Muscles - Diaphragm Flashcards
What is the diaphragm?
It is a double-domed musculotendinous sheet, located at the inferior-most aspect of the rib cage. It serves two main functions:
- Separates the thoracic cavity from the abdominal cavity
- Undergoes contraction and relaxation, altering the volume of the thoracic cavity and the lungs, producing inspiration and expiration.
Anatomical position?
The diaphragm is located at the inferior-most aspect of the ribcage, filling the inferior thoracic aperture. It acts as the floor of the thoracic cavity and the roof of the abdominal cavity. The attachments of the diaphragm can be divided into peripheral and central attachments. It has three peripheral attachments:
1) Lumbar and arcuate ligaments
2) Costal cartilages of ribs 7-10 (attach directly to ribs 11-12).
3) Xiphoid process of the sternum.
The parts of the diaphragm that arise from the vertebrae are tendinous in structure, and are known as the right and left crura:
1) Right crus - arises from L1-L3 and their intervertebral discs. Some of the fibres from the right crus surround the oesophageal opening, acting as a physiological sphincter to prevent reflux of gastric contents into the oesophagus.
2) Left crus - arises from L1-L2 and their intervertebral discs.
The muscle fibres of the diaphragm combine to form a central tendon. This tendon ascends to fuse with the inferior surface of the fibrous pericardium, the diaphragm ascends to form the left and right domes. At rest, the right dome lies slightly higher than the left - this is thought to be due to the presence of the liver.
Pathways through the diaphragm?
The diaphragm divides the thoracic and abdominal cavities. Thus, any structure that pass between the two cavities will pierce the diaphragm.
There are three openings that act as conduit for these structures:
1) Caval hiatus (T8) - IVC, terminal branches of the right phrenic nerve.
2) Oesophageal hiatus (T10) - oesophagus, right and left vagus nerves, oesophageal branches of the left gastric artery/vein.
3) Aortic hiatus (T12) - Aorta, thoracic duct and azygous vein.
Actions?
The diaphragm is the primary muscle of respiration. During inspiration, it contracts and flattens, increasing the vertical diameter of the thoracic cavity. This produces lung expansion, and air is drawn in. During expiration, the diaphragm passively relaxes and returns to its original dome shape. This reduces the volume of the thoracic cavity.
Innervation and vasculature?
The halves of the diaphragm receive motor innervation from the phrenic nerve. The left half of the diaphragm (known as a hemidiaphragm) is innervated by the left phrenic nerve, and vice versa. Each phrenic nerve is formed in the neck within the cervical plexus and contains fibres from spinal roots C3-C5.
The majority of the arterial supply to the diaphragm is selivered via the inferior phrenic arteries, which arise directly from the abdominal aorta. The remaining supply is from the superior phrenic, pericardiaphrenic and musculophrenic arteries. The draining veins follow the aforementioned arteries.
