9.5 - Defaecation

The Large Intestine

• Undigested food, dead epithelial cells, fibre and water enter the large intestine after nutrient absorption in the ileum.
• Peristalsis moves these materials slowly through the large intestine.
• Fibre consists of cellulose walls of plant cells.

Absorption of Water and Vitamins

• The large intestine absorbs water and mineral salts.
• It also absorbs bacterial metabolic by-products: vitamin B, vitamin K and folic acid.

Formation of Faeces

• After water absorption, the remaining semisolid waste forms faeces.
• Faeces contains dead cells from the inner intestinal layer, bile pigments, bacteria and toxic substances.
• Mucus from the large-intestine wall smoothens faeces movement to the anus.
• Faeces takes about 12–24 hours to reach the rectum.

Defaecation

• Faeces accumulates until rectal pressure triggers the need to expel it.
• Rectal muscles contract and expel faeces through the anus; this is defaecation.