Question 30

[McatCrusher]

Hi,
I think the correct answer here should be C. Solution says B but it does not make sense as thoracic actually gets smaller as diaphragm goes up!!!!I dont know...thats how i got the answer...

[admin]

The correct answer is B.

Clearly, using CPR is not natural so we have to work this out a bit. If you apply pressure to the abdomen upward, I think you will agree that this would lead to the diaphragm rising.

If there was no obstruction (blockage) of the airway, then air would flow out like water through a hose. But, because there is an obstruction, it is like someone standing on a soft hose, the hose will increase in volume because of the added pressure. The pressure rises until the obstruction is removed. Then we get a normal expiration with the volume of the chest cavity (thoracic) being reduced.

[jgeng03]

I still don't understand the explanation. I too think the correct answer is C. The water hose you were saying gets a larger volume because are you increasing pressure on the outside but there is even more pressure on the inside of the hose from the water rushing in. In this problem you are only applying external pressure but the amount of air (the water in the hose) is constant. Can you please help me understand this problem?

[admin]

Granted that the water/hose analogy is flawed because there is a source of more water. OK, let's move it to a balloon. Imagine a soft balloon with air inside (we are graduating from the water stuff!). Draw somewhere in the middle of the balloon a circumferential line (the balloon's "waist") such that the chest is above and the abdomen is below the line. Apply pressure from below (abdomen) thus the upper part gets more voluminous. Why? because the balloon is tied = the obstruction. Release the obtruction, the "chest" will decrease in size as the balloon "expires." (of course, for humans, when the diaphram rises, it is the liver, spleen and bowel which rise to occupy space in the thoracic cavity but the effect is similar to the balloon analogy)

The thoracic cavity MUST increase in size because the diaphram is moving up, occupying space but the air has nowhere to go yet. Once critical pressure has been obtained then the foreign body is dislodged and the "balloon" deflates. Of course, the chest wall is not anywhere as flexible as a balloon so of course the terms we are using are relative.

Analyze the reverse for a moment. If the diaphram went up, what mechanism explains the thorax getting smaller before an expiration? The only reason that physiologically your thorax decreases in size is precisely because you are expiring thus volume of air decreases as it leaves your body. That can't happen while the obstruction is still there.

[bjpatel2178]

The pressure changes, but the overall volume should not change. It is a closed control volume (until the food becomes dislodged of course). It may change shape but the overall volume should not change! It is the pressure which changes.

Re: the balloon example

If you squeeze a balloon the volume of that balloon doesn't change it still contains the same amount of gas, it just changes shape. Unless you are considering the gas's being compressible...

Please help me understand this also.

Thanks!!

[asfi]

so the only reason why the thoracic size increase is because something is dislodging the airway which is keeping the volume constant until it gets dislodged?

[StoneColdPS14]

the correct answer is indeed C.
So a particle is dislodged in the air way, blocking it. This means the air is not getting in or out. The whole point of Heimlich Manuver is to create pressure within the lungs and thoracic cavity in general so that this increased air pressure would push the object(obstructing the airway) out. Ask youself: Applying pressure to a closed gas system will do what?.... the obvious answer is that you will compress it. Its common sense. If you were to increase the volume of a closed system you would invariably lower the pressure and the stuck object would actually be pulled into the airway.

[mcat_premed3832]

The previous response has the following incorrect assumptions: (a) the thoracic cavity is an inflexible container; (b) upward abdominal thrust does not push bowel into the chest cavity thus reducing abdominal volume -> increasing thoracic volume until a maximum is achieved at which point pressure increases.

Consider:
(a) upward thrust -> (b) increased thoracic pressure -> (c) object dislodged

Let's consider what happens between (a) and (b): as the abdominal contents enter the thoracic cavity, lungs are compressed to a degree, the volume of the thoracic cavity increases to its maximum, now all the thrust is transmitted as increased pressure.

The correct answer is B.

[maxsati7049]

I was confused on that question for a while...I thought it was C too...but then when I read the discussion here and thought about it for a bit, it made sense. The balloon analogy works out. If you imagine a gas filled balloon with it's apex (tip) pointing up and tied up so air won't escape initially...then what would happen if you push on the bottom of the balloon and then open the balloon to the external environment? the air inside will rush out because of the applied pressure...if you apply that to the lungs, then the foreign particle will just pop outta the trachea or wherever it was stuck in...

