Pressure regulation and fluid dynamics of the respiratory sy...
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Pressure regulation and fluid dynamics of the respiratory sy...
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[post_date] => 2025-01-09 07:56:06
[post_date_gmt] => 2025-01-09 12:56:06
[post_content] => Practice Passage (Question 1-5)
*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.
During breathing, the body directs the flow of oxygen and nitrogen via coordinated motion of the diaphragm and lungs. In particular, during inhalation the diaphragm muscles are used to increase the volume of the lungs, leading to air intake in a manner analogous to the operation of a piston. During exhalation, the diaphragm muscles undergo an almost-reverse process, decreasing the volume of the lungs and pushing air back out of the body. A simple schematic of the respiratory system is shown in Figure 1.
During inhalation, the alveoli, capillary-rich terminals of pulmonary veins, expand as well, increasing the surface area of the capillary-air interface in order to more effectively accept oxygen into the bloodstream. The key mechanism by which this process occurs is gaseous diffusion, in which carbon dioxide diffuses out of de-oxygenated blood through a thin membrane, while oxygen diffuses into the blood. This process is aided by the high solubility of carbon dioxide in blood, which is nearly 20 times that of oxygen.
The dynamics of the repeated expansion and contraction of the alveoli during respiration are governed by the control of pressure provided by the diaphragm and the elastic response of the alveoli, which passively contract during exhalation without the need for active muscle stimulation. Rather, the diaphragm serves to actively modulate the pressure within the respiratory system, the lungs serve to transmit these pressure differences to the alveoli, and the alveoli respond to these pressure changes in a manner analogous to balloons. Chronic emphysema occurs when the tissues comprising the alveoli lose their elasticity, resulting in incomplete re-compression during exhalation that manifests as labored breathing.
Figure 1: A simplified diagram of the respiratory system.
[post_title] => Pressure regulation and fluid dynamics of the respiratory system
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[question] => Which of the following calculations gives the most accurate ratio of the diffusion rates of carbon dioxide molecules to oxygen molecules in the alveoli?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
The diffusion rate of a gas in a liquid is directly proportional to its solubility and inversely proportional to the square root of its mass.
The mass of a carbon dioxide molecule is
14amu + 2*16 amu= 44 amu, and the mass of an oxygen molecule is
2*16 amu = 32 amu.
Even without doing the calculation, only two answers are within a factor of 10 of the correct answer
The ratio of the diffusion rates is 
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[answers] => Array
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[0] => Array
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[each_answer] => A. 10
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[1] => Array
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[each_answer] => B. 1/20
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[2] => Array
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[each_answer] => C. 
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[3] => Array
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[each_answer] => D. 20
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which of the following principles governs the rate at which air enters and exits the lungs in response to the motions of the diaphragm?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
Air enters and exits the lungs due to the mechanical motion of the diaphragm inducing changes in the volume of the lungs
Sudden lung volume increases lead to pressure decreasing in the lungs relative to the air outside the body, leading to a pressure gradient that draws in air.
The inverse proportionality between pressure and volume is derived from the ideal gas law.
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[answers] => Array
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[0] => Array
(
[each_answer] => A. Graham’s Law
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[1] => Array
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[each_answer] => B. Fick’s Law
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[2] => Array
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[each_answer] => C. Henry’s Law
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[3] => Array
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[each_answer] => D. The ideal gas law
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Which of the following properties increases during exhalation?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
Intermolecular forces determine macroscopic properties like temperature and pressure, not vice-versa.
The solubility is independent of the amount of dissolved substance.
Pressure involves the frequency at which particles of a gas hit the walls of a container.
At high pressures, oxygen molecules dissolve more readily because they hit the blood-air interface more frequently.
The effective solubility of oxygen increases during exhalation due to increased pressure in the lungs.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The surface area of the alveoli into which oxygen can diffuse
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[1] => Array
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[each_answer] => B. The amount of oxygen in the bloodstream
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[2] => Array
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[each_answer] => C. The intermolecular forces acting on individual oxygen molecules
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[3] => Array
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[each_answer] => D. The solubility of oxygen in the bloodstream
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[3] => Array
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[quiz_unique_key] => 2261298308
[question] => Which of the following tissues mechanically stores and releases potential energy during respiration?
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[answer] => 1
[description] => Reason for the Correct Answer:
All living tissues chemically store potential energy, the question specifically asks about mechanical potential energy storage.
The two archetypal examples of potential mechanical energy are in the position of heavy objects in gravitational fields, and in compressed springs. Gravity is unimportant in this problem.
As stated in the passage, the diaphragm expands and contracts due to coordinated muscle movements, whereas the alveoli expand and contract due to pressure imbalances.
The alveoli store potential mechanical energy in the form of elastic energy during inhalation, and release it when they compress again during exhalation.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The alveoli
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[1] => Array
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[each_answer] => B. The diaphragm
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[each_answer] => C. The lungs
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[each_answer] => D. The respiratory tract
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[quiz_unique_key] => 574431310
[question] => Which of the following additional factors would reduce the rate at which oxygen molecules diffuse into the blood on the surface of the alveoli?
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[answer] => 1
[description] => Reason for the Correct Answer:
This question requires considering the total amount of time that oxygen molecules spend near the surface of the alveoli, which affects how readily they can be taken up.
Increased surface area would increase the amount of oxygen that can be taken into blood with each breath.
A thermal gradient would push molecules towards lower temperature areas, increasing the opportunities for oxygen molecules to be taken up into blood.
Gravitational effects are negligible over the length and mass scales found in biological systems, but, if anything, they would increase the pull of oxygen into blood.
Turbulent flow reduces the efficiency with which oxygen can be pulled into blood
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[answers] => Array
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[0] => Array
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[each_answer] => A. Turbulence disturbing flow along the surface
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[1] => Array
(
[each_answer] => B. Lower temperatures near the cells
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[2] => Array
(
[each_answer] => C. Gravitational effects pulling molecules towards the surface
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[3] => Array
(
[each_answer] => D. Additional increase in the capillary-rich surface area exposed to air in the alveoli
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Figure 1:
