Lung volume studies
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Lung volume studies
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[post_date] => 2024-12-23 18:30:06
[post_date_gmt] => 2024-12-23 23:30:06
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
A doctor is studying respiratory function in a group of male patients. The doctor begins by comparing the respiratory rates of the males with their inspiratory volumes. Participants are hooked up to a machine that measures both of these parameters. Inspiratory volume is measured as the amount of air that enters the lungs with each breath the person takes. Respiratory rate is measured as the number of breaths per minute.
Figure 1 Inspiratory volume vs. respiratory rate
The doctor then measures residual lung volume in the individual patients. Each patient is asked to breathe in air mixed with gas X (a gas that is insoluble in blood). Patients first expire as much air as possible; they then take their deepest breath of air mixed with gas X, and the inspiratory volume is measured. After holding their breath for five seconds, allowing for gas X to mix with the residual air, they breathe out. The doctor measures the partial pressure of gas X in the exhaled air. The results for three patients are shown in Table 1.
Table 1 Patient data
The physician understands that the dilution factor for gas X equals the total lung volume divided by the volume of inspired air.
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[quiz_unique_key] => 602779517
[question] => The data support which conclusion regarding respiratory rate and inspiratory volume?
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[answer] => 2
[description] => Reason for Correct Answer:
The graph shows an inverse relationship, where inspiratory volume decreases as respiratory rate increases.
The amount of air delivered to the lungs would be: respiratory rate (breaths per minute) x inspiratory volume (mL per breath). The inverse relationship indicates that the amount of air delivered to the lungs is kept relatively constant, since one variable increases as the other decreases.
In other words, respiratory rate and inspiratory volume compensate for one another.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Respiratory rate and inspiratory volume are independent of one another.
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[1] => Array
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[each_answer] => B. Respiratory rate and inspiratory volume compensate for one another.
)
[2] => Array
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[each_answer] => C. Respiratory rate is a more important indicator of oxygen delivery than inspiratory volume.
)
[3] => Array
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[each_answer] => D. Inspiratory volume is a more important indicator of oxygen delivery than respiratory rate.
)
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Which of the following describes the pressure inside the alveoli relative to the outside air when a person is taking a deep breath?
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
Which way does a gas flow if there are pressure differences? It goes from high to low pressure.
The pressure in the alveoli is considered to be the pressure in the lungs.
For gas to enter your lungs, you need the pressure in the lungs to be less than the pressure outside. You accomplish this by using your muscles to create negative pressure: during inspiration, the diaphragm contracts and moves down, the pressure in the lungs becomes negative (relative to outside air), and air is pulled in.
https://commons.wikimedia.org/wiki/File:Muscles_involved_in_forceful_breathing_in_and_out.jpg
During exhalation, the diaphragm relaxes and moves up; the pressure in the lungs becomes positive (relative to outside air), and air is pushed out.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Greater when breathing in, lesser when breathing out
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[1] => Array
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[each_answer] => B. Greater when breathing in, greater when breathing out
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[2] => Array
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[each_answer] => C. Lesser when breathing in, greater when breathing out
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[each_answer] => D. Lesser when breathing in, lesser when breathing out
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => In Patient 1, what fraction (in moles) of the inspired air is composed of gas X? You can assume that the gas X–air mixture is delivered at atmospheric pressure.
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
Atmospheric pressure is 1 atm, or 760 mmHg.
The partial pressure of a gas is equal to its mole fraction multiplied by the total pressure. The mole fraction is the number of moles of gas X divided by the total number of moles of gas in the air.
The mole fraction of a gas, then, is equal to the partial pressure of the gas divided by the total pressure of the air.
In this case, gas X has a partial pressure of 18mmHg, and atmospheric (total) pressure is 760 mmHg.
This means that the mole fraction of gas X is 18/760.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. 700 x 18/760
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[each_answer] => B. 700 x 18/1000
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[2] => Array
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[each_answer] => C. 18/760
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[each_answer] => D. 18/1000
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[quiz_unique_key] => 1403770772
[question] => What is a result of the fact that we have residual volume in our lungs?
[value] => Array
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[answer] => 4
[description] => Reason for Correct Answer:
Residual volume refers to the space or volume in our lungs BEYOND the volume of air that can be exhaled (vital capacity). The residual volume remains in the lungs all the time, even during forced inspiration and expiration.
https://commons.wikimedia.org/wiki/File:2317_Spirometry_and_Respiratory_Volumes.jpg
Because there is already air sitting in the lungs when we inhale, this air mixes with inspired air on inspiration.
The air that’s already sitting in the lungs will have a lower oxygen content than atmospheric air, therefore it dilutes the oxygen content in the inspired air.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Nitrogen leaves the body more quickly.
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[1] => Array
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[each_answer] => B. Carbon dioxide does not pass through the alveoli.
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[2] => Array
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[each_answer] => C. Oxygen can more easily diffuse into the blood.
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[each_answer] => D. Oxygenated air we breathe in is diluted by de-oxygenated air already in the lungs.
)
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[quiz_unique_key] => 1403770772
[question] => Why did the doctor choose a gas which is insoluble in blood?
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[answer] => 1
[description] => Reason for Correct Answer:
Gases enter the body by diffusing across the alveolar membrane and then entering the blood.
If the gas was soluble in blood, some of it would enter the blood on inspiration.
Without knowing how much dissolves into the blood, it’s not possible to calculate the residual volume that remains in the lungs. As a result, it’s helpful to choose a gas that is insoluble in blood so that very little will leave the lungs into the bloodstream.
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[answers] => Array
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[0] => Array
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[each_answer] => A. So that very little will leave the lungs when the patient breathes in
)
[1] => Array
(
[each_answer] => B. So that it will not prevent oxygen from entering the blood
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[2] => Array
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[each_answer] => C. So that they can measure how much crosses the alveolar membrane
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[each_answer] => D. So that the inspirational volume is accurate
)
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[quiz_unique_key] => 1997864699
[question] => According to the data, which patient has the largest residual volume?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
The patients in this study undergo forced expiration followed by a deep inspiratory breath of air containing gas X, so their measured “inspiratory volume” approximates their vital capacity, which equals the total lung volume minus the residual volume. The partial pressure of exhaled gas X is measured and compared to its original value.
The last sentence of the passage clarifies that “the dilution factor for gas X equals the total lung volume divided by the volume of inspired air.” (The dilution factor equals the pressure in exhaled air divided by the pressure in the original air.) This is because the residual volume of the lung (the volume not included in maximum inspiration and expiration) dilutes the gas. So, the larger the dilution factor, the bigger the residual volume compared to the measured inspiratory volume.
Patients 2 and 3 have the same inspiratory volume, but patient 2’s dilution factor (written on the right here) is larger. Therefore, he likely has a greater residual volume than patient 3.
Patients 1 and 2 have the same dilution factor, so they have the same ratio of total lung volume to inspiratory volume. However, patient 2 has a greater inspiratory volume.
This means that everything is proportionally bigger for patient 2 – he has a greater total lung volume and residual volume.
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[each_answer] => A. Patient 1
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[each_answer] => B. Patient 2
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[each_answer] => C. Patient 3
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[each_answer] => D. Patients 2 and 3 both have the largest
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Figure 1
