The central tendon of the diaphragm

Thoracic diaphragm

In the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...

 is a thin but strong aponeurosis

Aponeurosis

Aponeuroses are layers of flat broad tendons. They have a shiny, whitish-silvery color, are histologically similar to tendons, and are very sparingly supplied with blood vessels and nerves. When dissected, aponeuroses are papery, and peel off by sections...

 situated slightly anterior to the vault formed by the muscle, resulting in longer posterior muscle fibers.

It is inferior to the fibrous pericardium

Fibrous pericardium

The fibrous pericardium is the most superficial layer of the pericardium. It is made up of dense connective tissue, a loose connective tissue which acts to protect the heart, anchoring it to the surrounding walls, and preventing it from overfilling with blood...

, which fuses with the central tendon of the diaphragm via the pericardiacophrenic ligament.

The caval opening

Caval opening

The caval opening is a hiatus in the diaphragm of humans through which passes the inferior vena cava, the wall of which is adherent to the margins of the opening, and some branches of the right phrenic nerve....

 (at the level of the T8 vertebra) passes through the central tendon. This transmits the inferior vena cava

Inferior vena cava

The inferior vena cava , also known as the posterior vena cava, is the large vein that carries de-oxygenated blood from the lower half of the body into the right atrium of the heart....

 and right phrenic nerve

Phrenic nerve

The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves in humans....

.

Structure

It is shaped somewhat like a trefoil

Trefoil

Trefoil is a graphic form composed of the outline of three overlapping rings used in architecture and Christian symbolism...

 leaf, consisting of three divisions or leaflets separated from one another by slight indentations.

The right leaflet is the largest, the middle, directed toward the xiphoid process

Xiphoid process

The xiphoid process, or xiphisternum or metasternum, is a small cartilaginous process of the lower part of the sternum which is usually ossified in the adult human. By age 15 to 29, the xiphoid usually fuses to the body of the sternum with a fibrous joint. Unlike the synovial articulation of major...

, the next in size, and the left the smallest.

In structure the tendon is composed of several planes of fibers, which intersect one another at various angles and unite into straight or curved bundles—an arrangement which gives it additional strength.

Action during respiration

During respiration the diaphragm contracts causing the central tendon to be drawn inferiorly which partially flattens the domes. The result is an enlargement of thoracic cavity and reduction in intra-thoracic pressure. Physiologically this means that air enters the lungs and venous return to the heart is enhanced. During inspiration the central tendon retains its shape due to its tendonous nature and prevents constriction of the inferior vena cava or aorta, however the oesophagus is surrounded by muscle at the oesophageal hiatus and is constricted.