Gastrointestinal system

The gastrointestinal tract extends from the mouth to the anus. It derives from the foregut, midgut and hindgut and accordingly the blood supply and lymphatic drainage will differ.

Esophagus

It is a muscular tube composed of skeletal muscle in the upper one-third, mixture of skeletal and smooth muscles in the middle third and smooth muscle in the distal one-third, which are arranged in a circular internal layer and outer longitudinal layer. The esophagus is about ten inches long. It is divided into cervical, thoracic and abdominal segments based on its location. The cervical segment begins at the lower border of the cricoid cartilage at the level of C6 vertebra, it becomes the thoracic esophagus at the level of the suprasternal notch, running behind the left main bronchus before entering the abdomen at the esophageal hiatus at the level of T10. It has two sphincters, upper esophageal sphincter or UES and lower esophageal sphincter or LES. The UES or pharyngoesophageal sphincter is a musculocartilagenous structure, being composed of thyropharyngeus and cricopharyngeus muscles and cricoid cartilage. The LES is at the esophagogastric junction. It shows high resting pressure to prevent reflux from the stomach. Smooth muscle fibres at the LES are arranged in a “clasp and sling” fashion with contributions from both esophageal and gastric smooth muscle. The diaphragmatic crura also contribute to sphincter action at the LES. The esophagus is lined by non-keratinized stratified squamous epithelium. The parasympathetic innervation from the vagus nerve while sympathetic innervation is from thoracic and cervical sympathetic trunks (T1-10). Parasympathetic system relaxes the sphincters while the sympathetic system will constrict the sphincters. Cervical segment is supplied by the inferior thyroid artery, thoracic segment is supplied by esophageal arteries (branches of thoracic aorta via bronchial arteries) and the abdominal segment is supplied by the left gastric and left phrenic arteries.

Stomach

1. Gross anatomy: It has four main parts - cardia, fundus, body and pylorus. The cardia is situated right next to the gastroesophageal junction, the dome shaped fundus is located just under the left dome of the diaphragm, the body is the largest part while the pylorus has a wider pyloric antrum and a narrower pyloric canal. The transpyloric plane passes at the L1-2 level. Folds called rugae consisting of mucosa and submucosa can be seen in an empty stomach. The convex curvature on the left is called greater curvature while the concave curvature on the right is called the lesser curvature. The lesser omentum is attached to the lesser curvature while the greater omentum is attached to the greater curvature.

2. Histology and glands: The stomach has four layers, from inside to outside, these are mucosa, submucosa, muscularis externa and serosa. The muscularis externa has three layers in the stomach - an inner oblique, middle circular and outer longitudinal layers. The submucosa has the Meissner’s or submucosal plexus while the muscularis externa has the Auerbach’s or myenteric plexus. The mucosal epithelium is simple columnar. The stomach has numerous surface mucus cells and millions of tiny dot-like openings called gastric pits. The gastric pits are the entrance to the gastric glands which have a variety of secretory cells. The table below summarizes the types of secretory cells in the stomach. All gastric mucosal cells arise from stem cells. The stem cells are located just below the gastric pits, in the isthmus.

The cardia and pylorus have mainly mucus secreting glands, glands in the pyloric antrum secrete mucus and hormones mainly gastrin, glands in fundus and body produce most secretions like HCl, pepsinogen, mucus etc.