Respiration is the continuous process of moving air between the atmosphere and the lungs, divided into inspiration (inhalation) and expiration (exhalation). This gas exchange supplies oxygen and removes carbon dioxide from the bloodstream.
The mechanism of breathing involves cyclical changes in the volume of the chest cavity, which alters the pressure within the lungs. Understanding this process requires determining if the body expends energy through muscle contraction—an active process—or relies solely on natural physical forces—a passive process. The answer depends entirely on the phase of the breath and the depth of the effort required.
The Muscles of Quiet Inspiration
Inspiration, drawing a normal, quiet breath, is fundamentally an active process requiring energy expenditure through muscle contraction. The primary muscle responsible is the diaphragm, a large, dome-shaped sheet located beneath the lungs. When the diaphragm contracts, it flattens and moves downward, significantly increasing the vertical dimension of the chest cavity.
The external intercostal muscles, located between the ribs, also contract during quiet inspiration. Their contraction lifts the rib cage upward and outward, expanding the thoracic cavity laterally and anterior-posteriorly. This combined action maximizes the increase in the overall volume of the chest cavity.
According to Boyle’s Law, increasing the volume of a container decreases the pressure inside. The expansion of the chest cavity causes the pressure inside the lungs to drop below atmospheric pressure. This pressure difference creates a gradient that forces air to rush into the lungs until the pressures equalize.
The energy expenditure from these muscular contractions defines quiet inspiration as active. The diaphragm must pull against the natural resistance of the rib cage and return from its resting dome shape. This continuous work ensures a steady supply of air during routine activities.
Quiet Expiration: The Passive Contrast
Quiet expiration, following the active inspiration phase, is a largely passive event requiring no muscular energy expenditure. This phase is initiated by the simple relaxation of the muscles active during inhalation. The diaphragm relaxes and returns to its upward, dome-shaped resting position.
The external intercostals also relax, allowing the rib cage to fall back to its previous position. The ability of the lungs and the chest wall to return to their original size without muscle effort is due to their natural elastic recoil. The stretched lung tissue and compressed rib cage act like a fully expanded spring.
As the lungs and chest wall recoil, the volume of the thoracic cavity decreases. This reduction in volume causes the pressure inside the lungs to rise above the atmospheric pressure. The resulting pressure gradient pushes the air out of the lungs and into the atmosphere.
This reliance on stored potential energy within the stretched tissues, rather than metabolic energy consumption for muscle contraction, defines passive breathing. Under normal, resting conditions, the respiratory cycle alternates between an active phase to draw air in and a passive phase to expel it.
Forced Breathing: When Expiration Also Becomes Active
The balance between active inspiration and passive expiration changes when the body requires a greater volume of air or a more forceful expulsion, such as during intense exercise or a cough. Under these conditions, both inspiration and expiration become active processes that recruit additional muscle groups, requiring significantly greater energy expenditure.
To achieve a deeper, or forced, inspiration, the body recruits accessory muscles to further increase chest cavity volume. The scalenes, a group of neck muscles, lift the upper ribs, while the sternocleidomastoid muscles elevate the sternum.
The activity of these accessory inspiratory muscles, typically inactive during quiet breathing, maximizes chest expansion. This large volume increase creates a strong pressure gradient, allowing a rapid and large volume of air to be drawn into the lungs.
Forced expiration, unlike its quiet counterpart, is also an active process that uses powerful muscles to rapidly decrease thoracic volume. This is necessary when trying to blow out a candle or during sustained vocal effort.
The internal intercostal muscles, located beneath the external intercostals, contract to pull the ribs downward and inward. The strong abdominal muscles, including the rectus abdominis and the internal and external obliques, also contract vigorously. This action increases abdominal pressure, pushing the diaphragm forcefully upward into the chest cavity, resulting in a maximal and rapid decrease in lung volume.