Small Intestine: Anatomy & Physiology of the Digestive System

The major events of absorption occur in the small intestine.

• A long tube from the pyloric sphincter, coiling through the central and lower part of the abdominal cavity.
• Eventually opens into the large intestine.
• Averaging 2.5 cm in diameter and up to 6.4 m long.
• The major movement of the small intestine is known as “segmentation” which is the contraction of the circular muscles (rather than peristalsis which is the contraction of the longitudinal muscles).
• The partially digested mass of food that passes from the stomach to the small intestine is known as chyme.
• The chyme is mixed with digestive juices and brought into contact with mucosa for absorption.
• Next, muscle fibers encircling the middle of each segment contract, sloshing the chyme back and forth.
• Peristalsis pushes the chyme forwards.
• Chyme remains in the small intestine for approximately three to five hours.
• Peristalsis and segmentation are controlled by the autonomic nervous system.

The small intestine is categorized into three sections:

1. Duodenum — the shortest part approx 25 cm.
2. Jejunum — approx. 2.5 m
3. Ileum — approx. 3.6 m, joins the large intestine at the ileocecal sphincter.