Friday, April 16, 2021

Human Anatomy:The Esophagus

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The oesophagus is a fibromuscular tube, approximately 25cm in length, that transports food from the pharynx to the stomach. The oesophagus begins in the neck, at the level of C6. Here, it is continuous superiorly with the laryngeal part of the pharynx (the laryngopharynx).

It descends downward into the superior mediastinum of the thorax, positioned between the trachea and the vertebral bodies of T1 to T4. It then enters the abdomen via the oesophageal hiatus (an opening in the right crus of the diaphragm) at T10.

The abdominal portion of the oesophagus is approximately 1.25cm long – it terminates by joining the cardiac orifice of the stomach at level of T11.

Anatomical Structure

The oesophagus shares a similar structure with many of the organs in the alimentary tract:

  • Adventitia – outer layer of connective tissue.
    • Note: The very distal and intraperitoneal portion of the oesophagus has an outer covering of serosa, instead of adventitia.
  • Muscle layer – external layer of longitudinal muscle and inner layer of circular muscle. The external layer is composed of different muscle types in each third:
    • Superior third – voluntary striated muscle
    • Middle third – voluntary striated and smooth muscle
    • Inferior third – smooth muscle
  • Submucosa
  • Mucosa – non-keratinised stratified squamous epithelium (contiguous with columnar epithelium of the stomach).

Food is transported through the oesophagus by peristalsis – rhythmic contractions of the muscles which propagate down the oesophagus. Hardening of these muscular layers can interfere with peristalsis and cause difficulty in swallowing (dysphagia).

Oesophageal Sphincters

There are two sphincters present in the oesophagus, known as the upper and lower oesophageal sphincters. They act to prevent the entry of air and the reflux of gastric contents respectively.

Upper Oesophageal Sphincter

The upper sphincter is an anatomical, striated muscle sphincter at the junction between the pharynx and oesophagus. It is produced by the cricopharyngeus muscle. Normally, it is constricted to prevent the entrance of air into the oesophagus.

Lower Oesophageal Sphincter

The lower oesophageal sphincter is a physiological sphincter located in the gastro-oesophageal junction (junction between the stomach and oesophagus). The gastro-oesophageal junction is situated to the left of the T11 vertebra, and is marked by the change from oesophageal to gastric mucosa.

The sphincter is classified as a physiological (or functional) sphincter, as it does not have any specific sphincteric muscle. Instead, the sphincter is formed from four phenomena:

  • The oesophagus enters the stomach at an acute angle.
  • The walls of the intra-abdominal section of the oesophagus are compressed when there is a positive intra-abdominal pressure.
  • The folds of mucosa present aid in occluding the lumen at the gastro-oesophageal junction.
  • The right crus of the diaphragm has a “pinch-cock” effect.

During oesophageal peristalsis, the sphincter is relaxed to allow food to enter the stomach. Otherwise at rest, the function of this sphincter is to prevent the reflux of acidic gastric contents into the oesophagus.

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