of surfactant
Increases compliance of lung
Increases stability of the lung
Reduces tendency to alveolar oedema
How does surfactant increase pulmonary compliance
Reduced surface tension at lower areas so easier to inflate
Higher surface tension at higher areas so elastic recoil
How does surfactant increase stability of lung
Due to lower surface tension at lower areas and higher at higher areas, you do not see smaller alveoli inflating larger alveoli as you might expect according to Laplace’s law
How does surfactant reduce tendency to alveoli oedema
Reduced pressure in alveoli
Therefore reduced capillary all tension surrounding alveoli
Therefore reduced hydrostatic pressure of capillary and fluid more likely to move into capillary and less likely to form oedema
Effect of lung if no surfactant was present
Significant atelectasis
As seen in premature new-borns where surfactant not yet produced
Infant respiratory distress syndrome
No surfactant - commonly in premature babies
Treat by giving artificial surfactant
Regional differences of lung ventilation at FRC
Weight of lung results in different pressures in apex vs base of lung
Greater negative pressure at apex so on different part of curve
Base at a steeper part so change in pressure results in greater change in volume compared with apex
Regional differences of lung ventilation at residual volume
Pressures are less negative
Pressure at base above atmospheric pressure
Therefore no ventilation at bases occur until pressure falls below atmospheric pressure
More likely to have airway closure at higher volumes with increasing age or diseased lungs
Patterns of gas flow in tubes
Move from A -> B -> C as flow rate is increased
Poiseuille equation for laminar flow
Note resistance is inversely proportional to radius to 4th power
Velocity profile of laminar flow
Flow rate at centre of tube is higher than elsewhere in the tube
Reynold’s number
Likelihood of turbulent flow
Pressure / Volume / Flow changes during breathing cycle
Number of airways with each generation
Exponentially increases
Therefore very large number of smaller airways
Where in airways is there the most resistance
Medium sized airways
Higher resistance with smaller radius but due to vastly high numbers, the resistance is shared more and therefore less for each airway
What happens to airway resistance as volume increases
Airway resistance decreases as volume increases
This is because radial traction from lung parenchyma increases radius of airways and pulls them open (similar concept to blood vessel effects)
Explains why patients with lung disease (eg COPD) breath with higher lung volumes
Flow-Volume curves with spirometry
A = maximal effort
C = not maximal effort
B = low effort then maximal effort at the end
Cannot exceed downslope flow, always meets it
Isovolume pressure-flow curves
At different lung volumes, expiratory flow is independent of effort exerted
I.e, reach a maximum expiratory flow which cannot be exceeded
Reason that expiratory flow is independent of effort exerted at fixed lung volumes
Dynamic compression
During inspiration as intrapleural pressure decreases, airways are pulled open with radial traction
This is not present in expiration and in fact has opposite effect as transpulmonary pressure increases inwards causing airway compression / collapse
Increase in transpulmonary pressure matches increase in alveolar pressure and therefore flow remains constant regardless of effort
Tissue resistance
Resistance from tissue sliding over another
Not as important / notable as airway resistance
Work done by the lung during inspiration
In absence of airway resistance, work done would be area from line AEC to D to 0
Additional work done with airway resistance is represented by the shaded curve
