At what point in the breathing cycle would the pleural cavity pressure be going down while the a

At what point in the breathing cycle would the pleural...

Key Concepts

Alveolar Pressure

Alveolar pressure is the pressure within the lung’s airspaces that drives airflow in and out of the alveoli. It fluctuates during the breathing cycle, declining below atmospheric pressure during the early part of inspiration to allow air to flow into the lungs and then rising during expiration—sometimes exceeding atmospheric pressure slightly—to force air out. This dynamic pressure change is central to understanding the mechanics of pulmonary ventilation.

Pleural Pressure

Pleural pressure is the pressure within the pleural cavity surrounding the lungs. It is normally subatmospheric (negative relative to atmospheric pressure) and becomes even more negative during inspiration as the thoracic cavity expands due to muscle contraction. This increasingly negative pressure helps pull the lung tissue outward, contributing to lung expansion while maintaining the tight coupling between the chest wall and lung surfaces.

Transpulmonary Pressure

Transpulmonary pressure is defined as the difference between alveolar pressure and pleural pressure. It is the effective distending force of the lung and is crucial in determining lung volume. During inspiration, even though alveolar pressure is initially lower than atmospheric to draw air in, the continual drop in pleural pressure expands the lungs. Later during inspiration, as the lung’s elastic recoil begins to counter further expansion, alveolar pressure starts to rise while pleural pressure continues to become more negative, temporarily increasing transpulmonary pressure.

Phases of the Breathing Cycle

The breathing cycle consists primarily of inspiration and expiration. At the beginning of inspiration, diaphragmatic and intercostal muscle contraction causes the pleural pressure to fall (becoming more negative) to create a pressure gradient that draws air into the alveoli. As the lungs fill, elastic forces cause the alveolar pressure to start rising back towards atmospheric levels. Conversely, during expiration, relaxation of these muscles and the elastic recoil of the lungs increase alveolar pressure above atmospheric levels to drive air out. In quiet breathing, the alveolar pressure is most positive relative to atmospheric pressure at the end of expiration.

Transcript

00:01 So here i've drawn a simple diagram of the lungs inside the chest cavity as well as the diaphragm at the bottom.

00:12 So as you know, the contraction or the relaxation of the diaphragm is responsible for a change in the pressure inside the chest cavity and allows us to breathe and exhale.

00:26 So one thing to keep in mind for this is that pressure is going to be inversely proportional to volume.

00:39 So this means that as pressure increases or that as volume decreases, the pressure will increase.

00:49 Or as if volume increases, then the pressure of the gas inside will decrease.

00:55 So here we have three different scenarios.

00:59 We're going to start with an original lung, and in green you can see the alveoli inside.

01:09 So this would be right before inhalation.

01:14 As we inhale, the diaphragm will contract and sort of flatten out the space.

01:20 And of course, this means the volume inside the chest cavity will increase, meaning that the pressure will decrease.

01:28 This has an opposite effect on the alveoli and is going to cause the influx of air into the alveolus.

01:37 So inside the alveolus, you're going to have the pressure increase as we inspire through this method.

01:46 So for the first part of this question, the max point at which our alveoli are filling is going to be during the end of inhalation...

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