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Mechanics of Flow
Physiology and Pharmacology
| 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 |
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