When you inhale, your diaphragm contracts and flattens downward, expanding the chest cavity and dropping the air pressure inside your lungs so that air flows in. This single muscle, a dome-shaped sheet separating your chest from your abdomen, is the primary driver of every breath you take.
How the Diaphragm Creates Each Breath
At rest, the diaphragm sits in a relaxed, dome-like shape beneath your lungs. The moment you begin to inhale, the muscle fibers contract and the dome flattens, pulling downward like a piston in a cylinder. This downward motion increases the volume of your chest cavity. As that space grows larger, the air pressure inside the tiny air sacs of your lungs drops below the pressure of the air outside your body, and air rushes in through your nose or mouth to equalize the difference.
During a quiet, resting breath, the diaphragm moves about 1 to 2 centimeters downward. During a deep breath, that movement jumps to roughly 5 to 6 centimeters on average, and can reach up to 11 centimeters in some people. That difference explains why a deep breath feels so much more expansive than the shallow breathing you do without thinking.
What Happens Below the Diaphragm
Your abdomen is essentially a sealed cavity filled with organs and fluid that can’t be compressed. When the diaphragm pushes down into this space, it increases the pressure on everything below it: your stomach, liver, intestines, and other organs get gently squeezed. You can feel this if you place a hand on your belly during a deep breath. Your abdomen pushes outward because the only flexible wall of the abdominal cavity is the front, and the rising pressure forces it to stretch.
This abdominal pressure isn’t just a side effect. It actually helps breathing. The increased pressure pushes outward against the lower ribs where the diaphragm attaches, helping to expand the rib cage from below. If something compromises the abdominal wall, like a large hernia, the diaphragm loses some of this mechanical advantage and can’t work as efficiently.
The Muscles That Help
The diaphragm does most of the work during normal breathing, but it doesn’t act alone. The external intercostal muscles, small bands of tissue running between your ribs, contract during inhalation to pull the rib cage upward and outward. Think of them as a rowing team pulling in sync to widen the chest from the sides while the diaphragm expands it from the bottom. During heavier breathing, like exercise, these muscles take on a bigger share of the effort.
What Triggers the Contraction
You don’t have to think about breathing because the process runs on autopilot. A cluster of nerve cells in the brainstem generates a rhythmic signal, firing with every breath cycle. That signal travels down the spinal cord to the phrenic nerve, which originates from the third through fifth cervical vertebrae in your neck. The phrenic nerve carries the “inhale now” message to the diaphragm, triggering contraction. This is why spinal cord injuries high in the neck can affect the ability to breathe independently: they can interrupt the pathway before it reaches the phrenic nerve.
While breathing is automatic, you can also override the system consciously. Holding your breath, taking a deliberate deep breath, or singing all involve voluntary control layered on top of the automatic rhythm.
Inhalation vs. Exhalation
Inhalation is the active phase of breathing. The diaphragm and intercostal muscles contract, spending energy to expand the chest. Exhalation during quiet breathing is passive. The diaphragm simply relaxes, springs back up into its dome shape, and the elastic recoil of your lungs and chest wall pushes air out without any muscular effort. This is why breathing at rest feels effortless: half the cycle requires no work at all. Forced exhalation, like blowing out candles, does recruit abdominal and internal intercostal muscles, but normal resting exhalation is purely a release.
How Diaphragmatic Breathing Affects Your Body
The diaphragm’s movement has effects well beyond getting air into your lungs. The vagus nerve, a major nerve connecting your brain to your heart, lungs, and digestive organs, is influenced by the rhythm of your breathing. Vagus nerve activity is suppressed during inhalation and increases during exhalation. This is the mechanism behind the calming effect of slow, deep breathing: longer exhalations relative to inhalations boost vagus nerve activity, which lowers heart rate and shifts the nervous system toward its rest-and-digest mode.
Diaphragmatic breathing, where you deliberately breathe into your belly rather than shallowly into your upper chest, amplifies this effect. It’s the physiological basis for breathing techniques used in meditation, yoga, and stress management. The calming sensation isn’t just psychological. It reflects a measurable shift in nervous system activity driven by how deeply and slowly the diaphragm moves.