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What causes regional variation in ventilation?
1. Topographical differences
2. Variation in local distensibility
3. Variations in airway resistance
4. Dilation of of the airways in the region of the respiratory bronchioles leading to increase distance of diffusion of gases to and from the alveoli
Explain how topographical differences lead to regional difference in ventilation of the lung?
Intrapleural pressure is less negative at the base of the lung because the weight of the lung needs a higher (less negative) pressure to support it.
This leads to a small expanding pressure at the base of the lung resulting in a small resting volume. This region is then on the steep part of the pressure-volume curve → expands well on inspiration.
There reverse is true of the apex
What happens to the regional variation in ventilation at low lung volumes?
e.g. at full expiration
At this point the intrapleural pressure at the base now exceeding airway/atmospheric pressure.
The base of the lung is therefore compressed and ventilation is impossible.
The apex of the lung is now on the steep part of the pressure volume curve and ventilates well.
What formula can be used to determine if flow is laminar or turbulent in airways?
(pie) x driving pressure x radius⁴/ 8 x length x viscosity
How can flow resistance be calculated?
resistance = driving pressure / flow
Therefore by rearranging the H-P equation
R = 8 x length x viscosity / (pie) x radius⁴
What formula can be used to predict if flow in airways will be laminar or turbulent?
What factors affect intra-pleural pressure changes during inspiration?
Elastic recoil of the lung
At which site in the lungs is airway resistance at its highest?
The medium size bronchi (around the 7th generation)
The reason it is this region rather than the smaller airways as would be predicted by the H-P equation is due to the vast number of smaller airways.
What factors affect airway resistance?
1. Lung volumes - the calibre of small airways is increased at higher lung volumes and therefore resistance falls. At small lung volumes airways may close.
2. Bronchial smooth muscle tone
3. Density and viscosity of inspired gas
What factors affect bronchial smooth muscle tone ? (6)
Motor control of the smooth muscle is by the vagus nerve
1. Stimulation of the β₂adrenoceptors → dilation
2. Stimulation of receptors in the trachea and large bronchi by irritants e.g. smoke → constriction
3. Parasympathetic activity and ACh → constriction
4. Fall in Pco2 → constriction
5. Histamine → constricition at the level of alveolar ducts
6. Density and viscosity of gas e.g. helium used to reduce density
What is dynamic compression of the airways and why does it occur?
In the normal lung:
Compression of the airways by intra-thoracic pressure during maximal expiration.
During normal inspiration and expiration the intra-pleural always remains more negative than the alveolar/airway pressure
During forced expiration both the intra-pleural and alveolar pressure increase. However, because of the pressure drop across the airway the pressure in the airway becomes less than the intra-pleural pressure, tending to close the airway.
What factors exaggerate dynamic compression of the airways?
1. Increase in airway resistance → greater pressure drop → decrease intra-bronchial pressure
2. Low lung volumes - reduced driving pressure
3. Reduced radial traction (which helps keep the airways open) e.g emphysema.
4. Reduced recoil pressure e.g. emphysema
What pattern is seen in restrictive lung disease?
Reduced FEV₁but with a normal or increased FEV₁/FVC ratio
What is pulmonary resistance?
Total resistance which includes airway resistance plus tissue resistance (the resistance caused by pressure required to overcome the viscous forces of the tissues as they slide over each other)
Which are the accessory muscles of respiration?
1. Scalene muscles (move the first two ribs)
2. Sternomastoid (move the sternum)
3. Alae nasi (flare the nostrils)
4. Small muscles of the head and neck
What muscles are involved in active expiration?
1. Abdominal muscles (rectus abdominus, external oblique, internal oblique and transveresus abdominus
2. Internal intercostal muscles
What is hystersis?
The dependance of a system not only on its current environment but also on its past environment
The volume change per unit pressure change
The slope of the pressure volume curve
What factors reduce the compliance of the human lung?
1. High expanding pressures
2. Alveolar oedema
3. Increase in fibrosis tissue
5. Increase in surface tension
What is responsible for the elastic behavior of the human lung?
The geometric arrangement of fibers of elastin and collagen
Define surface tension
The forece acting across the an imaginary line 1 cm in length in the surface of the liquid
What causes surface tension?
The fact that the attractive forces between the molecules of a liquid are greater than the between the liquid and gas. The result is that the liquid surface area becomes as small as possible.
Describe the pressure-volume curve of the lung?
Nonlinear with the lung becoming stiffer at high volumes
What is surfactant?
A substance secreted by type II alveolar cells containing dipalmitoyl phosphatidylcholine (DPPG)
How does surfactant reduce surface tension?
The molecules of DPPG are hydrophobic at one end and hydrophilic at the other end
The DPPG oppose the attractive forces between the molecules of the liquid
This effect is exaggerated as the liquid is compressed
What are the physiological advantages of surfactant?
1. Increase in compliance of the alveoli - lower surface tension reduces the pressure work of expanding the alveoli
2. Promotes stability of the alveoli - without surfactant smaller alveoli would collapse into larger alveoli. With surfactant the surface tensions are the same therefore the pressure inside the alveoli is inversely proportional to its radius. Therefore the small alveoli have a larger pressure compared to the larger alveoli.
3. Keeps the alveoli dry by reducing the hydrostatic pressure and therefore reduces translocation of fluid
What are the consequences of a lack of surfactant?
1. Low compliance
3. Alveoli filled with transudate
(Features of the infant respiratory distress syndrome)
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