Diaphragm

Figure. The diaphragm, inferior view.

A musculotendinous sheet separating the thoracic and abdominal cavities consisting of muscle fibres, attached around the thoracic outlet, which converge to a central trefoil-shaped tendon. The lumbar part of the diaphragm arises in part from two cmra which attach to the anterolateral aspects of the bodies of the lumbar vertebrae: the larger right cms from the bodies and intervening discs of L1-L3, the smaller left cms from the bodies and disc of L1 and L2. The aorta passes into the abdomen behind the two crura as they cross one another, and in front of the body of T12. At this point the two crura are connected by a tendinous band, the median arcuate ligament. Fibres of the right crus generally pass towards the left, separating to surround the oesophagus before inserting into the cen-tral tendon. Fibres of the left crus may also pass behind the oesophageal opening, separating it from the aortic opening. From near the oesophageal opening, the suspensory ligament of the duodenum arises from the right crus to attach to the terminal part of the duodenum.

The diaphragm, inferior view

The remainder of the lumbar part of the diaphragm arises from the medial and lateral arcuate ligaments which are immediately lateral to the crura. The medial arcuate ligament is a thickening of the fascia covering psoas major and runs from the side of the body of L2 to the transverse process of LI. The lateral arcuate ligament is a thickening of the anterior layer of the thoracolumbar fascia covering quadratus lumborum and runs from the transverse process of LI to the tip of the 12th rib. Lateral to the arcuate ligaments the costal part of the diaphragm arises from the inner suiface of the lower six ribs and their costal cartilages, interdigitating with transversus abdominis, to insert into the anterolateral part of the central tendon.

The most anterior sternal part of the diaphragm arises by two slips from the posterior surface of the xiphoid process of the sternum.

All the muscle fibres arch upwards and medially towards their insertion into the central tendon which is situated towards the front of the muscle. Consequently, the short anterior fibres and longer posterior fibres give the appearance of an inverted letter J when viewed from the side. When viewed from the front, two small domes (cupolae) on either side of the central tendon can be seen, that on the right being at a slightly higher level than that on the left; the central part lies opposite the xiphisternal joint.

The upper surface of the diaphragm is covered with parietal pleura, which lines the thoracic cavity. A potential space, the costodiaphragmatic recess, separates the parietal and visceral pleurae, the latter covering the lungs. The fibrous pericardium enclosing the heart is firmly attached to the central tendon. The inferior surface of the diaphragm is lined by the parietal layer of peritoneum. This surface is related on the right side to the right lobe of the liver and right kidney, and on the left side to the left lobe of the liver, the fundus of the stomach and left kidney.

Several structures pass between the thorax and abdomen, doing so by either passing through or behind the diaphragm. The major tubular structures (inferior vena cava, oesophagus and aorta) do so by named openings; they maybe accompanied by nerves and/or other vessels. The caval opening is in the central tendon just to the right of the midline, and transmits the inferior vena cava and right phrenic nerve. The opening is level with the lower border of T8, with the wall of the vena cava firmly adherent to its margin. Consequently, the inferior vena cava is constantly held open.

The oesophageal opening, at the level ofTIO, is to the left of the midline surrounded by fibres of the right and left crura. As well as the oesophagus, the trunks of the vagus nerves (now known as the gastric nerves) and the oesophageal branches of the left gastric vessels pass through this opening. The left phrenic nerve pierces the muscular part of the diaphragm near to the oesophageal opening in front of the left part of the central tendon.

The aortic opening lies behind the diaphragm, in front of T12, as the two crura cross each other. The aorta and thoracic duct pass in and out of the abdomen at this opening. The azygos vein is partly covered by the right crus. The greater and lesser splanchnic nerves pierce the crura as they enter the abdomen running to the coeliac ganglion.

Behind the medial and lateral arcuate ligaments pass the sympathetic trunk and subcostal nerve respectively.

Anteriorly, between the sternal and costal attachments of the diaphragm, the superior epigastric artery passes to enter the rectus sheath and supply the upper part of rectus abdominis.

Nerve supply

The diaphragm is supplied with motor and sensory innervation by the left and right phrenic nerves (root value C3, 4 and 5). Additional sensory fibres to the peripheral part of the diaphragm are supplied by the lower six intercostal (thoracic) nerves.

Action

The diaphragm is the major muscle of inspiration. Downward movement of the diaphragm, elevation of the ribs and forward movement of the sternum all increase the dimensions of the thorax causing air to be drawn into the lungs. From its resting position, sequential contraction of the diaphragm can be described in the following stages. Contraction of the peripheral muscular portion of the diaphragm against the fixed ribs flattens the two cupolae and pulls the central tendon down from the level of T8-T9. Further descent is prevented by compression of the abdominal viscera, which is prevented from bulging outwards by tone in the abdominal muscles and, to a lesser extent, tension on the pericardium. At this point the central tendon becomes the fixed point and further contraction of the muscle fibres causes movement of the ribs and sternum. The lower ribs are lifted upwards and outwards to increase the lateral diameter of the thorax while the upper ribs are raised to push the body of the sternum forwards and so increase the anteroposterior diameter of the thorax. These movements of the ribs are known as ‘bucket-handle’ and ‘pump-handle’ respectively and occur at the costovertebral, costotransverse, sternocostal and interchondral joints. In shallow respiration the descent of the diaphragm can be as little as 1.5 cm, whereas in deep inspiration it can be as much as 10 cm. This descent, together with movement of the ribs and sternum, produces a very efficient mechanism for drawing air into the lungs. From the position of full inspiration the diaphragm relaxes to control the rate of expiration produced by the elastic recoil of the lungs.

The diaphragm also plays an important role in increasing intra-abdominal pressure, where it resists upward movement of the abdominal contents when the abdominal muscles contract, to produce the expulsive acts (defecation, vomiting, micturition, parturition). This action is also important in supporting the lumbar spine during lifting activities by creating a pneumatic cushion to support it. It is likely that compression of the oesophagus by the fibres of the right crus prevents regurgitation of food from the stomach.

The changes in pressure in tlie thoracic and abdominal cavities, caused by movement of tlie diaphragm, assist venous and lymphatic drainage from the abdomen to tlie thorax.

Palpation

The diaphragm is too deep to be directly palpable, but the effects of its contraction can be seen and felt. With the subject sitting and relaxed, the examiner’s flat hand should be placed over the subcostal angle. After full expiration, as a breath is taken the abdominal wall can be felt pushing outwards. If the hands are now placed along the lower ribs these will be felt rising upwards and outwards at the same time as the sternum moves forwards.

Frequently, accessory muscles of respiration are used during breathing. In many instances this leads to apical breathing, in which only the upper parts of the lungs are used. Instruction in the correct method of diaphragmatic breathing greatly increases the efficiency of breathing, and is essential in activities which require breathing control, such as singing.

Video: Diaphragm – Definition, Function, Muscle & Anatomy