Anatomy · Muscles

Diaphragm

The main muscle of breathing

Thorax and abdomen · partition between the cavities

A dome-shaped sheet of muscle and tendon that separates the chest from the abdomen and is the main muscle of quiet breathing. When it contracts it flattens and pulls down, enlarging the chest cavity and drawing air into the lungs. The oesophagus, the aorta and the inferior vena cava all pass through it in three named openings.

How to spot them

▸A double-domed muscular sheet, right dome higher than the left because of the liver beneath it
▸Muscle fibres run out from a central tendon to the xiphoid process, the lower six ribs and the upper lumbar vertebrae
▸Three large openings at T8, T10 and T12 for the inferior vena cava, the oesophagus and the aorta
▸Supplied by the phrenic nerves from C3, C4 and C5

What to know

A dome-shaped partition between the chest and the abdomen, with a central tendon fused to the base of the pericardium

Diaphragm — the primary muscle of respiration.

Three openings at the levels of the eighth, tenth and twelfth thoracic vertebrae carrying the vena cava, the gullet and the aorta

Diaphragm — the caval (T8), oesophageal (T10) and aortic (T12) openings.

Supplied by nerves that arise from the neck, from the fourth cervical segment mostly, and run down the front of the mediastinum

Diaphragm — the phrenic nerves, C3 to C5 ('C3, 4, 5 keeps the diaphragm alive').

Irritation of its central part is felt as pain at the tip of the shoulder

Diaphragm — referred pain, because the phrenic nerve shares roots with the shoulder skin (C4).

Part of the stomach can push up through the muscular ring around the gullet, causing reflux

Diaphragm — a hiatus hernia through the oesophageal hiatus.

A sudden spasm of it, with the vocal cords snapping shut, gives a familiar noise

Diaphragm — a hiccup.

Its right half sits higher than the left, because it rides over the liver

Diaphragm — the right dome reaches about the level of the fifth rib in quiet expiration.

Its name comes from the Greek for a partition or wall

Diaphragm — from the Greek diaphragma.

Key ideas

When it contracts it flattens and descends, which increases the vertical dimension of the chest; the pressure inside falls and air flows in. Quiet expiration is passive, the elastic recoil of the lungs and the relaxing diaphragm pushing air out.
Its crura, the tendinous pillars attached to the upper lumbar vertebrae, wrap round the oesophagus, and the right crus acts as a physiological sphincter that helps prevent reflux.

Quick-fire facts

•Origin: three parts: the sternal part from the back of the xiphoid process, the costal part from the inner surfaces of the lower six costal cartilages and ribs, and the lumbar part from the arcuate ligaments and the lumbar vertebrae (right crus L1 to L3, left crus L1 to L2)
•Insertion: the central tendon, which fuses with the base of the fibrous pericardium
•Action: contracts and flattens, raising thoracic volume in inspiration; relaxes in expiration
•Nerve supply: motor from the phrenic nerves (C3 to C5); sensory from the phrenic nerve centrally and from the lower intercostal nerves at the edge
•The three main openings: caval at T8 (inferior vena cava, branches of the right phrenic nerve), oesophageal at T10 (oesophagus and the vagal trunks), aortic at T12 (aorta, thoracic duct and often the azygos vein)
•A memory aid counts the letters: vena cava 8, oesophagus 10, aortic hiatus 12
•The aortic opening is strictly behind the diaphragm, under the median arcuate ligament, so the aorta is not squeezed when the muscle contracts
•Blood supply: mainly the inferior phrenic arteries from the abdominal aorta, plus the superior phrenic, pericardiacophrenic and musculophrenic arteries
•At rest the right dome lies slightly higher than the left, owing to the liver
•Damage to the phrenic nerve paralyses one half; paralysis of both is life-threatening, and a cervical cord injury above C3 needs a ventilator

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Sources

TeachMeAnatomy — The diaphragm