Gastrointestinal system

The gastrointestinal tract extends from the mouth to the anus. It develops from the foregut, midgut, and hindgut, so its blood supply and lymphatic drainage vary by region.

Esophagus

The esophagus is a muscular tube with different muscle types along its length:

• Upper one-third: skeletal muscle
• Middle one-third: mixed skeletal and smooth muscle
• Distal one-third: smooth muscle

Its muscle layers are arranged as an inner circular layer and an outer longitudinal layer.

The esophagus is about 10 inches long. It’s divided into cervical, thoracic, and abdominal segments based on location:

• Cervical segment: begins at the lower border of the cricoid cartilage at the level of the C6 vertebra
• Thoracic segment: begins at the level of the suprasternal notch; it runs behind the left main bronchus
• Abdominal segment: enters the abdomen through the esophageal hiatus at the level of T10

The esophagus has two sphincters:

• Upper esophageal sphincter (UES), also called the pharyngoesophageal sphincter: a musculocartilaginous structure composed of the thyropharyngeus and cricopharyngeus muscles and the cricoid cartilage
• Lower esophageal sphincter (LES): located at the esophagogastric junction; it maintains a high resting pressure to prevent reflux from the stomach

At the LES, smooth muscle fibers are arranged in a “clasp and sling” pattern, 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.

Innervation:

• Parasympathetic: vagus nerve
• Sympathetic: thoracic and cervical sympathetic trunks (T1-10)

Functionally, the parasympathetic system relaxes the sphincters, while the sympathetic system constricts them.

Arterial supply:

• Cervical segment: inferior thyroid artery
• Thoracic segment: esophageal arteries (branches of the thoracic aorta via bronchial arteries)
• Abdominal segment: left gastric and left phrenic arteries

Stomach

1. Gross anatomy: The stomach has four main parts: cardia, fundus, body, and pylorus. The cardia lies next to the gastroesophageal junction. The dome-shaped fundus sits just under the left dome of the diaphragm. The body is the largest part. The pylorus includes a wider pyloric antrum and a narrower pyloric canal. The transpyloric plane passes at the L1-2 level. In an empty stomach, folds called rugae (made of mucosa and submucosa) are visible. The convex left border is the greater curvature, and the concave right border is the lesser curvature. The lesser omentum attaches to the lesser curvature, and the greater omentum attaches to the greater curvature.

2. Histology and glands: The stomach wall has four layers (from inside to outside): mucosa, submucosa, muscularis externa, and serosa. In the stomach, the muscularis externa has three layers: inner oblique, middle circular, and outer longitudinal. The submucosa contains Meissner’s (submucosal) plexus, and the muscularis externa contains Auerbach’s (myenteric) plexus. The mucosal epithelium is simple columnar. The stomach contains many surface mucus cells and millions of tiny openings called gastric pits. Gastric pits lead into gastric glands, which contain several types of secretory cells. The table below summarizes the types of secretory cells in the stomach. All gastric mucosal cells arise from stem cells located just below the gastric pits, in the isthmus.

The cardia and pylorus mainly contain mucus-secreting glands. Glands in the pyloric antrum secrete mucus and hormones (mainly gastrin). Glands in the fundus and body produce most secretions, including HCl, pepsinogen, and mucus.