5 - Pulmonary Mechanics Flashcards
What provides the pressure differences required for air movement?
The action of the respiratory pump muscles: diaphragm, intercostals, abdominal)
For air to more, the airway must be open.
What is interaction between the lungs and the chest wall?
The lungs are elastic structures whos equilibrium potential is total collapse.
The chest wall is also elastic and and its equilibrium position is 60% of vital capacity (maximum air we can inhale in a breath).
The equilibrium point for the lung and chest wall as a functional unit is about 40% of vital capacity. This equilibrium point is called funcitonal residual capacity or volume (FRC or FRV).
What are the three different pressures in the thoracic cavity?
Alveolar pressure: pressure within alveoli (equal to atmospheric when there’s no air flow)
Pleural pressure: pressure within pleural space (subatmospheric when ther’s no air flow)
Transpulmonary or recoil pressure: difference between pleural and alveolar pressures (reflects elastic recoil pressure of the lung).
What is alveolar, pleural, and transpulmonary pressure at the end of expiration?
At end of expiration (FRC) when there’s no respiratory muscle activity and airflow, alveolar pressure (PA) is the same as atmosphereic pressure and thus it is zero.
At FRC the chest wall attempting to expand to its equilibrium volume creates a sub-atmospheric pleural pressure (PPL) that pulls the lungs outwards thereby preventing complete deflation of the lungs.
PPL is -5
Pa is 0
Trnspulmonary pressure (recoil pressure) is 5.
How do the pressures in the lung change with inspiration?
The force generated by the respiratory muscles expands the pleural space to decrease intrapleural pressure.
This decrease in pressure overcomes lung recoil to expand the lung and decrease PA below atmospheric pressure resulting in inspiration.
What is the sequence of events that occurs during inspiration?
- Brain initiates diaphragm and/or intercostal muscle contraction
- Pleural space increases as chest wall expands
- When pleural space increases, the PPL becomes more negative countering the precoil pressure and thereby expanding the alveoli
- Alveolar pressure falls below atmospheric pressure
- Air flows into alveoli to equalize alveolar and atmospheric pressure
What is the sequence of events that occurs during normal expiration?
- Brain ceases inspiratory command
- inspiratory muscles relax
- Thoracic volume decreases causing PPL to become less negative and decreasign the alveolar pressure gradient
- Alveolar elastic recoil increases alveolar pressure above atmospheric providing airflow until alveolar and atmospheric pressure equalizes.
Why does the alveolar pressure chaneg less than the change in pleural pressure during inspiration and expiration?
Because a portio of the pleural pressure change (or energy) is required to overcome lung elasticity.
What is the relationship between pleural pressure and lung volumes during inspiration and expiration?
Lugn vol increases as pleural pressure becomes more negative during inspiration and decreases as the pleural pressure becomes less negative during expiration.
At the end of inspiration and expiration there’s no air flow and thus alveolar pressure is zero which means that pleural pressure reflects the lung elastic recoil pressure.
How do we calculate lung compliance?
Computed as change in volume/change in transpulmonary pressure
PTP cannot be directly measured, but when airflow is zero PPL reflects PTP.
If a line is drawn to connect the zero flow values, the slope of that line reflects lung compliance.
How do you figure out alveolar pressure from the graph depicted? How do you calcular airway resistance?
Any point on the line where PPL exceeds PTP, the remaining pressure is PA, which generates airflow.
If you measure airflow at that point, then airway resistance can be computed as R=PA/airflow.
The amount of airflow for an PA is determined by airway resistance, which is determined primarily by airway diameter.
At high lung volumes, the long becomes _____ elastic.
Less elastic.
What happens to surfactant molecules during expiration?
They move closer together; thus, as the lung volume decreases, the concentration of surfactant increases and thus surface tension decreases as lung volume decreases.
The effect is important in preventing small alveoli from emptyhing into large alveoli.
Elastic reoil pressure is ______ than normal in emphysema and _____ than normal in fibrosis.
Elastic recoil pressure is less than normal in emphysema and greater than normal in fibrosis.
What would happen to the chest wall if the lungs were removed?
The volume would equal 60% of vital capacity (VC).
Airway resistance _______ dramatically as lung volume increases.
Decreases.
This change is likely due to alveolar elastic tissue acting on neighboring airways like a spring, and causing progressive dilation as long volume increases.
Why is flow rate at long lung capacities effort independent?
At some point, the pressure inside the airway = pressure outside the airway.
At low lung volumes, the pressure inside the airway falls below PPL and thus the airways become compressed. When this point is reached, then the effect of an increased effort to increase the PA is offset by increased compression which increases resistance.
This point is reached when the drop in pressure down the airway exceeds the elastic recoil contribution to PA.
How does the flow/volume curve change with airway obstruction and constrictive lung disease?
During emphysema, there’s both reduced contribution of recoil pressure to PA and increased airway resistance due to reduced elasticity.
Contricted lungs result from fibrosis which causes the lung to be very “stiff” and difficult to expand.
What happens to our air usage during exercise?
We prefer to use inspiratory reserve (as opposed to expiratory reserve) because of airway compression at long lung volumes.
