Diaphragm

At a glance

English

Diaphragm

Latin (TA2)

Diaphragma

System

Muscle

Region

Trunk

Side

Single, on the midline

About the diaphragm

The diaphragm is the domed sheet of muscle and tendon that closes the lower opening of the ribcage, separating the chest above from the abdomen below. Its upper surface carries the heart and the bases of the lungs, and its lower surface roofs the abdominal cavity over the liver, stomach and spleen. The right dome usually sits a little higher than the left, mainly because the liver lies beneath it.

Its muscle fibres start all round the edge of the thoracic outlet and climb inward to a central tendon, a thin but tough aponeurosis set nearer the front than the back. The sternal part comes from the back of the xiphoid process; the costal part from the inner surfaces of the lower costal cartilages and ribs; and the lumbar part from the medial and lateral arcuate ligaments and from two crura that run down onto the upper lumbar vertebrae, the right crus reaching lower than the left. A median arcuate ligament joins the crura in front of the aorta.

Several openings let structures cross between chest and abdomen. The inferior vena cava crosses within the central tendon at about the level of T8; the oesophagus passes through a sling of right-crus fibres at about T10; and the aorta slips behind the diaphragm, between the crura, at about T12. Smaller gaps carry nerves and vessels. The muscle is supplied by the phrenic nerve from C3 to C5, which explains why diaphragmatic irritation can be felt as shoulder-tip pain; its outer rim also receives sensory fibres from the lower intercostal and subcostal nerves.

Contraction flattens the domes, lengthening the chest cavity and lowering the pressure inside it so that air flows into the lungs; relaxation and the elastic recoil of the lungs let air out. By raising pressure in the abdomen it also helps in vomiting, defecation, urination and bracing the trunk for heavy lifts. Damage to a phrenic nerve paralyses the same half, which then rides high on a chest radiograph. A hiatus hernia lets part of the stomach slide or roll up through the oesophageal opening, and a congenital diaphragmatic hernia, usually on the left, allows abdominal contents into the chest and can limit lung growth.

Attachments, action and supply

Origin

Back of the xiphoid process, inner surfaces of the lower costal cartilages and ribs, the arcuate ligaments, and the upper lumbar vertebrae through the right and left crura.

Insertion

Central tendon.

Action

Main muscle of inspiration: contracting, it flattens and enlarges the thoracic cavity; it also raises intra-abdominal pressure for expulsive efforts and bracing.

Nerve supply

Phrenic nerves (C3, C4, C5) for motor supply; the periphery also receives sensory fibres from the lower intercostal (T5 to T11) and subcostal (T12) nerves.

Blood supply

From above, the pericardiacophrenic and musculophrenic branches of the internal thoracic artery, the superior phrenic arteries and lower intercostals; the inferior phrenic arteries reach its underside.