[noobcat]

This thread is so frustrating that I actually went through forum registration just to post here.

C is the correct answer. Here's my reasoning:

We all know that our lungs expirate passively, meaning that air is pushed out by the recoil of the chest cavity after the cage muscle and diaphragm relax. How does the recoil push the air out exactly? By exerting pressure on the lungs and thereby reducing the volume of the lungs. So logically, it makes sense that to breathe out any air, the size of the lung/cavity must be smaller than before.

The same conclusion is reached with the ideal gas law, PV=nRT. First, we shall agree that the upward thrust increases the pressure inside the lungs to expel the food. If you increase the pressure P, V must go down for the ideal gas law to hold. This is consistent with passive expiration. Let's just say for some magical reason the upward thrust increases the lung volume (as in choice B). Pressure in this case would actually have to go down. In other words, P and V cannot increase at the same time.

[mcat_premed3832]

You correctly applied the relationship with PV and you correctly described what happens in passive respiration but the problem is your conclusion is not what the answer is saying.

"Let's just say for some magical reason the upward thrust increases the lung volume (as in choice B)."

This is not what answer choice B is suggesting (thoracic volume is very different from lung volume). In fact, I agreed with you above, that the lung volume must decrease then pressure increases until the object is dislodged. But how do you you decrease lung volume with someone who is not breathing? Keep in mind, the lung is changing volume, but it's not in a hardened container. It's in a cavity that can increase to a degree or decrease to a degree. The question is, with abdominal contents suddenly being pushed into the chest, what will the chest cavity naturally do? Increase to a degree.

Oh, about the passive breathing model: as you mentioned passive breathing (expiration for example) means that lung volume decreases as the diaphragm relaxes and the thoracic cavity reduces in size ("recoil"); but, if you can't breathe then those events cannot occur in the natural way. So this manoeuvre forces the diaphragm up, pushes abdominal contents into the chest with the objective to reduce lung volume, but as a consequence of this violent act, the thoracic cavity gives way to its maximum size until the increased thrust is only transmitted to increase pressure in the "collapsing" lung.

[study-hard]

[mcat_premed3832]

The correct answer is still B!

Note the quote from the Explanation: "The events in expiration normally include decreasing the size of the thoracic cavity and relaxation or raising of the diaphragm (BIO 12.4)."

That is "normal" expiration. Having someone apply pressure to your abdomen resulting in your bowels/stomach/liver being pushed up into your chest cavity does not constitute "normal" expiration. The details are described above as to how this increased "stuff" being pushed in the chest leads to increased chest volume (increased thoracic cavity) followed by/in concert with compression of the lungs leading to forced expiration.

The real MCAT loves to take something you know well, and then twist one thing about it to see if you can make the adjustment on your feet. If that's all you get out of this question, then that's more than good enough.

[quadalpha]

So how is "thoracic volume" defined? Is it the volume enclosed by the diaphragm, or the volume enclosed by an imaginary plane somewhere below the ribs?

If it is the latter (imaginary plane), then the argument that your abdominal contents enter the thorax and increases its volume would make sense.

If it is the former (enclosed by diaphragm), then the only way you can get increased pressure to remove the blockage is by decreasing the volume. While it may look like the volume is increasing because the chest rises, the overall volume must decrease for there to be any increase in pressure.

mcat_premed's argument from 15-Jul seems to imply that lung pressure (which must increase) is separate from the pressure in the cavity between the lungs and the chest wall, which is then supposed to enable the thoracic volume to increase while still increasing the lung pressure. A diagram will show the impossibility of this notion.

Of course, all this rests on the definition of thoracic volume.

[mcat_premed3832]

You put your finger on the issue. During the discussion above the words used in the question became something else. The issue is much more simple: let's try an experiment! Take your fist and hand and perform an auto-Heimlich but not too fast! If you try it on yourself, what would you say happens to your thoracic size? Bigger? Definitely.

The question never asked about thoracic volume (that came up in the discussion in this thread and was never clarified).