Inspiration, or inhalation, is the active phase of breathing that draws air into the lungs. This mechanical process is governed by physics, not simply “sucking” air in. Air movement is fundamentally a passive response to pressure changes created by the contraction of specific muscles. The body orchestrates a sequence of events to ensure the pressure inside the lungs briefly drops lower than the air pressure outside the body.
The Driving Principle of Respiration
The flow of air into and out of the lungs is driven by the pressure gradient. Air, like all gases, naturally moves from a region of higher pressure to a region of lower pressure. This movement continues until the pressures between the two areas equalize. In breathing, air moves because a pressure difference exists between the atmosphere outside the body and the air within the lungs.
The mechanism for creating this pressure difference is explained by Boyle’s Law, which describes the inverse relationship between the volume and pressure of a gas within a closed container. If the container’s volume increases, the pressure exerted by the gas inside decreases, assuming constant temperature. Conversely, if the volume decreases, the pressure increases. Since the chest cavity and lungs function as a closed system, the body must actively change the chest volume to manipulate internal pressure.
Mechanical Action of Inspiration Muscles
The first step in inspiration is the coordinated contraction of the primary respiratory muscles, which increases the volume of the thoracic cavity. The most significant muscle is the diaphragm, a thin, dome-shaped sheet separating the chest cavity from the abdomen. When the diaphragm contracts, its central tendon is pulled downward, causing the dome to flatten. This movement lengthens the chest cavity from top to bottom, contributing the largest portion of the volume increase during quiet breathing.
Simultaneously, the external intercostal muscles, located between the ribs, also contract. These muscles elevate the rib cage, pulling the ribs upward and outward like bucket handles swinging up. This action expands the chest cavity from front to back and side to side.
The combined action of the flattened diaphragm and the elevated rib cage results in a substantial expansion of the entire thoracic volume. Since the lungs are held tightly against the inner chest wall by the pleural membranes, they are forced to stretch and expand as the cavity volume increases. The expansion of the chest, therefore, directly dictates the expansion of the lung tissue.
How Air Flows Into the Lungs
The expansion of the lungs immediately triggers the physical mechanism that draws air inward. The increase in lung volume, dictated by Boyle’s Law, causes the pressure of the air inside the lungs (intra-alveolar pressure) to drop. This pressure momentarily falls slightly below the atmospheric pressure outside the body. During normal, restful breathing, this pressure drop is small, usually only about one to two millimeters of mercury (mmHg) below the surrounding air pressure.
This established pressure difference creates the necessary gradient for airflow. Air from the atmosphere, which is at a higher pressure, rushes into the lungs through the conducting airways. The air continues to flow down this pressure gradient until the intra-alveolar pressure equalizes with the atmospheric pressure. At this point, the flow of air ceases, and the inspiratory phase of the breath concludes. This entire event is a mechanical consequence of muscle contraction changing volume, which then changes pressure, resulting in the movement of air into the body.