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Official Guide to MCAT ...Discrete

Discussion in 'MCAT Study Question Q&A' started by SaintJude, May 1, 2012.

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  1. SaintJude

    SaintJude

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    SDN Members don't see this ad. (About Ads)
    No. 35.

    During the respiratory cycle, contraction of the diaphragm causes:

    A. exhalation
    B. elevation of the ribs
    C. the intrapleural pressure to increase
    D. the intra-alveolar pressure to decrease.

    Answer: D
  2. MedPR

    MedPR

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    You inhale air because the diaphragm contracts and elongates (increases the volume of) the thoracic cavity. Increased volume = decreased pressure and fluid flows from higher pressure to lower pressure.

    A. Wrong because relaxation of the diaphragm causes exhalation (see explanation above).
    B. Wrong because the external intercostals cause the ribs to elevate. Also, if I'm not mistaken, technically the rib cage expands, not elevates.
    C. Wrong. If intrapleural pressure increases (too much), the lungs collapse. In a healthy individual, the intrapleural pressure is always less than the alveolar (intrapulmonary) pressure. Think pneumothorax.

    Edit: Also, this tripped up a lot of the people in my physio class. Is the diaphragm smooth muscle or skeletal muscle?
    Last edited: May 1, 2012
  3. chiddler

    chiddler

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    Can you explain the difference between C and D?

    How does interpleural pressure change as breathing occurs?
  4. SaintJude

    SaintJude

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    Darn, you're right. I was struggling b/w B & D. It is the external intercostal muscles that elevate the ribs, but I thought they were only employed during strenuous activity??

    Intrapleural volume increase. Boyle's law dictates that volume is inversely proportional to pressure. So as a result of the volume of the thoracic cavity increasing, intrapleural pressure will decrease.
  5. MedPR

    MedPR

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    The intraalveolar pressure is the pressure in the alveoli (the air sacs). If you decrease the pressure in there, then atmospheric pressure "pushes" air into your lungs just like when you "suck" something into a straw.

    Immediately surrounding your lungs is visceral pleura, then serous fluid (in the pleural space), then parietal pleura, then your chest wall. The intrapleural pressure is the pressure within the pleural space. As you increase this pressure, you apply more pressure onto the lung and eventually cause it to collapse.

    If that doesn't make sense, here's a simpler analogy. Think about an ink pen and how there is the outer plastic ring (the thing you hold while you write), then there is a space, then there is the outer plastic part of the ink thing, then the ink, then the plastic part of the ink thing, then a space, then the outer plastic ring again.

    The space between the palstic ring (that you hold) and the plastic part of what encloses the ink is analogous to the intrapleural space. The plastic ring = your chest wall, and the plastic part of the ink = the outer layer of your lung. What would happen if you increased the pressure in that space? The plastic ink thing would be crushed.
  6. SaintJude

    SaintJude

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    Is the diaphragm a smooth muscle??? Kaplan says inspiration isn't an active process usually...

    I can't believe intra-alveolar pressure decreases

    Edit: I guess it makes sense since it's the pressure gradient that leads to diffusion of oxygen. The pressure in the alveoli has to be relatively lower than the trachea, so oxygen diffuses from the trachea to the alveoli.
  7. MedPR

    MedPR

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    I'm not sure about the strenuous activity part, but I think you are correct.

    The terminology is beyond the MCAT, but the intrapleural volume does not increase when you inhale. The intrapleural volume is actually shrinking as your lungs expand, thereby increasing the intrapleural pressure and causing your lungs to shrink again (exhalation).
  8. MedPR

    MedPR

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    It's skeletal. Inspiration isn't usually an active process because you breathe without thinking about it. However, if you wanted to breathe (or not breathe) you could easily "tell" your diaphragm not to contract. Compare this to your heart beating. You can increase your heart rate by breathing harder or exercising or whatever, but you can't "tell" your right atrium to stop or start pumping. It just happens.
  9. chiddler

    chiddler

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    i understand what you are describing with your analogy but i don't understand how to look at it in terms of physio.

    so let me be more specific with my question

    1. As we breathe, does the pleural pressure remain constant or does it change?

    2. I don't understand how alveolar pressure changes. Shouldn't it be more or less constant since gases exchange via diffusion, not really changing their concentration? Specifically total pressure stays the same. Partial pressures of the gases changes though for sure.. But the answer doesn't indicate this. Just says pressure.
  10. MedPR

    MedPR

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    1. Intrapleural pressure changes as you breathe. When you inhale, intrapleural pressure increases and intrapulmonary (alveolar) pressure decreses. Opposite when you exhale.

    2. In order to get air into and out of the alveoli, there must be a pressure gradient with the atmosphere. When you inhale you are creating a vacuum within your alveoli. By decreasing the alveolar (intrapulmonary) pressure, you allow air to flow into your lungs. When you reach the end of your inhale, intrapulmonary pressure (alveoli) equals atmospheric pressure, but now your intrapleural pressure is higher than both atmospheric and intrapulmonary and it is pressing onto your lungs, thus making you exhale.
  11. chiddler

    chiddler

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    Isn't contraction of diaphragm = inhaling?
  12. MedPR

    MedPR

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    Yes.
  13. SaintJude

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    Yes, & what is making you think that negates what MedPr is saying?
  14. chiddler

    chiddler

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    "contraction of the diaphragm causes"

    "C. the intrapleural pressure to increase "

    in OP

    medpr wrote

    "When you inhale, intrapleural pressure increases"

    but C is wrong.
  15. ezsanche

    ezsanche

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    Intrapleural pressure decreases during insipiration. Intrapleural pressure increases during expiration.

