View Full Version : Aveolar/Pleural Pressures...

03-30-2008, 10:58 AM
If anyone can help me, I am having trouble getting the facts straight with positive/negative aveolar/pleural pressures with inspiration and expiration.

In inspiration I know that the aveolar pressure goes to -1 in mid-inspiration then to 0 at the end. In expiration it goes to +1 mid-expiration and then 0 again at the end.

I guess I am just having trouble with pleural pressure and really just the main point. With pleural is it just comparing with the atmospheric pressure to determine whether you will have inspiration or not?

Sorry I know if must seem so basic but it's a tricky point for me. Thanks in advance!

08-18-2008, 08:34 PM
Old question, so hopefully you're still around for me to answer some of your questions about the significance of pleural pressure:

Your pleural pressures usually range from between -9 cmH2O in inspiration and back down to -5 cmH2O in expiration. In other words, pleural pressure is ALWAYS negative.

Your chest wall has a natural tendency to want to expand outwards, and your lungs have a natural tendency to want to recoil inwards. Because each of these structures are attached together by your pleura, they will continue to pull apart from each other until a negative pressure develops between them in the pleural cavity. This negative pressure eventually builds high enough that the lungs and chest wall reach an equilibrium where they do not collapse or expand any further, respectively.

When you take a deep breath, your chest wall expands out further generating an even more negative intrapleural pressure. This causes the lungs to expand along with the chest to offset lower pleural pressure, which then causes alveoli to expand, which then causes a decrease in alveolar pressure (leading to air entry).

Transpulmonary pressure reflects the difference between pleural and alveolar pressure. When this pleural-alveolar pressure gradient becomes very high, it reflects that the patient is needing to generate much more negative pressures to get air in, and thus it reflects underlying pulmonary dysfunction. Increased transpulmonary pressures means an increase in work of breathing.

You should also be familiar with the concept of pneumothoraces. When there is a break in the integrity of the chest wall or the lung parenchyma, air will be drawn into that negative pleural space. As more and more air draws into the intrapleural space, the normal negative intrapleural pressure which is required to keep the lungs expanded against the chest wall is neutralized, and your lung collapses. This is called a pneumothorax. As more and more air enters the pleural space and becomes trapped, you start to develop a positive intrapleural pressure, and this can lead to tension pneumothorax. A tension pneumo causes high intrathoracic pressures that decrease your venous return, cardiac output, and blood pressure.

11-26-2008, 02:09 AM
Here's an excerpt that might be of use

Pleural Effusions is the accumulation of excess fluids in the pleural cavity, which inhibits the normal expansion/contraction of the lungs and can impair breathing. Normally, very small amounts of fluids are present around the pleural spaces and are generally not detectable. The Pleura is a Sac which houses the lungs, and consists of a thin membrane called the 'mesothelium.' The mesothelioma is a vital part of the lungs because it enables them to expand and contract when breathing by secreting a fluid. This fluid is located in the lungs and inside of the rib cage. The official definition of pleural effusions is that it occurs when the rate of fluid formation exceeds the rate of fluid absorption, resulting in excess fluids clogging up the lungs and causing pulmonary signs and symptoms. Normal human beings have the capacity of 20-25ml of fluids in each pleural space. Fluids enter the pleural space via the capillaries in the parietal pleura or through the peritoneal cavity through small holes in the diaphragm. Excess fluids that have not been absorbed are normally removed by lymphatics in the parietal pleura that have the capacity to absorb upto 20 times more fluid than is produced. When this capacity is overwhelmed, pleural effusions develops.
Source: themesotheliomalibrary.com

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