How We Breathe – The Role Of Pressure Gradients

I figure as an anesthesiologist and intensivist, I should have a post on the basic mechanism of breathing. Here’s my attempt to simplify this complex process while maintaining some of the important details! 🙂

The atmospheric pressure (P_atm) at sea level is approximately 760 mmHg. The alveolar pressure, go figure, is the pressure within the alveolus (P_alv). The intrapleural space is the area between the visceral pleura (a serous membrane lining the lung itself) and the parietal pleura (the lining on the inner chest wall). The intrapleural pressure (P_ip) is important in determining air flow through the tracheobronchial tree and is a consequence of balancing the chest wall’s tendency to recoil outward with the alveoli’s tendency to collapse inward. Esophageal balloon manometry is sometimes used as a surrogate for P_ip. Finally, P_br is the varying bronchial pressure at several points in the airway (which I’ve labeled A, B, C, and D) due to airway resistance.

By convention, pulmonary pressures are given with reference to a normal atmospheric pressure (760 mmHg). For example, P_alv of 775 mmHg will be +15 whereas a Pip of 754 mmHg will be -6.

The transpulmonary pressure (TPP) is the difference between P_alv and P_ip. This pressure is responsible for maintaining alveolar inflation. In other words: TPP = P_alv – P_ip. In the case of a pneumothorax, the TPP = 0 since P_alv = P_ip. The lung will collapse on itself due to its elastic recoil.

In normal circumstances, the P_ip is always negative (-4 to -6 mmHg), P_alv is slightly negative during inspiration and slightly positive during expiration, and the TPP is always positive. See these descriptions reflected schematically on the diagram.

Fluids (liquids and gases) move from areas of higher pressure to those of lower pressure down a pressure gradient. When the diaphragm, the primary muscle of inspiration, contracts downward, the chest cavity’s volume increases. For a fixed temperature, increasing volume will decrease pressure. This is Boyle’s Law. This decreases P_ip which, in turn, decreases P_alv slightly. This creates a pressure gradient with the atmosphere subsequently drawing air into the lung. On expiration, intrapleural pressure becomes more positive to promote air movement out of the lungs.

Drop me a comment below with questions! 🙂