When you breathe in, your diaphragm contracts and flattens downward, enlarging the chest cavity and creating a vacuum that pulls air into your lungs. This single muscle is responsible for about 75% of the work of normal breathing, making it the most important muscle in respiration.
How the Diaphragm Changes Shape
At rest, the diaphragm is dome-shaped, curving upward into the chest cavity like an upside-down bowl. When it receives the signal to contract, its muscle fibers shorten and pull the central portion (a tough, flat sheet of connective tissue called the central tendon) downward. The dome flattens out, pushing the floor of the chest cavity lower and increasing the vertical space inside the thorax.
During a normal, relaxed breath, the diaphragm moves about 1 to 2 centimeters downward. Ultrasound measurements in healthy adults put the average closer to 2.3 centimeters on each side. During a deep breath, that excursion jumps dramatically, ranging from about 5 to 6 centimeters on average and reaching up to 7 to 11 centimeters in a maximal effort. That difference explains why a deep breath feels so much more expansive than quiet breathing.
Why Air Flows In
The diaphragm doesn’t push air into your lungs. Instead, it creates the conditions for air to flow in on its own. As the diaphragm flattens and descends, the chest cavity gets larger in three dimensions: vertically (from the diaphragm dropping), side to side (from the lower ribs lifting outward), and front to back. This expansion increases the volume of the sealed thoracic space, which drops the air pressure inside it below atmospheric pressure. Air then rushes in through your nose or mouth to equalize that pressure difference, inflating the lungs.
This is why breathing is sometimes called “negative pressure ventilation.” The lungs themselves are passive. They don’t have muscles of their own. They simply stretch open because they’re attached to the chest wall by a thin fluid layer, and they follow wherever the chest cavity goes.
What Triggers the Contraction
The diaphragm is controlled by the phrenic nerves, one on each side, which originate from the spinal cord at the C3 through C5 vertebral levels in the neck. Rhythm-generating neurons in the brainstem send excitatory signals down through the spinal cord to phrenic motor neurons. These signals arrive in a cyclic pattern, depolarizing the phrenic motor neurons during each inspiratory phase, which triggers the diaphragm to contract.
This connection is direct and monosynaptic, meaning the brainstem respiratory centers communicate to phrenic motor neurons through a single relay. That streamlined pathway provides the primary “inspiratory drive” on a breath-by-breath basis. It’s why breathing is automatic: you don’t have to think about it, because the brainstem handles the timing. Additional inputs from other neural sources can modulate how strongly the diaphragm contracts, adjusting for things like posture, speech, or physical exertion.
The C3 to C5 origin also explains why spinal cord injuries at or above the mid-neck can paralyze the diaphragm and eliminate the ability to breathe independently.
What Happens Below the Diaphragm
When the diaphragm descends during inspiration, it doesn’t just affect the chest. It pushes downward on the abdominal organs, displacing them toward the pelvis and increasing pressure inside the abdominal cavity. This is why your belly rises when you breathe in, especially during relaxed or diaphragmatic breathing.
That increase in abdominal pressure isn’t just a side effect. The diaphragm plays a real role in stabilizing your spine. When you lift something heavy or move your limbs quickly, the diaphragm contracts tonically (sustained, not just with each breath) alongside the abdominal and pelvic floor muscles. This co-contraction pressurizes the abdominal cavity like a firm cylinder around the spine, providing mechanical support. Studies measuring abdominal pressure during repetitive arm movements found that tonic diaphragm activity raised pressure by an average of about 26 centimeters of water pressure compared to quiet breathing alone.
During inspiration, as the diaphragm shortens and pushes the abdominal contents downward, the deep abdominal muscles lengthen to accommodate. During expiration, the reverse happens: the diaphragm relaxes upward and the abdominal muscles shorten. This alternating coordination keeps breathing smooth even while the core is engaged for postural tasks.
What Happens During Expiration
Normal expiration is largely passive. The diaphragm simply relaxes, returning to its domed shape through elastic recoil of the lungs and chest wall. As the chest cavity shrinks back to its resting size, pressure inside rises above atmospheric pressure, and air flows out. During exercise or forced breathing, the abdominal muscles actively contract to push the diaphragm upward faster, squeezing more air out and setting up a larger inspiration on the next breath.