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alveolar pressure is alternately made less than and greater than atmospheric pressure. These alveolar pressure changes are caused, as we shall see, by changes in the dimensions of the lungs. To understand how a change in lung dimensions causes a change in alveolar pressure, you need to learn none more basic concept—Boyle's law. At constant temperature, the relationship between the pressure exerted by a fixed number of gas molecules and the volume of their container is as follows: An increase in the volume of the container (lungs) decreases the pressure of the gas, whereas a decrease in the container volume increases the pressure. It is essential to recognize the correct causal sequences in ventilation: During inspiration and expiration the volume of the "container"—the lungs—is made to change, and these changes then cause, by Boyle's law, the alveolar pressure changes that drive air flow into or out of the lungs. Our descriptions of ventilation must focus, therefore, on how the changes in lung dimensions are brought about. There are no muscles attached to the lung surface to pull the lungs open or push them shut. Rather, the lungs are passive elastic structures—like balloons—and their volume, therefore, depends upon: (1) the difference in pressure—termed the transpulmonary pressure—between the inside and the outside of the lungs; and
(2) How stretchable the lungs are. The rest of this section and the next three sections focus only on transpulmonary pressure; stretchability will be discussed later in the section on lung compliance. The pressure inside the lungs is the air pressure inside the alveoli (Palv), and the pressure outside the lungs is the pressure of the intrapleural fluid surrounding the lungs (Pip). Thus,
Transpulmonary pressure = Palv _ Pip
At the end of inspiration, the nerves to the diaphragm and inspiratory intercostal muscles decrease their firing, and so these muscles relax. The chest wall is no longer being actively pulled outward and upward by the muscle contractions and so it starts to recoil inward to its original smaller dimensions existing between breaths. This immediately makes the intrapleural pressure less sub atmospheric and hence decreases the transpulmonary pressure. Therefore, the transpulmonary pressure acting to expand the lungs is now smaller than the elastic recoil exerted by the more expanded lungs, and the lungs passively recoil to their original dimensions. As the lungs become smaller, air in the alveoli becomes temporarily compressed so that, by Boyle's law, alveolar pressure exceeds atmospheric pressure. Therefore, air flows from the alveoli through the airways out into the atmosphere. Thus, expiration at rest is completely passive, depending only upon the relaxation of the inspiratory muscles and recoil of the chest wall and stretched lungs. Under certain conditions (during exercise, for example), expiration of larger volumes is achieved by contraction of a different set of intercostal muscles and the abdominal muscles, which actively decreases thoracic dimensions. The "expiratory" intercostal muscles (again a functional term, not an anatomical one) insert on the ribs in such a way that their contraction pulls the chest wall downward and inward. Contraction of the abdominal muscles increases intraabdominal pressure and forces the relaxed diaphragm up into the thorax.