ANATOMY AND PHYSIOLOGY
The small intestine is a convoluted tube, with two concentric layers of smooth muscle, that extends from the pyloric sphincter to its junction with the large intestine at the ileo-caecal valve. It is approximately 6 m long with a 3.5 cm diameter and lies in the central and lower part of the abdominal cavity. The small intestines consist of three sections:
• duodenum • jejunum • ileum
The first 30 cm of the small intestine is the C-shaped muscular duodenum, which begins at the pyloric sphincter in the stomach. The common bile duct empties into the duodenum at the ampulla of vater. After the duodenum, the proximal two-fifths of the small bowel (2.5 m) is known as the jejunum. The distal three-fifths of the small intestine, known as the ileum (3.5 m), extends from the jejunum to the ileo-caecal valve. This important physiological valve controls the flow of chyme into the large intestine and prevents backflow or reflux into the small intestine. No anatomical feature separates the jejunum from the ileum and their structure is consistent with that of the duodenum.