| Question | Answer |
| Types of flow | Two basic types of flow depending on reynolds number
Laminar - Re<2000, orderly flow, parabolic profile
Flow proportional to pressure gradient
Turbulent - Re>2000, disordered motion, flat profile
Flow proportional to square root of pressure gradient |
| Flow in the lungs | Conditions are almost always laminar flow
Lungs complicated branching pattern means there may be eddies and the flow is non-steady
Turbulent flow seen in trachea during exercise |
| Measuring airway resistance | Subject in sealed box - body plethysmograph
When they breathe in the alveolar pressure fall and pressure in the box rises
Alveolar pressure cant be measured by can be estimated using P1V1=P2V2
R = P2-P1/V |
| Distribution of resistance in the lung | Although airway resistance is much greater in smaller tubes than larger, this is more than countered by the increasing total cross sectional area at each generation of the airways
Segmental bronchi site of maximum resistance |
| Resistance at different lung volumes | At higher lung volumes resistance decreases as the radius of airways increases |
| Maximum expiratory flow | At a certain point flow becomes effort independent - same flow regardless of pressure
There is a maximal transmural gradient of 11 cmH20 before airway collapse occurs
When this happens, the same pressure gradient through the same segment occurs |
| Clinical importance of MEF | High resistance gives low maximal expiratory flows e.g. asthma
Low lung elasticity gives poor airway support and easy collapse e.g. emphysema
Can be treated by increasing pressure to above atmospheric to hold the airways open |
| Airway closure | At RV the lower lung is compressed-airways close and gas is trapped
Beginning inspiration the upper lung inflates at -ve pressure lower down
Loss of elasticity gives +ve pressures and airway closure at higher volumes-hyperinflation and poor ventilation |
| Work of breathing | Work = pressure x volume
This is the area under the pressure volume graph
Split into elastic work - associated with stretching to give passive expiration
And viscous work - associated with moving air in inspiration, lots lost in friction |
| Work in stiff lungs | Tend to breathe shallowly as work goes up rapidly with volume
E.g. in pulmonary fibrosis
Increased pressure needed to inflate the lungs |
| Work with increased resistance | Tend to breathe deeply to minimise wasted ventilation (dead space)
Resistance is lass at higher lung volumes
Increases viscous work |
| Why is there a maximal expiratory flow | Always an 8 cmH20 pleural pressure maintained no matter the pressure gradient
The pressure created generates a transmural pressure across the vessels
When this reaches -11 cmH20 it tends to collapse the airways, meaning the same pressure is formed |
