When your diaphragm contracts, it flattens and pulls downward, expanding your chest cavity and creating a pressure drop that draws air into your lungs. This single muscle action is the primary driver of every breath you take, repeating roughly 20,000 times a day without conscious effort. But the diaphragm does more than just move air. Its contractions also influence blood flow, stabilize your core, and help you generate force for everything from coughing to lifting heavy objects.
How the Diaphragm Moves During a Breath
At rest, the diaphragm sits like a dome-shaped parachute at the base of your ribcage, separating your chest from your abdomen. When it contracts, muscle fibers pull the central portion of that dome downward and flatten it out, increasing the vertical space inside your chest. During quiet breathing, the diaphragm moves about 2.3 centimeters, roughly the width of a nickel. During a deep breath, that distance more than doubles to around 5 to 5.5 centimeters. At maximum effort, excursion can reach 7 to 11 centimeters.
The signal to contract travels through the phrenic nerve, which originates from nerve roots in the neck at the C3 through C5 level of the spinal cord. This nerve runs a surprisingly long path, descending through the neck and chest past the heart and lungs before reaching the diaphragm. That lengthy route is why a neck injury high on the spinal cord can paralyze the diaphragm and stop spontaneous breathing, while injuries lower down may leave breathing intact.
The Pressure Changes That Move Air
The reason a flattening diaphragm pulls air into your lungs comes down to a basic physics principle: when a container gets bigger, the pressure inside it drops. As the diaphragm descends and your chest cavity expands, the pressure surrounding your lungs (called intrapleural pressure) falls to about negative 8 centimeters of water pressure. Inside the air sacs of your lungs, pressure drops by about 1 centimeter of water below atmospheric pressure.
That tiny pressure difference, roughly equivalent to 2 millimeters of mercury, is enough to pull about 500 milliliters of air into your lungs with each normal breath. Air simply flows from the higher-pressure atmosphere outside your body into the lower-pressure space inside your lungs. When the diaphragm relaxes, it returns to its domed shape, the chest cavity shrinks, pressure rises, and air flows back out. Normal exhalation is entirely passive: the diaphragm simply stops contracting, and the elastic recoil of your lungs does the rest.
The Muscles That Help
The diaphragm doesn’t work alone. The external intercostal muscles, which run between your ribs, contract in sync with the diaphragm during inhalation. Their primary job is to lift and expand the ribcage outward, adding even more volume to the chest cavity. The most active portion is along the back of the upper ribcage, where these muscles fire with every breath at a rhythm closely matching the diaphragm itself, around 10 to 12 nerve impulses per second.
During calm breathing, the diaphragm and external intercostals handle nearly all the work. When you need more air, such as during exercise or respiratory distress, additional muscles in the neck and upper chest kick in to pull the ribcage up even further. These accessory muscles are why you can see someone’s neck straining visibly when they’re struggling to breathe.
Why the Diaphragm Never Gets Tired
Your diaphragm contracts every few seconds for your entire life, so it needs to resist fatigue in ways most skeletal muscles don’t. About 55% of diaphragm muscle fibers in adults are slow-twitch (type I) fibers, the kind built for endurance. The remaining 45% are fast-twitch fibers split between two subtypes: roughly 21% are moderately fatigue-resistant, and 24% are built for quick, powerful bursts. This mix gives the diaphragm both the stamina for continuous breathing and the explosive capacity for forceful actions like coughing or sneezing.
Interestingly, babies are born with far fewer of those endurance fibers. Premature infants have only about 10% slow-twitch fibers in their diaphragm, and full-term newborns about 25%. The adult proportion of 55% develops over months and years, which partly explains why premature infants are more vulnerable to breathing fatigue.
How the Diaphragm Moves Blood
Every time your diaphragm descends, it compresses the organs in your abdomen, raising the pressure there. This squeeze pushes blood from the large veins and blood-rich organs in your belly, particularly the liver and spleen, toward your heart. At the same time, the dropping pressure in your chest helps pull that blood upward through the large vein (the inferior vena cava) that returns blood to the heart.
During quiet breathing at rest, this pumping action shifts about 50 to 75 milliliters of blood in and out of the abdominal blood reservoir with each breath. During intense straining, like a forceful cough or heavy lift, abdominal pressure can spike dramatically and push more than 600 milliliters of blood toward the heart in a single effort. Researchers have described the diaphragm’s role in circulation as an “auxiliary heart,” and its contribution becomes even more important during exercise, when large swings in abdominal and chest pressure help meet the body’s increased demand for blood flow.
Contraction Beyond Breathing
Your diaphragm contracts forcefully during many activities that have nothing to do with gas exchange. When you bear down to lift something heavy, have a bowel movement, or push during childbirth, the diaphragm locks in a contracted position against a closed airway. This is called the Valsalva maneuver: you take a breath, close your mouth and nose, and push against that sealed space. The result is a sharp spike in both chest and abdominal pressure that stabilizes your torso and helps generate force.
Hiccups are another form of diaphragm contraction, this time involuntary. A hiccup is a sudden spasm of the diaphragm that triggers a sharp inhalation, followed almost immediately by the vocal cords snapping shut. That abrupt closure of the airway is what produces the characteristic “hic” sound. The reflex travels through the same phrenic nerve that controls normal breathing, along with the vagus nerve and sympathetic nerve fibers in the chest. Anything that irritates these pathways, from eating too fast to stomach distension to certain medications, can trigger the spasm.
Coughing, sneezing, vomiting, and laughing all rely on rapid, powerful diaphragm contractions coordinated with other muscles. In each case, the diaphragm’s ability to rapidly change pressure inside the chest and abdomen is what makes the action effective.