# AAMC FL2 CP #25

6

#### 663697

I am aware of the Venturi effect where velocity is increased in areas of lower pressure (ie. the opening in the figure). The answer mentions static pressure and I'm not sure how this plays into the question. Why doesn't diffusion explain the air being drawn in?

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#### Nugester

2+ Year Member
I think it is referring to static pressure of the air outside the mask as air is a fluid. The person causes a lower or "negative" pressure within the mask in respect to ambient air, hence air rushes into the opening and mixes with the oxygen from the tank.

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6

#### 663697

I think it is referring to static pressure of the air outside the mask as air is a fluid. The person causes a lower or "negative" pressure within the mask in respect to ambient air, hence air rushes into the opening and mixes with the oxygen from the tank.
That was pretty much how I thought about it when answering the question. I thought that diffusion just means the air flowed down its pressure gradient similar to the mechanism of negative pleural pressure in the lungs drawing air in. Is diffusion just the wrong word here?

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#### aldol16

5+ Year Member
That was pretty much how I thought about it when answering the question. I thought that diffusion just means the air flowed down its pressure gradient similar to the mechanism of negative pleural pressure in the lungs drawing air in. Is diffusion just the wrong word here?

Diffusion is generally the spreading of a substance from a location where it is highly concentrated to an area where it is less highly concentrated. You could make a case for the air being 'concentrated' at high pressure and less 'concentrated' at lower pressures but usually when we talk about diffusion, we don't really talk about pressure differentials. I usually like to imagine a fart as an example of diffusion. You fart and if the air is stagnant, the smell only very slowly spreads. You can actually use the diffusion equations to calculate the rate of spread of the air. Usually, it's really tiny - the fact that we observe it spreading at all means that there are more forces than only diffusion at play. But anyway, the 'best' answer in this case is the more specific one.

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#### akimhaneul

2+ Year Member
Diffusion is generally the spreading of a substance from a location where it is highly concentrated to an area where it is less highly concentrated. You could make a case for the air being 'concentrated' at high pressure and less 'concentrated' at lower pressures but usually when we talk about diffusion, we don't really talk about pressure differentials. I usually like to imagine a fart as an example of diffusion. You fart and if the air is stagnant, the smell only very slowly spreads. You can actually use the diffusion equations to calculate the rate of spread of the air. Usually, it's really tiny - the fact that we observe it spreading at all means that there are more forces than only diffusion at play. But anyway, the 'best' answer in this case is the more specific one.

Is the answer Venturi effect because inhaling oxygen creates a higher velocity and thus lower pressure, allowing the static air to flow in?

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#### babyoreo

I am aware of the Venturi effect where velocity is increased in areas of lower pressure (ie. the opening in the figure)

Think of it the other way. Fluids that flow at higher velocities are lower in pressure.

Aldol16 did a good job explaining diffusion. I would emphasize that diffusion deals with concentration gradients, which often involves a solute, and the driving force behind diffusion is a concentration gradient (differences in concentration; eg, oxygen atoms diffusing across the capillary wall from high to low concentrations). You are talking about fluid pressure gradients, which simply results in fluid flow.

Is the answer Venturi effect because inhaling oxygen creates a higher velocity and thus lower pressure, allowing the static air to flow in?
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Yes. Oxygen is flowing from the tube through the mask into the mouth. Because the velocity of the air in the mask is greater than that of the ambient air, the pressure inside the mask is lower (this is known as the Venturi effect). Air flows from high to low pressures; the static air flows in to the mask.

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8

#### 865558

The AAMC is actually wrong here: Venturi masks actually work through the Bernoulli principle and constant pressure jet-mixing.
"Venturi masks entrain air by the Bernoulli principle and constant pressure-jet mixing.10 This physical phenomenon is based on a rapid velocity of gas (e.g., O2) moving through a restricted orifice. This action produces viscous shearing forces that create a decreased pressure gradient (subatmospheric) downstream relative to the surrounding gases. The pressure gradient causes room air to be entrained until the pressures are equalized."

Air moving down a " decreased pressure gradient downstream relative to the surrounding gases" is best described as DIFFUSION.

Source: Benumof and Hagberg's Airway Management (Third Edition)

#### coooolllbeEAns

The AAMC is actually wrong here: Venturi masks actually work through the Bernoulli principle and constant pressure jet-mixing.
"Venturi masks entrain air by the Bernoulli principle and constant pressure-jet mixing.10 This physical phenomenon is based on a rapid velocity of gas (e.g., O2) moving through a restricted orifice. This action produces viscous shearing forces that create a decreased pressure gradient (subatmospheric) downstream relative to the surrounding gases. The pressure gradient causes room air to be entrained until the pressures are equalized."

Air moving down a " decreased pressure gradient downstream relative to the surrounding gases" is best described as DIFFUSION.

Source: Benumof and Hagberg's Airway Management (Third Edition)

You are right Vanturi masks do work based on Bernoulli principle but the Venturi Effect explains HOW this principle operates. I do not recall exactly what the Vanturi principle states word for word but to sum it up its basically saying that an increase in velocity due to decrease in the cross sectional area will also result in the decrease of pressure. Look at the mask, the cross section of the hose attached to the oxygen reserve is small, velocity of that oxygen is HIGH but pressure is significantly lower. Using that in conjecture with how gasses follow pressure gradients (via diffusion) basically what the Venturi mask does is introduce a new gas into the system (the mask) through a small hole open to the environment with a pressure that is higher than the pressure created inside the mask. Because the pressure of the air/oxygen outside the mask is higher than that of the dynamic (flow) pressure inside the mask, air mixes with the oxygen inside the mask via the process of entertainment. (yes that is what it's called ‍♀️).

