When you lower your diaphragm, you create the pressure change that pulls air into your lungs. The diaphragm is a dome-shaped muscle sitting beneath your lungs, and when it contracts, it flattens downward, expanding your chest cavity and dropping the air pressure inside it below the pressure of the air outside your body. That pressure difference is what drives every breath you take.
How the Diaphragm Changes Shape
At rest, the diaphragm sits in a relaxed dome shape, curving upward into the chest like an inverted bowl. When your brain signals it to contract, the muscle fibers shorten and the dome flattens, pushing downward toward your abdomen. During quiet breathing, this movement is subtle: the diaphragm descends roughly 2.3 centimeters. During a deep breath, it can drop more than 5 centimeters, nearly doubling the expansion of your chest cavity from top to bottom.
When you exhale, the diaphragm doesn’t actively push back up. It simply relaxes, and the natural elastic recoil of your lungs and chest wall returns it to its resting dome position. This passive return is why exhaling during normal breathing feels effortless compared to a deliberate deep inhale.
The Pressure Drop That Moves Air
The physics behind this are straightforward: when a container gets bigger, the pressure inside it drops. Your chest cavity is essentially a sealed container, and when the diaphragm descends, the volume inside increases. The air pressure in your lungs, which at rest equals atmospheric pressure, drops by about 1 centimeter of water pressure below the outside air. That small difference is enough to pull air through your nose or mouth, down your airways, and into the tiny air sacs where oxygen enters your blood.
At the same time, the pressure in the space between your lungs and chest wall (the pleural space) drops even further, to about negative 8 centimeters of water pressure at the end of a full inhale. This increasingly negative pressure is what keeps your lungs expanded against the chest wall and prevents them from collapsing inward as they stretch.
The whole system reverses on exhale. The diaphragm relaxes, chest volume shrinks, pressure rises above atmospheric, and air flows back out.
The Nerve Signal Behind Every Breath
The diaphragm is controlled by the phrenic nerve, which originates from the spinal cord in the neck at the level of the third through fifth cervical vertebrae. This is why spinal cord injuries high in the neck can paralyze the diaphragm and stop a person from breathing independently. The connection is so direct that electrically stimulating the phrenic nerve causes the diaphragm to contract, a technique that has been used since the 1800s and is now an established treatment for people who cannot breathe on their own due to certain neurological conditions.
What Happens in Your Rib Cage
The diaphragm doesn’t work alone. The muscles between your ribs, called the external intercostals, contract at the same time, pulling the rib cage upward and outward. This adds even more volume to the chest cavity. During quiet breathing, the diaphragm does most of the work, but deeper breaths rely more heavily on the rib cage muscles working in coordination with it.
There’s also an indirect rib-expanding effect from the diaphragm itself. As it pushes down into the abdomen, it increases the pressure on the abdominal contents below. That rising abdominal pressure pushes outward against the lower ribs (the part of the diaphragm that sits against the inner rib wall), helping to flare them open. So the diaphragm expands the chest in two directions at once: vertically by descending, and horizontally by pushing the lower ribs out through the abdomen.
The Effect on Your Abdomen
Your abdominal cavity is essentially a fluid-filled space with the diaphragm as its ceiling. When the diaphragm descends, it acts like a piston pressing into a cylinder, compressing the organs below. This is why your belly pushes outward when you take a deep breath. The liver, stomach, and intestines shift slightly downward and forward to make room.
The abdominal contents also push back. They provide resistance that limits how far the diaphragm can descend, and this resistance actually helps the diaphragm function effectively. People with large abdominal wall hernias, where the belly wall offers less resistance, can paradoxically have weaker diaphragm function because there’s less pushback to work against. The system depends on a balance of pressures between the chest and abdomen.
Core Stability and the Pelvic Floor
Lowering the diaphragm does more than move air. The rise in abdominal pressure it creates is a key part of how your body stabilizes your spine. When the diaphragm and abdominal muscles contract simultaneously, they generate internal pressure that stiffens the trunk, giving your spine a supportive “cylinder” of pressure to brace against. This is why you instinctively hold your breath during heavy lifting: you’re locking the diaphragm down to maximize core stability.
The pelvic floor muscles at the bottom of the abdomen are part of this system. They contract in response to the rising abdominal pressure created by the diaphragm, working to contain and control that pressure from below. Research has shown that pelvic floor contraction can actually limit how far the diaphragm descends, reducing its movement by as much as 5.6 centimeters in some measurements. The diaphragm, abdominal wall, and pelvic floor don’t operate independently. They function as a coordinated pressure system that serves both breathing and postural support, constantly adjusting to whatever your body is doing at any given moment.