Small Intestine Anatomy and Function

Structure and Function of the Small Intestine

• The small intestine extends from the pyloric sphincter to the ileocecal valve, where it empties into the large intestine.
• It is the part of the gastrointestinal tract where the majority of digestion and absorption of food takes place.
• The small intestine finishes the process of digestion, absorbs the nutrients, and passes the residue on to the large intestine.

Associated Organs

• The liver, gallbladder, and pancreas are accessory organs of the digestive system that are closely associated with the small intestine.

Divisions of the Small Intestine

• The small intestine is divided into three parts: the duodenum, jejunum, and ileum.

Wall Structure

• The wall of the small intestine has a mucosa with simple columnar epithelium, submucosa, smooth muscle with inner circular and outer longitudinal layers, and serosa.

Absorptive Surface Area

• The absorptive surface area of the small intestine is increased by plicae circulares, villi, and microvilli.

Cell Types and Secretions

• Exocrine cells in the mucosa of the small intestine secrete mucus, peptidase, sucrase, maltase, lactase, lipase, and enterokinase.
• Endocrine cells secrete cholecystokinin and secretin.

Regulation of Secretions

• The presence of chyme is the most important factor for regulating secretions in the small intestine.
• This is a local reflex action in response to chemical and mechanical irritation from the chyme and in response to distention of the intestinal wall.

Dimensions

• The small intestine is approximately 2.5cm wide and 6m long.
• It fills most of the abdomen.