16th February 2024

Gastrointestinal tract

Oesophagus

The oesophagus starts from the hypopharynx at the lower border of the cricoid cartilage, passing in through the superior mediastinum, then the posterior mediastinum just to the left of the midline and through the diaphragm at the level of T10 to enter the stomach at the abdomen.

There are important port-systemic anastomoses at the gastro-oesophageal junction and lower oesophagus level.

Stomach

The stomach has five parts: cardia, fundus, body, antrum and pylorus. The oesophagus enters the stomach at the cardia, the site of the physiologic sphincter, preventing food from regurgitating. The fundus is a rounded part above the cardia and typically contains gas. 

The body is the largest section of the stomach. The body leads to the antrum, which leads to the pylorus. The pylorus is the distalmost part of the stomach connecting with the duodenum and containing the pylorus sphincter. The stomach has two curvatures, the greater and lesser curvatures.

Small intestines

The duodenum is a 20-30 cm C-shaped small bowel region. The pancreatic head usually lies inside the C-shape of the duodenum. The initial 2-3cm of the 1st part of the duodenum is intraperitoneal. The second part of the duodenum receives the pancreatic and common bile duct at the ampulla of Vater. Just inferior to the ampulla is the junction of the foregut and midgut. The duodenum ends at the ligament of Treitz aka duodenojejunal (DJ) flexure continuing as jejunum.

The remainder of the small bowel is divided into the jejunum, the proximal 2/5th, and the ileum, the latter 3/5th. Small bowel loops contain mucosal folds called valvulae conniventes, which start at the second part of the duodenum, becoming the thickest at the jejunum and then fading away distally towards the terminal ileum. 

These folds make complete circles in the bowel wall and give a characteristic appearance on plain radiographs or barium studies. The ileum enters the caecum at the ileo-caecal valve.

Large intestine

The caecum is an intraperitoneal part of the large bowel and has three longitudinal muscular bands (Tinea coli), which converge inferiorly towards the base of the appendix. The caecum continues as the retroperitoneal ascending colon. 

The transverse colon starts at the hepatic flexure from the ascending colon and ends at the splenic flexure to continue as the retroperitoneal descending colon. The sigmoid colon connects the descending colon to the rectum and has a variable length. It is intraperitoneal and is also suspended by its mesentery. 

Rectum and anus

The rectum, which is retroperitoneal, starts at the level of S3, passing anterior to the sacrum. The tinea coli converges into a single muscular layer at the rectum. The rectum leads to the anus, the distal end of the GI tract.

References

Wineski, L. E., & Snell, R. S. (2019). Snell’s clinical anatomy by regions. 

Chaurasia, B. D. (2019). Human anatomy: Regional and applied, dissection and clinical. New Delhi, India: CBS Publishers & Distributors.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Moore’s Clinically Orientated Anatomy (7th ed.). China: Lippincott.