Large Intestine

Large Intestine

The large intestine includes motility, regulation of motility, digestion and absorption, and emptying of the rectum.

The cecum

is a blind diverticulum at the beginning of the colon.

• is inconspicuous in carnivores.

• is a large blind tube in ruminants.

• is huge in equines.

Three parts: base, main body, and apex.

• The ileum opens into the cecum, colon, or both (depending on species)

Four parts: 

• Cecum

• Colon

• Rectum

• Anus

Listening to GI sounds in equine

Left dorsal quadrant: Pelvic flexure (typically quieter but present)

Left ventral quadrant: Large/small intestine

Right dorsal quadrant: base of cecum (typically the loudest) can hear “cecal flush” caused by ileocecal valve muscles

Right ventral quadrant: cecum

Segmentation of Colon and Cecum 

• Segmentation in the colon and cecum involves sacculations called haustra.

• Muscles form bulging sac-like structures.

• Areas contract, then relax

• then different areas contract and relax.

• This prolongs the time contents stay in the large intestine.

• It increases time for absorption and microbial digestion.

Motility of the Large Intestine

• Ingesta travels slowly.

• Ingesta must be thoroughly mixed to allow contact with absorptive surface.

• Motility patterns vary with species.

•  Segmentation

•  Peristalsis

•  Antiperistalsis

•  mass movement.

Regulation of Large Intestine Motility

• Slow waves are initiated by pacemaker cells.

• These slow waves are under the influence of the autonomic nervous system.

• There are alternating periods of activity and rest.

• Depolarizing waves sent toward the stomach create antiperistaltic movements.

• Depolarizing waves sent toward the rectum create peristaltic movements.

Digestion and Absorption in the Large Intestine

Functions 

• absorbs water and ions.

• completes carbohydrate or protein microbial digestion and absorption.

• In horses, the fermentation center “hindgut” is positioned after the stomach, where enzymatic digestion occurs.

Emptying the Rectum

• Chyme passes through the large intestine.

• Much of the water is absorbed.

• It leaves as a semisolid material called feces.

Sensory receptors stimulated when feces are transported to rectum

• defecation reflex is initiated 

•  colon and rectum contract 

• inner sphincter relaxes = need to defecate is perceived = outer (voluntary) anal sphincter muscles relax