The human diaphragm is made of skeletal muscle and connective tissue. Its outer portion is a dome-shaped sheet of muscle fibers, while its center is a flat plate of dense connective tissue called the central tendon, composed primarily of interwoven collagen fibers. Together, these two tissue types create the thin, powerful wall that separates your chest cavity from your abdomen and drives every breath you take.
Muscle Fiber Composition
The muscular portion of the diaphragm radiates outward from the central tendon to anchor on your lower ribs, breastbone, and spine. Like all skeletal muscle, it’s made of bundled fibers, but the diaphragm has a distinctive mix tailored for nonstop work. About 55% of its fibers are slow-twitch (type I), the kind built for endurance. These fibers resist fatigue and keep firing rhythmically for a lifetime of breathing without conscious effort. The remaining 45% are fast-twitch fibers: roughly 21% are type IIA (moderately fatigue-resistant) and 24% are type IIX (built for quick, powerful contractions). That fast-twitch portion is what lets you cough forcefully, sneeze, or push air out during heavy exercise.
This ratio isn’t fixed from birth. Premature infants have only about 10% slow-twitch fibers in their diaphragm, rising to 25% at full term and reaching the adult proportion of 55% as the muscle matures. That low percentage in premature babies is one reason they’re more vulnerable to breathing fatigue.
The diaphragm’s muscle cells are also packed with mitochondria, the structures inside cells that produce energy using oxygen. This high density of mitochondria gives the diaphragm an exceptional capacity to burn fuel aerobically, which is essential for a muscle that contracts roughly 20,000 times a day. In people with chronic lung conditions like COPD, the diaphragm actually adapts to its heavier workload by increasing its oxidative capacity and mitochondrial function even further.
The Central Tendon
At the middle of the diaphragm sits the central tendon, a thin, clover-shaped sheet of dense connective tissue. Unlike the surrounding muscle, this tissue doesn’t contract. It’s made of tightly woven collagen fibers arranged in multiple directions, giving it strength without bulk. The collagen fibers are somewhat wavy and unevenly distributed, which allows the tendon to absorb and distribute the mechanical forces generated by the surrounding muscle during breathing. The central tendon also serves as the anchor point that the muscular fibers pull against when the diaphragm contracts and flattens downward.
How Thin It Actually Is
Despite being your most important breathing muscle, the diaphragm is remarkably thin. Ultrasound measurements in healthy adults show that at the end of a normal exhale, the diaphragm is only about 1.4 millimeters thick on average, roughly the width of a pencil lead. It thickens when it contracts during inhalation, but even at peak effort it remains just a few millimeters. Its power comes not from bulk but from its broad surface area and the efficiency of its fiber arrangement.
Where It Attaches
The diaphragm’s muscle fibers originate from three distinct areas, which is why anatomy texts describe it as having sternal, costal, and lumbar parts. The sternal fibers attach to the back of the breastbone’s lower tip. The costal fibers connect to the inner surfaces of the lower six ribs and their cartilages, forming the largest portion. The lumbar fibers arise from the upper lumbar vertebrae via two thick muscular bands called the crura, which flank the spine like pillars.
All of these fibers converge inward and upward to insert into the central tendon. This radial arrangement means the diaphragm pulls its center downward when it contracts, expanding the chest cavity and drawing air into the lungs.
Three Built-In Openings
Because the diaphragm forms a complete partition between the chest and abdomen, it has three major openings that allow vital structures to pass through.
- Caval opening (at the T8 vertebral level): carries the body’s largest vein, the inferior vena cava, which returns blood to the heart from below the diaphragm.
- Esophageal opening (T10 level): allows the esophagus to pass from the chest into the abdomen, along with branches of the vagus nerves that help control digestion.
- Aortic opening (T12 level): lets the aorta, the body’s main artery, pass behind the diaphragm into the abdomen. The thoracic duct, which carries lymph fluid, also passes through here.
The esophageal opening is surrounded by muscle fibers that act as a sphincter, squeezing the esophagus during breathing to help prevent stomach acid from refluxing upward. When this mechanism weakens, it can contribute to a hiatal hernia.
Nerve and Blood Supply
The diaphragm gets its motor commands from the phrenic nerve, which originates from spinal cord segments C3 through C5 in the neck. This is the only nerve that makes the diaphragm move. It also provides sensation to the diaphragm itself, the protective lining around the heart (pericardium), and parts of the membrane lining the abdomen. This shared nerve pathway is why irritation of the diaphragm sometimes causes pain that feels like it’s in the shoulder, since the same spinal cord segments supply skin sensation to that area.
Blood reaches the diaphragm from above and below. The inferior phrenic arteries, which typically branch from the abdominal aorta or its nearby trunk, supply the underside. These arteries also send small branches to neighboring organs including the esophagus, stomach, and adrenal glands. The upper surface receives blood from smaller branches of the internal thoracic and musculophrenic arteries.
How It Forms Before Birth
The diaphragm doesn’t develop as a single structure. During embryonic growth, it assembles from at least four separate tissue sources that gradually fuse together. The transverse septum forms the floor of the early heart cavity and eventually becomes the central tendon. A pair of membranes grow inward from the body wall to form the lateral edges. Meanwhile, surrounding tissue grows in to complete the seal between the chest and abdominal cavities. This complex assembly process explains why congenital diaphragmatic hernias, where a gap remains in the diaphragm at birth, occur in roughly 1 in 2,500 births. The defect represents a failure of these embryonic components to fully merge.