Respiration - Please Help

[Rolling]

So I'm reading the Kaplan Book and it says the following

Inhalation
The chest wall/thoracic cavity expand causing the intrapleural space to increase in volume, thereby causing a subsequent decrease in pressure in the intrapleural space. As a result, it says that results in a relatively lower pressure in the intrapleural space than the lungs, and air will run into the lungs.

Exhalation
The diaphragm relaxes, and the chest wall contracts, causing the intrapleural pressure to increase, because the volume of the intrapleural space decreases. As a result it says the pressure of the intrapleural space will exceed the pressure of the alveoli (which is at atmospheric pressure) and will push the air out of the lungs.

I've been very bothered by this explanation and find it odd on all counts. As a result, I've read a lot of information online, and NONE of it suggests that the intrapleural pressure exceeds atmospheric pressure, infact it is stated that the intrapleural pressure will NEVER exceed the alveoli because that would cause the alveoli to collapse.

Can someone explain to me how atmospheric pressure, alveolar pressure, and intrapleural pressure are all related and change during exhalation/inhalation? I feel so confused now.

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[Silverfalcon]

Hint: Consider Ideal Gas Law and movement of pressures.

 

[Charles_Carmichael]

Think about PV = PV.

If the volume inside the thoracic cavity increases, the pressure decreases accordingly. This pressure will then be lower than atmospheric pressure and air will flow into the lungs.

When exhaling, you're decreasing the intrathoracic volume. This decrease in volume increases pressure and the pressure becomes greater than atmospheric pressure. So, air flows out of the lungs.

What helps me conceptualize this stuff is PV = PV and recognizing that gas flows down the pressure gradient (ie. gas flows from areas of high pressure to areas of low pressure).

Hope this helps.

 

[Rolling]

I understand all of that. However, as I said, the Kaplan book claims that the INTRAPLEURAL pressure exceeds atmospheric pressure, which simply cannot be true.

Again, I specifically want to talk about the intrapleural pressure, alveolar pressure, and atmospheric pressure during inhalation/exhalation.

 

[Charles_Carmichael]

I understand all of that. However, as I said, the Kaplan book claims that the INTRAPLEURAL pressure exceeds atmospheric pressure, which simply cannot be true.

Again, I specifically want to talk about the intrapleural pressure, alveolar pressure, and atmospheric pressure during inhalation/exhalation.

It is true that intrapleural pressure exceeds atmospheric pressure during exhalation (it's less than atmospheric pressure during inhalation).

The IP pressure is not going to be lower/greater than atmospheric pressure by a TON. It'll be a few cmH2O lower/higher. And that pressure gradient is enough to drive inhalation/exhalation.

Again, it all has to do with PV = PV:

As the volume of the thoracic cavity increases, the IP and alveolar pressure decreases. This creates a pressure gradient that drives air into the lungs. During exhalation, when the volume of the thoracic cavity decreases, the IP and alveolar pressure increases, creating a pressure gradient that drives air out of the lungs.

 

[Rolling]

Taken from Wikipedia:

If 'transpulmonary pressure' = 0 (alveolar pressure = pleural pressure), such as when the lungs are removed from the chest cavity or air enters the pleural space (a pneumothorax), the lungs collapse as a result of their inherent elastic recoil. Under physiological conditions the transpulmonary pressure is always positive; pleural pressure is always negative and relatively large, while alveolar pressure moves from slightly positive to slightly negative as a person breathes.

Again, intrapleural pressure is ALWAYS below atmospheric pressure and always below intralveolar pressure. Your diagram shows the exact same thing, so my question is how does intrapleural pressure actually affect inhalation/exhalation?

If intrapleural pressure does increase during exhalation, why does that cause alveolar pressure to increase?

 

[Charles_Carmichael]

Taken from Wikipedia:

If 'transpulmonary pressure' = 0 (alveolar pressure = pleural pressure), such as when the lungs are removed from the chest cavity or air enters the pleural space (a pneumothorax), the lungs collapse as a result of their inherent elastic recoil. Under physiological conditions the transpulmonary pressure is always positive; pleural pressure is always negative and relatively large, while alveolar pressure moves from slightly positive to slightly negative as a person breathes.

Again, intrapleural pressure is ALWAYS below atmospheric pressure and always below intralveolar pressure. Your diagram shows the exact same thing, so my question is how does intrapleural pressure actually affect inhalation/exhalation?

If intrapleural pressure does increase during exhalation, why does that cause alveolar pressure to increase?

Again, everything goes back to PV = PV. Really, you can use that to explain the respiration cycle.

You're right, I was thinking of the changing IP pressure with respect to the resting IP pressure (ie. the zero mark), not the atmospheric pressure. That's my mistake.

At rest, the IP pressure is negative compared to atmospheric pressure. This is because lungs are trying to collapse while the chest wall is trying to expand. Look up anatomical pictures or read up in a physio book if you want confirmation.

During inspiration, the thorax volume increases (due to the contraction of the diaphragm). This increases the lung volume and thus, decreases the pressure inside the lungs (again, Boyle's law). The IP pressure is more negative that it normally is at rest and the alveolar pressure becomes negative. This draws air into the lungs (air flows down the pressure gradient).

During exhalation, alveolar pressure increases above atmospheric pressure (the lung is recoiling back to its original state and thus, the pressure increases). Air flows out of the lungs and the volume of the lungs decreases to resting value again (Boyle's law all over again).

Really though, PV = PV pretty much explains this entire concept. Think of it in that manner.

Edit: Are you mainly asking why the pressure inside the lungs increases during exhalation? The alveolar pressure increases during exhalation because the lung is recoiling (the volume of the lung is being compressed). Remember, the lung normally wants to recoil while the chest wall wants to expand (this is what creates that negative IP pressure in the first place). At the end of inhalation, the lung starts to recoil back to its resting state.

Think of it like squeezing a filled balloon. You're increasing pressure inside (ie. increasing alveolar pressure) and forcing the air to flow out of the balloon (ie. down the pressure gradient). This is essentially what's happening in the lungs during exhalation.

Hope this helps.

 

[Rolling]

So why does kaplan say that the intrapleural pressure is greater than atmospheric? Unbelievable.

 

[SN2ed]

Thread moved. This type of question belongs in the Study Q&A forum.