9.5 - Defaecation

The Large Intestine

After absorption of nutrients in the ileum is complete, the remaining undigested food, fiber, and secretions pass into the large intestine.

Absorption of Water and Nutrients in the Large Intestine

• Water and Minerals: The colon absorbs water and mineral salts back into the bloodstream.

• Vitamins: Symbiotic bacteria (such as Escherichia coli) in the large intestine synthesize vitamin K and vitamin B (including ${\text{B}}_{12}$ and biotin), which are then absorbed by the colon walls.

Formation of Faeces

• As water is absorbed, the remaining mixture becomes increasingly semi-solid, forming faeces.

• Faeces consist of undigested dietary fiber (cellulose), dead cells from the intestinal wall, bacteria, bile pigments (giving it a brown colour), and mucus.

• Mucus is secreted by the colon wall to lubricate the passage of faeces.

Defaecation

• Faeces are moved along the colon by peristalsis and stored temporarily in the rectum.

• As the rectum fills, pressure increases, stimulating sensory receptors and triggering the urge to defaecate.

• Contraction of the rectal muscles and relaxation of the anal sphincter expel the faeces through the anus.