Intrapleural pressure

In physiology, intrapleural pressure refers to the pressure within the pleural cavity. Normally, the pressure within the pleural cavity is slightly less than the atmospheric pressure, in what is known as negative pressure.[1] When the pleural cavity is damaged/ruptured and the intrapleural pressure becomes equal to or exceeds the atmospheric pressure, pneumothorax may ensue.

Note this is different from intrathoracic pressure. The intrathoracic cavity is the space that includes the pleura, lungs and heart, while the pleural space is only the space between the parietal and visceral pleura surrounding the lungs.

Intrapleural pressure depends on the ventilation phase, atmospheric pressure, and the volume of the intrapleural cavity.[2]

At rest we have a negative intrapleural pressure. This gives us a transpulmonary pressure expanding the lungs. In simpler terms, if we didn't maintain a slightly negative pressure even when exhaling, our lungs would collapse on themselves because all the air would rush towards the area of lower pressure. Intra-pleural pressure is sub-atmospheric. This is due to the recoil of the chest and lungs away from each other.

Müller's maneuver can temporarily significantly decrease the intrapleural pressure.[1]

The logic in intra-pulmonary pressure and the intra-pleural pressure is that the pressure becomes more negative during inspiration and allows air to get sucked in (Boyle 's law.) P vs V relationship and during expiration, the pressure becomes less negative(Note: still less than atmospheric pressure, also take note of the partial pressure of carbon dioxide) and air is given out. The only difference in the pressures are intra-pleural pressure is more negative than intra-pulmonary pressure.

Factors affecting are:

Physiological effects:

1. Müllers maneuver (forced inspiration against a closed glottis results in negative pressure)
2. Deep inspiration

Pathological effects:

1. Emphysema
2. Pneumothorax Condition

A person breathing at rest inhales and exhales approximately half a litre of air during each respiratory cycle, this is tidal volume. The respiratory rate is directly affected by concentration of carbon dioxide in blood. Lungs do not collapse after forceful respiration because of residual volume.