    Intrapleural pressure is always less than alveolar pressure unless during forced expiration. Here is a link that shows you the ventilation cycle and how each thing changes during inspiration and expiration.
    http://www.google.com/imgres?q=vent...13&start=16&ndsp=22&ved=1t:429,r:7,s:16,i:126
    Last edited: May 1, 2012
  16. DrRichand1

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    don't forget about the elasticity of the lung, this also creates pos pressure when exhaling. The intraplural fluid pressure is needed to keep the lungs from collapsing on themselves when exhaling. Hence a colapsed lung when this area is disturbed by unwanted chest stabbings.
  17. MedPR

    MedPR

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    The diaphragm contracting causes intrapulmonary pressure to decrease, which causes the lungs to fill, which causes the lungs to expand, which causes the intrapleural pressure to increase.
  18. SaintJude

    SaintJude

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    According to Kaplan, intrapleural volume does increase when as you inhale--that's what accounts for the decrease in intrapleural pressure.

    Maybe it's a simplification, but Kaplan must be explaining this way b/c it gets you the right answer.
  19. chiddler

    chiddler

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    I want to talk more about #2 as well.

    Air flows. K. But what causes gas exchange? Is it not that the partial pressure gradient of the gases? Gas exchange is not caused by simply changing the pressure in our lungs - the purpose of that is to bring in new air. But actual blood-air gas exchange is due to difference in partial pressures.
  20. ezsanche

    ezsanche

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    That is pretty much what you need to know for the MCAT. Any additional information will most likely be provided to you in a passage.
  21. chiddler

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    yes i think medpr, you made a mistake. As lungs inhale, pressure becomes low which would cause the intrapleural space to try to increase. This tendency would decrease pressure since P α 1/V.

    mistake in explanation not in answering the question anyway.
  22. MedPR

    MedPR

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    Ah yea, I did have that confused. Pleural pressure decreases during inspirateion.

    Sorry for the confusion :/.
  23. chiddler

    chiddler

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    alrighty. lets talk about #2
  24. SaintJude

    SaintJude

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    Not so fast, the OP is a discrete from AAMC's official guide !! And just knowing that didn't get me the answer!

    So although the intra-alveolar pressure decrease, it is still relatively higher pressure than that of the intrapleural pressure, correct?
  25. MedPR

    MedPR

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    Gas exchange because of the composition of the blood in the capillaries near the alveoli. Deoxygenated blood from the right atrium passes through those capillaries. The deoxygenated blood has a high PCO2 and a low PO2, while the alveoli have a high PO2 and a low PCO2, so everybody moves down its pressure gradient.
  26. MedPR

    MedPR

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    Yea, the intra-alveolar should always be higher than intrapleural. Both of them decrease when you inhale. I made a mistake above. This information is beyond the MCAT though.

    All you had to know for the question was that contracting the diaphragm increases thoracic volume; increasing volume = decreasing pressure; and fluid flows from high pressure to low pressure.
  27. MedPR

    MedPR

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    Thank you for the correction. I have an essay exam on pulmonary and digestion on Thursday!
  28. ezsanche

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    Yes gas exchanges is dependent on the alveoli and blood interface. Gas will diffuse and equlibrate at different partial pressure concentration.
  29. chiddler

    chiddler

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    ok i need help understanding lung anatomy!

    so given this, the yellow portion is exposed to the outside environment or is that part of our innards?
  30. ezsanche

    ezsanche

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    This question was a little bit tricky.

    But yes you are correct although alveolar pressure decreases during inspiration it does not decrease more than intrapleural pressure which is at a much more negative pressure.

    What would happen if alveolar pressure was less than intrapleural pressure? The airway would collapse.
  31. SaintJude

    SaintJude

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    Ahahah--what is that?? Here, please let me help you Can you rephrase your question...

    .hahaha--chiddler I will give you 100 bucks to quite the road to med school, enroll in art school and become a famous computer renditions artists

    [​IMG]
  32. chiddler

    chiddler

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    LOL that was supposed to be an alveolus

    I use to teach middle school kids. sometimes i'd lose their attention as they stare at my drawing. "what are you guys staring---oh....". and then immature giggles...
  33. MedPR

    MedPR

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    The lung would collapse, not the airway.
  34. ezsanche

    ezsanche

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    The lung will definitely collapse but will the airway remain patent?
  35. MedPR

    MedPR

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    Maybe my definition of airway is wrong. When I think airway, I think trachea. The trachea isn't in the pleural cavity, the pleural pressure shouldn't affect it. Even if it was in the pleural cavity, wouldn't the c-rings keep it open?
  36. MedPR

    MedPR

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    Alveoli are considered external because when air gets to them it has not yet crossed any membranes. Gas exchange at the alveoli is called external respiration.
  37. chiddler

    chiddler

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    did own research i've answered my question.

    ty
  38. SaintJude

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    Just wanted to get on record to say I was wrong about this: Inhalation is an active process and actually always involves the external intercostal muscles and contraction of diaphgram.

    It's actually exhalation, a passive process, that can be sped up by using the internal intercostal muscles during highly active task/strenuous activity. Didn't want anyone to get misled in future possibly.
  39. Morsetlis

    Morsetlis SGU MS-3

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    Air flows down a pressure gradient. Thus, you must either make your thorax more negative compared to atm, or make the incoming air more positive (ventilator/ambubag) in order to inhale.

    Yes, you could also go learn respiratory physiology but why not just stick to common sense ;p

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