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Transpulmonary pressure is the difference between the alveolar pressure and the intrapleural pressure in the pleural cavity. During human ventilation, air flows because of pressure gradients. P tp = P alv – P ip. Where P tp is transpulmonary pressure, P alv is alveolar pressure, and P ip is intrapleural pressure.
Dynamic compression of the airways results when intrapleural pressure equals or exceeds alveolar pressure, which causes dynamic collapsing of the lung airways. It is termed dynamic given the transpulmonary pressure (alveolar pressure − intrapleural pressure) varies based on factors including lung volume, compliance, resistance, existing pathologies, etc. [1]
At rest, there is a negative intrapleural pressure. This provides a transpulmonary pressure, causing the lungs to expand. If humans didn't maintain a slightly negative pressure even when exhaling, their lungs would collapse on themselves because all the air would rush towards the area of lower pressure. Intra-pleural pressure is sub-atmospheric.
The partial pressure of carbon dioxide, along with the pH, can be used to differentiate between metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values – greater than 45mmHg.
Once air enters the pleural cavity, the intrapleural pressure increases, resulting in the difference between the intrapulmonary pressure and the intrapleural pressure (defined as the transpulmonary pressure) to equal zero, which cause the lungs to deflate in contrast to a normal transpulmonary pressure of ~4 mm Hg. [28]
Static lung compliance is the change in volume for any given applied pressure. [1] Dynamic lung compliance is the compliance of the lung at any given time during actual movement of air. Low compliance indicates a stiff lung (one with high elastic recoil ) and can be thought of as a thick balloon – this is the case often seen in fibrosis .
On the other hand, gravity causes a gradient in blood pressure between the top and bottom of the lung of 20 mmHg in the erect position (roughly half of that in the supine position). Overall, mean pulmonary venous pressure is ~5 mmHg. Local venous pressure falls to -5 at the apexes and rises to +15 mmHg at the bases, again for the erect lung.
Image illustrating transpulmonary, intrapleural and intra-alveolar pressure. Alveolar pressure (P alv) is the pressure of air inside the lung alveoli.When the glottis is opened and no air is flowing into or out of the lungs, alveolar pressure is equal to the atmospheric pressure, that is, zero cmH 2 O.