Special Sub-Topic: Lung Mechanics: Statics & Dynamics
|During spirometry (measurement of gas coming in and out of the lungs), IRV, or inspiratory reserve volume, is measured at which point?|
End of maximum inspiration. The inspiratory reserve volume (IRV) is measured at the end of maximum inspiration. In a healthy male weighing 70kg, this figure will be about 3000ml. The expiratory reserve volume (ERV) is measured at the end of maximum expiration, giving a figure of about 1000ml.
The difference in volume at the end of normal inspiration and expiration is known as the tidal volume. This is usually about 500ml.
As you cannot expirate all the gas in the lungs, there is a residual volume left inside which cannot be measured by spirometry. This is believed to be around 1200ml.
|At functional residual capacity (FRC), the lung tries to contract outwards and the chest wall tries to contract inwards.|
f. The functional residual capacity is the values from the expiratory reserve volume and residual volume added together.
At this point, the lungs are trying to contract inwards and the chest wall outwards. They are trying to reach elastic recoil equilibrium.
The pleural sac is a fluid filled layer which aids movement, but also acts as cohesion between the lung and chest wall. The force on this is due to a negative intrapleural pressure.
|What is transpulmonary pressure?|
The difference in pressures inside & outside the lung. The distending pressure across the lung, or transpulmonary pressure, is the difference in pressures inside and outside the lung.
In other words, transpulmonary pressure is the difference between the alveolar pressure and intrapleural pressure.
|In a normal adult, the recoil of the chest wall and the lungs are equal, creating a intrapleural pressure of -5 cm H2O. What happens to the recoil in a person with emphysema?|
Recoil of the lung decreases. In emphysema, there is decreased recoil of the lung, but the chest wall recoil can balance this out, leading to a less negative intrapleural pressure of -3. This can cause "barrel chest" as the chest may spring outwards due to less lung recoil.
In fibrosis, there is increased lung recoil, and the chest wall recoil can balance this out leading to a more negative intrapleural pressure of -7. This will cause the FRC to decrease.
|To generate gas flow during inspiration, the pressure in the atmosphere must be higher than the pressure in the alveoli.|
t. Shortening the diaphragm is the primary mechanism of inspiration. The external intercostal muscles also pull the ribs upwards and forwards. The scalene and sternomastoid muscles are known as accessory muscles of respiration.
The work of these muscles causes the lung volume to increase, lowering the pressure and allowing gas to flow in.
For gas flow during expiration to take place, the pressure in the atmosphere must be lower than the pressure in the alveoli.
At rest, this is a passive process which occurs due to the elastic recoil of the lung. However, during exercise and voluntary hyperventilation, the muscles of the abdominal wall contract, raising interabdominal pressure.
Also, the internal intercostal muscles pull the ribs down and inwards. The work of these muscles causes the pressure in the alveoli to increase, and allows expiration.
|What must happen to the transpulmonary pressure to bring about a change in lung volume?|
It must become more negative. If a graph showing a transpulmonary pressure-volume relationship was drawn, the shape of the line would determine the compliance of the lung under static conditions.
Instead of increasing pressure inside, the lung decreases pressure outside, thus increasing volume.
Lung compliance is defined as the change in volume per unit of distending pressure.
For example, in an adult during quiet breathing, the volume inspired would be 600ml. The change in intrapleural pressure would be from -4 to -7, so the lung compliance would be 600/3 = 200ml/cm H2O
You can think of compliance as "stiffness". Lungs that are not compliant will be stiff.
The same change is present with intrapleural pressure, but when the lung is closer to the total lung capacity, (instead of the functional residual capacity), the lung volume will only be changed by a smaller amount. Pressure can be increased but lung capacity cannot be increased, so at a high lung volume, compliance decreases.
|A major determinant of compliance is surface tension. If two fluid molecules are adjacent, compliance will be increased.|
t. Surface tension is measured in dynes/cm because it is the force acting across a line in the surface of the liquid.
The attractive forces between adjacent molecules in the liquid are much greater than those between the liquid and the gas. This results in the liquid surface area tending towards its smallest possible value.
The pressure generated by these forces can be calculated from Laplace's Law, where pressure is equal to 2 multiplied by the surface tension, divided by the radius of the alveoli. According to this law, a small radius will equal a large pressure.
|What is the name of the liquid lipoprotein which lowers surface tension?|
Surfactant & Pulmonary Surfactant & Dipalmitoylphosphatidylcholine. For example, if there was a small and a large alveolus, the one with the smaller radius would have a larger pressure. Therefore the gas would flow down the pressure gradient to the large alveolus, causing the small alveolus to collapse.
Pulmonary surfactant, secreted by type II alveolar epithelial cells, lives on the surface on the alveoli and lowers pressure when the radius is small, and therefore stabilises the alveoli.
Therefore it increases lung compliance and decrease the work of breathing.
|Resistance to lamina flow can be calculated with the driving pressure and the flow rate. What is this law known as?|
Poiseuille's Law. The distending pressure is where to maintain a lung volume above the functional residual capacity, the transpulmonary pressure must be great enough to overcome the elastic recoil of the lung. If there is an equal pressure there will be no gas flow as a pressure gradient is needed.
To generate gas flow into or out of the lung, the driving pressure (change in pressure, or the difference between the pressure in the atmosphere and pressure in the alveoli), must always overcome airway resistance.
Poiseuille's Law shows us that as the radius expands, the resistance lowers very quickly.
Airways resistance can be increased by contractile activity of bronchiolar smooth muscle, such as in asthma, or by secretion of mucus, such as in chronic bronchitis.
|At the onset of forced expiration, the intrapleural pressure and pressure in the alveoli are both negative.|
f. During dynamic compression of the airways, the flow, of most of the expired volume, is independent of effort.
During pre-inspiration, the pressure in the alveoli is equal to the pressure in the atmosphere. Therefore the intrapleural pressure is negative, causing a 5cm H2O pressure to hold the lungs open.
During and at the end of inspiration, pressures that tend to keep the lung open increase, and so at the onset of forced expiration, the intrapleural pressure and pressure in the alveoli are both positive.
At some point along the airway, there is a pressure drop as the intrapleural pressure exceeds airways pressure, which causes the airways to close.
During airway compression, the flow limiting factor is the pressure around the airway, i.e. the intrapleural pressure.
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