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GASTRIC ORIFICES

Cardiac orifice and gastro-oesophageal junction

The opening from the oesophagus into the stomach is the cardiac orifice (Fig. 65.1). It is typically situated to the left of the midline behind the seventh costal cartilage at the level of the eleventh thoracic vertebra. It is typically 10 cm from the anterior abdominal wall and 40 cm from the incisor teeth. The abdominal oesophagus is continuous with the cardiac orifice; the right side is continuous with the lesser curvature, the left side with the greater curvature. There is no specific anatomically discernible cardiac sphincter related to the orifice.

Internally, the transition between oesophagus and stomach is difficult to define because mucosa of gastric fundal pattern extends a variable distance up into the abdominal oesophagus. It usually forms a ‘zig-zag’ squamo-columnar epithelial junction with the oesophageal epithelium above this Z line: for histological and endoscopic purposes, this is often referred to as the gastro-oesophageal junction. A sling of longitudinal and oblique gastric muscle fibres forms a loop on the superior, left, side of the gastro-oesophageal junction between the oesophagus and the lesser curvature, and this is taken as the external boundary of this junction. Bands of thickened circular smooth muscle in the upper wall of the greater curvature and the distal oesophagus are sometimes confusingly referred to as ‘clasp’ or ‘sphincter’ fibres.

Lower oesophageal sphincter and gastro-oesophageal reflux

At rest there is a gastro-oeosophageal pressure gradient due to the presence of negative intra-thoracic pressure (transferred to the thoracic oesophagus) and positive intra-abdominal pressure (transferred to the stomach and augmented by any contraction of the stomach wall itself). Several anatomical and physiological factors normally prevent gastro-oesophageal reflux. Minor factors include the folds of gastric mucosa present in the gastro-oesophageal junction, the mucosal rosette, which contribute to the formation of a fluid- and gas-tight seal and also help to ensure that even low levels of tone within the lower oesophageal wall muscles may occlude the lumen of the junction against low pressures of gastric gas; the angle of the cardiac orifice, which is formed, in part, by the pull of the long oblique fibres of the inner layer of the gastric smooth muscle and may help to form a type of ‘flap valve’; and the length of the abdominal oesophagus, which is buttressed externally by pads of adipose connective tissue at and below the level of the diaphragmatic hiatus. However, the major anti-reflux mechanisms are the tonic contractions of the specialized smooth muscle of the wall lower oesophageal and the encircling fibres of the right diaphragmatic crus, which, together, exert a radial pressure that can be measured by electromyography or manometric testing (Paterson 2001, Mittal 2006), and form an effective high pressure zone (HPZ). At and just below the level of the entry of the abdominal oesophagus into the stomach, the circular fibres of the intermediate layer of the muscularis externa lying over the upper lesser curvature are particularly pronounced and sometimes referred to as ‘clasp’ fibres and exert fairly constant myogenic tone (Fig. 65.6). Since the oesophagus passes obliquely into the stomach, with increasing gastric distension the tone in the clasp fibres rises and they may act to draw the anterior and posterior surfaces together, increasing the tone at the gastro-oesophageal junction, contributing to the HPZ. These anatomical and physiological features are together referred to as the lower oesophageal sphincter (LOS). If reflux is to be prevented, the pressure within the HPZ must exceed the difference between the pressures on either side of the junction. The oesophageal part of the LOS is controlled by the intramural plexuses of the enteric nervous system via the neural release of nitric oxide which relaxes the smooth muscle of the LOS. Tone is reduced in advance of the oesophageal peristaltic wave during swallowing and raised again after the food bolus has passed. During inspiration, the greater negative intrathoracic pressure which increases the gastro-oesophageal pressure gradient is offset by increased pressure in the HPZ due to contraction of the peri-oesophageal fibres of the right diaphragmatic crus. Activation of the crural diaphragm slightly before the costal fibres ensures that this increase precedes the increase in the gradient.

  

Fig. 65.6  Detail of the arrangements of the muscle layers of the oesophageal and gastric walls.
(Reprinted from Netter Anatomy Illustration Collection, © Elsevier Inc. All Rights Reserved.)

Reflux of gastric contents into the abdominal and lower thoracic oesophagus as a result of transient relaxation of the LOS occurs as a normal event in most individuals for a small percentage of their daily life. It also occurs as a result of a weak LOS, or of hiatus hernia which disrupts the normal anatomical barriers. Surgical procedures to prevent excessive reflux mainly focus on the restoration of a normal length of intra-abdominal oesophagus, by reduction of any hiatus hernia which carries the gastro-oesophageal junction into the thorax, and an increase in the tonic contraction surrounding the intra-abdominal oesophagus, which is usually achieved by partial wrapping of the fundus of the stomach around the intra-abdominal oesophagus to provide greater tone, ‘fundoplication’.

Barrett’s oesophagus

The squamous epithelium lining the lower oesophagus may be pathologically replaced by islands, strips, or circumferential patches of columnar, gastric type epithelium. This process is most likely to be the result of chronic reflux of gastric contents, acid or alkali, into the oesophagus, inducing a change in mucosal cell type. The abnormal columnar type epithelium, which may extend for a variable length up the lower part of the oesophagus, is referred to as Barrett’s epithelium.

Pyloric orifice

The pyloric orifice is the opening from the stomach into the duodenum, and typically lies 1–2 cm to the right of the midline in the transpyloric plane when the body is supine and the stomach empty. The pyloric sphincter is a muscular ring formed by a marked thickening of the circular gastric muscle interlaced with some longitudinal fibres. The circular pyloric constriction on the surface of the stomach usually indicates the location of the pyloric sphincter, and is often marked by a prepyloric vein which crosses the anterior surface vertically downwards.

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