Pulmonary ventilation is simply the physical process of moving air in and out of the lungs to allow gas exchange. It relies entirely on pressure differences between the atmosphere and the alveoli inside the chest. When a woman takes a breath during labor, air flows because of these changing pressures, not because the lungs pull air in directly.
Boyle's law states that pressure and volume are inversely proportional in a closed container: as volume increases, pressure decreases. When the thoracic cavity expands, its internal pressure drops below atmospheric pressure, forcing air inside. Conversely, when the chest cavity shrinks, internal pressure rises above atmospheric pressure, pushing air out. Midwives observe this mechanical principle every time a newborn takes its first chest-expanding gasp at birth.
Inspiration is an active process where the diaphragm flattens and external intercostal muscles pull the ribs up and outward. This enlarges the thoracic cavity, drops alveolar pressure, and draws air in. Expiration during quiet breathing is usually passive; the stretched muscles relax, the elastic lungs recoil, and air is pushed out as the chest cavity shrinks.
When a pregnant woman experiences respiratory distress or engages in heavy physical exertion, quiet breathing is no longer enough. Accessory muscles in the neck and upper chest—such as the scalenes and sternocleidomastoid—engage to lift the rib cage even higher. During forced expiration, abdominal wall muscles contract forcefully to push internal organs upward against the diaphragm, rapidly decreasing thoracic volume.
Midwives must routinely monitor respiratory rates to detect early signs of complications like postpartum hemorrhage, sepsis, or neonatal respiratory distress. A normal adult respiratory rate ranges from 12 to 20 breaths per minute, while a newborn should breathe between 30 and 60 breaths per minute. Observing chest movement patterns and watching for nasal flaring or sternal retractions helps identify oxygen deprivation quickly.