While Bernoulli's Principle P(i) + 1/2pv(i)^2 = P(f) + 1/2pv(f)^2 *** is applied. And this is the most confusing about the solution they gave: Pair=Pstatic + 1/2rv^2. Anyone looking at that would be confused, like what the f happened to the other 1/2v^2 on the other side? Well that is because they assume that we know that this mask works similarly to a manometer or that we understand stagnation pressure/total pressure of a flowing fluid. Idk about you guys but I do not remember any of this from physics.

*** (yes this is correct, there is no "pgh" because there is no height difference)

Here is more on that dumb ass equation if you care to read about it:

Another way to conceptualize that equation is like this: Stagnation Pressure = Pstatic + Pdynamic
Or even Ptotal = Pstatic + Pdynamic

Stagnation pressure/ total pressure is the pressure of a fluid that would be exerted "if" the flow was brought to a stop without loss of mechanical energy. Or rather the point where the "dead spot" occurs, that is an obstacle is in the way causing flow to divert around the object w/o disturbing the flow itself. It is a localized, reduction of flow. There would still be pressure at that spot but not due as much to the velocity of the fluid. In a way this is also "static" when it comes to a stop but the difference is that this is also the TOTAL not just the pressure of the static gas. (this is the most confusing part of this question for me)

Static pressure of a gas is the pressure that a fluid has when it is either stationary or is measured while moving with the flow or "among" the steady flow. This is typically the gauge pressure. It is the pressure that is experienced into "every direction".

Dynamic Pressure of a gas is the pressure of the gas where the gas is moving, that is it is the representation of the kinetic energy of a fluid while it is flowing. WHY the 1/2pv^2 is used as it's notation. This is the difference between the "total" pressure of a fluid and the completely static version of it.

ANYWAY back on topic. The Venturi mask effect works and explains the modified Bernoulli's principal equation question because the total pressure of air (or stagnant pressure or even total pressure of oxygen) was greater than the pressure of the static, steady flowing oxygen in the mask.

Idk if you can tell but I really obsessed over this one, because pressures are just the mf worst.

#### ScrubswithnoSleeves

##### SDN Gold Donor
2+ Year Member
Just found this because I got this exact question wrong for the exact same reason.

Now re-reading the question with a little more attention to detail, I think it is important to note that they ask what causes "static air" to be drawn into the mask rather than what causes O2 to be drawn into the mask (thus highlighting that they are talking about the fluid as a whole rather than molecules of O2).

Its a lil dirty by the ol' AAMC in my opinion to put 2 answer choices like that in this question, but whatever, we adapt to their rules they don't adapt to us.

#### Mr Donald Mouse

2+ Year Member
The velocity of static air is zero and the Venturi effect increases that velocity so that previously static air could move and be diffused into the body.

#### pretty_positron

I got this one because of a friend demonstrating this effect for me using a straw wrapper which he blew under and asked us if it will be pulled under or over. I wrote an explanation here because the AAMC explanation doesn't make much sense to me.

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#### ScrubswithnoSleeves

##### SDN Gold Donor
2+ Year Member
Following up after a lil more review of this, just know that the venturi effect essentially states that velocity and pressure have an inverse relationship. Thus in this example, when our man in the image breathes in, the air in the tube moves with a higher velocity and thus the pressure in the tube decreases to less than the pressure of the static air surrounding the mask. This pressure differential (Pmask<Pair) causes air to flow into the mask.

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#### pretty_positron

Following up after a lil more review of this, just know that the venturi effect essentially states that velocity and pressure have an inverse relationship. Thus in this example, when are man in the image breathes in, the air in the tube moves with a higher velocity and thus the pressure in the tube decreases to less than the pressure of the static air surrounding the mask. This pressure differential (Pmask<Pair) causes air to flow into the mask.

Yes, see my explanation above. It all comes from Bernoulli's equation which is basically just conservation of energy applied to ideal fluids.

#### letsgetstarted1234

2+ Year Member
following up on this.

the concentration of other non oxygen gas molecules would be lower in the mask than outside right?

like nitrogen.

so wouldnt nitrogen flow?

but the pressure is a graeter contributor so we go with the best answer.

#### thewarcow

I am aware of the Venturi effect where velocity is increased in areas of lower pressure (ie. the opening in the figure). The answer mentions static pressure and I'm not sure how this plays into the question. Why doesn't diffusion explain the air being drawn in?
This is a really simple question and you guys are over complicating it. The Venturi affect states that the pressure is DECREASED WHEN IT FLOWS THROUGH A CONSTRICTED SECTION. The constricted section is the tube. When you decrease pressure to a point of negative pressure, what happens? This is basic, things gets sucked in. Thereby, the patient in this case can suck in the oxygen. Hence why its the Venturi effect. Dispersion makes no sense. Diffusion also makes no sense. There is no talk about concentration differences. You can't assume that. Also, you don't have to know the difference between static and dynamic pressure. You need to reason this question out. A lot of these above comments make it seem too hard. You have to know this and that. Not true. You don't have to know anything on the mcat. The material on the mcat is really basic, its the reasoning and applying the basic material that they are testing. Not fancy stuff.

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