Air, like other fluids, moves from a region of higher pressure to one of lower pressure. Therefore, for air to be moved into or out of the lungs, a pressure difference between the atmosphere and the alveoli must be established. If there is no pressure gradient, no airflow occurs.
Air is normally moved from the atmosphere into the alveoli by causing alveolar pressure to fall sufficiently below atmospheric pressure to overcome the resistance to airflow offered by the airways. Because atmospheric pressure is conventionally referred to as 0 cm H2O in discussions of pulmonary physiology (measurements in cm H2O are used instead of mm Hg because the pressures are generally lower than those encountered in cardiovascular physiology), lowering alveolar pressure below atmospheric pressure is referred to as negative-pressure breathing. In clinical practice it is frequently necessary to deliver air or other gas mixtures to the alveoli by raising the pressure at the nose and mouth or in the airway above alveolar pressure. Such positive-pressure breathing is used on patients unable to generate a sufficient pressure gradient between the atmosphere and alveoli to move air through the airways. The cardiopulmonary effects of positive pressure ventilation during anesthesia-paralysis are discussed in Chapter 36. Expiration occurs when alveolar pressure exceeds atmospheric pressure by an amount sufficient to overcome the resistance to airflow offered by the conducting airways.
PRESSURE-FLOW RELATIONSHIPS IN BREATHING
The alveoli are not capable of expanding themselves. They expand passively in response to an increased distending pressure (i.e., an increased transmural pressure gradient). This is usually explained by saying that contraction of the muscles of inspiration increases the volume of the sealed thoracic cavity, which, according to Boyle’s law, decreases the pressure in the pleural space (the intrapleural pressure or, more correctly, pleural surface pressure), which pulls open the very distensible or compliant alveoli (Figure 26-1).
The interaction of the lung and chest wall. (A) At end expiration the muscles of respiration are relaxed. The inward elastic recoil of the lung is balanced by the outward elastic recoil of the chest wall. Intrapleural pressure is –5 cm H2O; alveolar pressure is 0. The transmural pressure gradient across the alveolus is therefore 0 cm H2O –(–5 cm H2O) or 5 cm H2O. Since alveolar pressure is equal to atmospheric pressure, no airflow occurs. (B) During inspiration, contraction of the muscles of inspiration causes intrapleural pressure to become more negative. The transmural pressure gradient increases, and the alveoli are distended, decreasing alveolar pressure below atmospheric pressure, which causes air to flow into the alveoli. (Reproduced with permission from Levitzky MG: Pulmonary Physiology, 9th ed. New York, NY: McGraw Hill; 2018.)
The intrapleural pressure is ...