Diaphragm

What it is

A double-domed sheet of muscle separating the chest from the abdomen — the floor of one and the roof of the other. It arises around the entire lower rim of the ribcage and from the lumbar spine by two tendinous pillars called the crura, and all of it converges inward on a central tendon rather than onto bone.

That is structurally unusual: a muscle inserting into itself. When the muscular rim contracts it pulls the central tendon downward, flattening the domes.

Three major structures pass through it — the aorta, the oesophagus and the inferior vena cava — each at its own level and its own opening.

It is supplied by the phrenic nerve, which arises from the third, fourth and fifth cervical nerve roots — high in the neck, a long way from the muscle. The old mnemonic is "C3, 4, 5 keeps the diaphragm alive", and it is the reason a high spinal cord injury can stop someone breathing.

What it does

It is the primary muscle of breathing in. Contracting flattens the domes, enlarging the chest cavity, lowering the pressure inside it and drawing air in. Quiet breathing out is largely passive — the tissue recoils.

Its second job is postural. Contracting downward against a braced abdominal wall raises pressure inside the abdomen, which stiffens the trunk. It works with transversus abdominis and the pelvic floor as the top of that pressurised container, and it does this at the same time as breathing.

What loads it

Breathing, continuously and for life. Its postural role is loaded by heavy lifting and by carrying.

What is commonly got wrong about it

"Belly breathing" as diaphragmatic breathing. The abdomen moves outward when the diaphragm descends and pushes the contents down. The movement of the belly is a consequence, not the mechanism, and you can move your belly without using your diaphragm well.

That most people breathe wrong. Breathing pattern advice is given with enormous confidence. Some breathing interventions have reasonable evidence for specific conditions; the general claim that ordinary people are breathing incorrectly and need retraining is not well supported.

Stitches, and what causes them. Exercise-related transient abdominal pain is often attributed to diaphragm spasm or reduced blood flow to it. Neither explanation is established, and irritation of the parietal peritoneum is currently considered more likely. The honest answer is that nobody is certain.

That it is not a skeletal muscle. It is — voluntary, striated, and under conscious control, despite running automatically.