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2 - Whole Lung Ventilation and Compliance


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What is minute ventilation? How is it calculated? What is a normal value?
*the amount of air inspired/exipired in one minute
*equal to tidal volume minus air trapped in physiological dead space times resp rate
*(0.5L - 0.15L) x (12bpm) = 4.2L/min
What are the two general types of resistance to air flow?
*airways resistance
*lung and chest wall compliance
What is lung compliance? How can it be calculated from a graph of volume vs. pressure?
*CPL is the ease with which the lung changes volume per unit pressure
*CPL would be the slope of this graph
How does compliance vary during inspiration?
CPL decreases during inspiration, becoming less compliant as maximum inspiratory capacity is approached.
How does elastic recoil of the lung work? In other words, what processes does this assist and which are hindered by it?
Elastic recoil assists in deflation and works against inflation.
An air-inflated lung has higher compliance during inspiration as compared to expiration.
What two factors are the primary determinants of lung compliance?
*elastic recoil
*surface tension
Why does inflation of an isolated lung with saline show higher compliance than one inflated with air?
Because in the saline-filled lung there is no surface tension.
How is surface tension created?
At the interface between two phases, molecules of one phase are unable to H-bond with molecules of the other phase - this creates a force on the surface.
How can surface tension be calculated? In which direction is this tension exerted?
*LaPlace equation: P = 2T/r
*the tension is directed inside - to collapse the alveoli
What are the two means of counteracting surface tension in the human lung?
*alveolar interdependence
How does surfactant work? What does it consist of?
*provides a layer of amphipathic molecules that can H-bond with both phases
*phosphotidylcholine and proteins
How does surfactant lessen surface tension during expiration?
*compression of the alveoli leads to an increase in the density of surfactant
*the more dense surfactant has less surface tension
*collapse is opposed
What is the primary cause of infant respiratory distress syndrome?
Immature type II alveolar cells do not produce enough surfactant leading to alveolar collapse.
How does alveolar interdependence oppose alveolar collapse?
*the walls of the alveoli are interconnected
*as any one gets smaller this stabilizes adjacent alveoli
How does lung compliance change in emphysema and fibrosis?
*emphysema leads to increased compliance
*fibrosis leads to decreased compliance
How can chest wall compliance be calculated? How is it affected by obesity?
*1/CPL(tot) = 1/CPL(lung) + 1/CPL(wall)
*obesity decreases chest wall compliance
What is airway pressure at mechanical equilibrium?
What occurs at mechanical equilibrium?
*the tendency of the ribs to spring out is balance by the tendency of the lung to collapse
*i.e. lung compliance is balanced by chest wall compliance
Describe the energy transfer between the chest wall and lung during inspiration.
*the chest wall moves to its preferred position and transfers energy to the lungs
*the lungs are moved away from their preferred position and thus require energy released from the chest wall
Describe the transfer of energy between the chest wall and the lungs during exhalation.
*lungs move from a position of high energy to one of low energy and transfer the difference to the chest wall
*the chest wall moves from a position of low energy to one of high energy and require energy
What lung volume defines total system equilibrium?
In normal patients, what percentage of the O2 taken in is used to power the work of respiration? How does this change during heavy exertion of exercise?
*5% of O2 taken in is used to power tidal breathing
*this can increase to as high as 30% during heavy exertion

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