Use the compliance curves of the lung and RESTING chest wall shown below to answ
ID: 64447 • Letter: U
Question
Use the compliance curves of the lung and RESTING chest wall shown below to answer questions. NOTE: if the glottis is open, that means that the respiratory muscles are holding the lung at the volume indicated in the question, and therefore Alveolar Pressure = 0 cmH2O = Atmospheric Pressure. ?
1b. (Use the above graph). This person inhales to a lung volume of 6 liters and holds that lung volume steady by maintaining contraction of his inspiratory muscles (with his glottis open), determine:
i. interpleural pressure in cmH2O
ii. transmural pressure of the lung in cmH2O
iii. transmural pressure of the chestwall in cmH2O
iv. alveolar pressure in cmH2O
(Hint: CW compliance curve changes due to expiratory effort, lung curve stays the same)
1c. If the person in question 1b above with a lung volume of 6 liters were to close his glottis and relax all his respiratory muscles, (still keeping lung volume at 6L, but with respiratory muscles relaxed):
i. what would interpleural pressure be now? ______________ cmH2O.
ii. what would alveolar pressure be now ? _______________ cmH2O.
iii. if person now opened his glottis, he would (exhale passively, inhale passively) circle right answer
(Hint: CW compliance curve returns to resting position when you relax your respiratory muscles)
Explanation / Answer
I) ressures and ow during the breathing cycle At rest the pressure in the alveoli of the lung is the same as at the mouth, ie zero with respect to atmospheric pressure. The intrapleural pressure is -5cmH2O. The volume of gas within the lungs is the FRC, and there is no gas ow into or out of the airways.
During inspiration,intrapleural pressure falls because of the activity of the inspiratory muscles expanding the chest wall.
This is transmitted across the lung, and alveolar pressure falls towards -1cmH2O. Since this is subatmospheric air ows from the mouth to the alveoli. At the end of inspiration the intrapleural pressure is -8 cmH2O, alveolar pressure is again atmospheric and gas ow to the lungs has increased, the result being 500ml of air has owed into the lungs.
b) Expiration commences when the activity of the inspiratory muscles stop.
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