Movement of Feces Through the Large Intestine

Functions of the Colon

1. Absorb water and electrolytes from liquid chyme to form solid feces
2. Store fecal matter until it is expelled

Two Types of Colonic Movements

1. Mixing Movements - "Haustrations"

• Large circular constrictions (~2.5 cm of circular muscle) contract, sometimes nearly occluding the lumen
• Simultaneously, the teniae coli (three longitudinal muscle strips) contract
• Together, these create haustrations - baglike outward bulges of the unstimulated colon wall
• Each haustration peaks in ~30 seconds and disappears in ~60 seconds
• They slowly "dig and roll" fecal material, exposing it to the mucosa for absorption
• Result: only 80-200 mL of feces expelled daily

2. Propulsive Movements - "Mass Movements"

• Occur 1 to 3 times per day, often within the first hour after breakfast
• A constrictive ring forms at a distended or irritated point (usually the transverse colon)
• The 20+ cm of colon distal to the ring loses haustrations and contracts as a unit
• This propels a large mass of feces forward in one go
• Contraction builds over ~30 seconds, then relaxes over 2-3 minutes
• A series of mass movements lasts 10-30 minutes, then ceases

• Gastrocolic and duodenocolic reflexes - distension of the stomach and duodenum after eating triggers these reflexes via the autonomic nervous system (that post-meal urge to defecate)
• Colonic irritation - e.g., ulcerative colitis can cause near-continuous mass movements

Defecation

Continence is maintained by two sphincters:

The Defecation Reflex (two components):

• Rectal distension triggers afferent signals through the myenteric plexus
• Peristaltic waves are initiated in the descending colon, sigmoid, and rectum, pushing feces toward the anus
• The internal anal sphincter relaxes
• If the external sphincter is also voluntarily relaxed, defecation occurs
• By itself, this reflex is relatively weak

• Rectal stimulation sends signals to the sacral spinal cord (S2-S4)
• Parasympathetic signals return via the pelvic nerves
• This greatly intensifies peristaltic waves and further relaxes the internal sphincter
• This reflex is essential for effective defecation

Voluntary Assistance

• The person can facilitate defecation by taking a deep breath, closing the glottis, and contracting abdominal muscles - this increases intra-abdominal pressure (the Valsalva maneuver)
• If defecation is consciously suppressed, the urge usually fades after a few minutes as the rectum relaxes

Transit Time

• Chyme takes 8 to 15 hours to travel from the ileocecal valve through the entire colon
• During this time it is converted from semifluid slush to semisolid feces

Source: Guyton and Hall Textbook of Medical Physiology, Chapter 64 - Movements of the Colon & Defecation