Equal Pressure Point In Lung Physiology

Before reading this post, it’s important to understand the basic physiology behind breathing. Normally, intrapleural pressure (Pip) isnegative, but during a forced expiration, the usage of expiratory muscles can drive the Pip upward. This, in turn, compresses the alveolus increasing pressure within the airway which, due to pressure gradients, exceeds the atmospheric pressure (Patm) causing airflow to move outward

In the diagram below, the normal lung on the left shows how the airway pressure (I’ve called it bronchial pressure, Pbr) drops from +20 to +18 to +16 and so on as air exits the alveolus in transit to the atmosphere. This pressure dropoff is due to resistance within the airway. If the patient has obstructive physiology (ie, chronic obstructive pulmonary disease), significant obstruction can cause a more precipitous drop in pressures along the distal airways.

In either case, one can imagine the Pbr stents open the airway and Pip is trying to collapse it. The point at which these two are equal is called the equal pressure point. In forced exhalation of normal lungs, this point occurs fairly proximal in the tracheobronchial tree where cartilage reinforces the airway and prevents collapse. In patients with COPD, this point occurs much more distal causing airway collapse near the alveolus (parts of the airway that are NOT reinforced with cartilage) leading to hypoventilation, hypoxemia, etc.

This explains why patients with COPD habitually exhale through tightly pursed lips. By limiting the rate at which air is exhaled, the patient is able to maintain a higher airway pressure (Pbr) to help stent the airway where it wants to collapse.

Drop me a comment below with questions! 🙂


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  1. Hey Dr. Rishi,

    So I believe I understand this correctly, but let me know if I am wrong. I have the basic concepts down the difficulty comes when you talk about the Pbr working against the Pip to keep the distal airways open upon forced expiration. Is the Pbr pressure keeping the airway open because as PiP increases the air is forced from the alveolus and that is the air forcing itself forward as well as against the walls of the distal airways?

    Then for the COPD patient you bring up, does the pursed lip breathing increase that Pbr by providing resistance to the outflow of air and increase the pressure in the system that way?

    Thank you for your articles! Always a very interesting read and very easy to grasp!


    1. Great questions, Matthew! I’ll start with your second one – yes, exactly. Pursing one’s lips maintains more air pressure within the airways (higher Pbr) to counteract the intrapleural pressure (Pip) that wants to collapse the airway.

      Now to address your first point, Pbr is higher than Pip during forced expiration at the alveolar level, but the farther air traverses the airway (ie, moving from the alveolus to the mouth), the more pressure drop off occurs. At some point, the Pbr (pressure within the airway) is less than that outside the airway (Pip) leading to airway collapse.

      1. Hi Dr Rishi!
        Just wondering why the obstruction in the small airways causes the pressure to drop off more quickly in a COPD patient?

        1. Patients with COPD have difficult breathing air OUT of their lungs, so I like to think that the airway pressure is higher in their distal airways (alveoli, respiratory bronchioles, etc.) compared to their more proximal airways (trachea, mainstem bronchi). With the dynamic obstruction, there’s some degree of airflow acceleration between the distal airways and proximal airways causing a larger pressure drop off (Bernoulli’s principle) to occur in the area with higher velocities (proximal airways). At least this is how I think about it… let me know if you find a better explanation! 🙂

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