4.1 Mechanics of Breathing

The mechanics of breathing, scientifically termed pulmonary ventilation, is the physical process by which air is continuously moved into and out of the lungs. This process relies on a strict relationship between the volume of the thoracic cavity and the air pressure within the lungs, governed by Boyle’s Law, which states that pressure and volume are inversely proportional. By manipulating the size of the chest cavity through coordinated muscle contractions, the body creates pressure gradients that force air to move from areas of high pressure to areas of low pressure.

Inspiration is the active process of breathing air into the lungs. At rest, this is initiated by the contraction of two primary muscles: the diaphragm and the external intercostal muscles. The diaphragm contracts and descends, flattening its dome-shaped profile, while the external intercostals contract to pull the rib cage upward and outward. This simultaneous movement increases the internal volume of the thoracic cavity, which automatically causes the intrapulmonary pressure inside the lungs to drop below atmospheric pressure. Because gases naturally flow down a pressure gradient, air rushes through the respiratory tract and into the lungs to equalise the pressure.

During physical exercise, the body's demand for oxygen increases, requiring a greater volume of air to be inspired. To achieve this, the body recruits additional, secondary inspiratory muscles to expand the thoracic cavity even further and faster. The sternocleidomastoid, scalenes, and pectoralis minor muscles contract to lift the sternum and clavicle with greater force. This extra lift drastically maximises the chest's vertical and lateral expansion, creating an even larger drop in internal pressure, which draws a significantly greater volume of air into the lungs with each breath.

In contrast, expiration at rest is entirely passive and requires no muscular effort. Instead, it relies on the natural elastic recoil of the lungs and surrounding thoracic tissues. The diaphragm and external intercostal muscles relax, causing the diaphragm to rise back into its dome shape and the rib cage to descend and inward under the influence of gravity. This reduction in thoracic volume compresses the air inside the lungs, raising the intrapulmonary pressure above atmospheric pressure. Consequently, air is expelled from the lungs into the environment.

However, during exercise, the passive recoil of the lungs is too slow to meet the demands of rapid breathing frequency, forcing expiration to become an active, muscular process. To empty the lungs quickly, the body recruits expiratory muscles, primarily the internal intercostal muscles and the abdominal muscles (such as the rectus abdominis and obliques). The internal intercostals contract to forcefully pull the ribcage downward and inward, while the abdominal muscles contract to compress the internal organs and push the diaphragm upward into the chest cavity. This active compression rapidly decreases thoracic volume and creates a massive, instantaneous pressure spike, forcefully ejecting carbon dioxide-rich air so the next breath can begin immediately.