Defaecation (also called egestion) is the removal of undigested and indigestible material from the alimentary canal through the anus. It is the last of the five processes of nutrition.
What is in faeces?
Faeces are formed in the large intestine and contain:
- Dietary fibre (cellulose) that humans cannot digest;
- Dead cells shed from the gut lining;
- Bacteria from the large intestine;
- Small amounts of water and bile pigments (which give faeces their colour).
Key idea
Defaecation ≠ excretion. The material removed was never absorbed into the body's cells, so it is not a metabolic waste product. Excretion removes wastes made by cells (e.g. urea, carbon dioxide).
Forming and removing faeces
In the colon, water and mineral salts are reabsorbed from the undigested remains, making them semi-solid. The faeces are stored in the rectum and expelled through the anus, which is controlled by muscular sphincters. Peristalsis moves the faeces towards the anus.
Importance of dietary fibre
Fibre adds bulk to the faeces and holds water, making them soft and easy to move. This stimulates peristalsis and prevents constipation. A diet low in fibre can lead to hard faeces and difficulty in defaecation.
The time material spends in the colon matters. If it moves too slowly, too much water is reabsorbed and the faeces become hard, causing constipation. If it moves too quickly, as in diarrhoea, too little water is reabsorbed and the faeces are watery, risking dehydration. A diet with enough fibre and water helps keep this balance normal.
Example
Eating fruits, vegetables and wholemeal foods increases fibre intake. The extra bulk keeps the colon active, so waste is removed regularly.
Remember
Faeces contain material that was never truly inside the body's cells — that is why defaecation is egestion, not excretion.