Algonquin College
13.3 - Lower GI Tract
The gastrointestinal (GI) tract is the site for the ingestion, digestion and absorption of nutrients. It is divided in the the upper and lower GI tracts.

The lower GI tract is the site for final chemical digestion, absorption and defecation. It begins at the opening to the small intestine and ends at the anus.

Small intestine

The small intestine is the site for final chemical digestion and the absorption of nutrients and some water. It has three sections: the duodenum, jejunum and ileum. The duodenum is the most proximal part, connecting to the stomach, and the ileum is the most distal and connects to the large intestine.

The lining of the small intestine has a very large surface area due to modifications to its mucosa. The walls of the small intestine are folded in large circular folds. The surface of the folds is covered in villi, finger-like projections of the mucosa. The epithelial cells of the villi (called brush border cells) have numerous cellular projections called microvilli.

Movement

Segmentation is a special type of peristalsis in the small intestine that causes the mixing and slow forward movement of chyme.
  • Segmental movements repeat several times a minute
  • Rhythm-generating cells in the walls of the small intestine cause alternating contractions of adjoining
    • sections
Digestion

Final chemical digestion occurs in the duodenum Mechanical digestion of fats occurs A salt and mucus secretion protects and lubricates the lining of the small intestine while providing hydrolysis enzymes with a water supply

Absorption Large intestine

The large intestine (colon) is the final site for the body to recover materials from ingested food before defecation. The entry to the large intestine is controlled by a one-way sphincter called the ileocecal valve, which is relaxed by the hormone gastrin when a new meal enters the stomach. Indigestible food residues, unabsorbed bile salts and fluids make up the chyme entering the large intestine.

The cecum is a small pouch for the storage of feces immediately after the ileocecal valve. The ascending colon travels superiorly from the cecum to behind the liver, where it turns to travel medially across the abdomen as the transverse colon. Once the colon has reached the other side of the body, it curves downwards and travels inferiorly as the descending colon. The sigmoid colon is the final section of the colon, curving medially towards the rectum; the storage site for feces before defecation.

Movement

Small pouch-like segments of the colon (haustra) contract in slow, rhythmic movements called haustral churning A few times a day mass movements contract the ascending and transverse colon simultaneously Defecation is stimulated when the stretch receptors in the rectum are activated The mucosa of the large intestine secretes an alkaline mucous to protect itself. The mucosa is secreted in response to mechanical and chemical stimulation.

Absorption