Defaecation is the physiological process of expelling faeces (undigested waste material) from the digestive tract through the anus. This process is controlled by a combination of involuntary and voluntary reflexes.
The reflex in adults involves both involuntary and voluntary components, requiring the integration of Neurons→ and Receptors→.
A person can initiate the reflex by:
This increases intra-abdominal pressure, forcing faeces into the rectum. This is often associated with the Valsalva maneuver, where forced expiration against a closed airway increases pressure.
When the rectum fills and its walls are stretched (distended), sensory nerve receptors are stimulated. This triggers the defaecation reflex.
The external anal sphincter is under conscious control. It is signalled to relax, allowing faeces to be expelled. A person can voluntarily inhibit defaecation by keeping the external anal sphincter contracted. If inhibited, the reflex subsides after a few minutes until the next peristaltic wave.
| Feature | Defaecation in Infants | Defaecation in Adults |
|---|---|---|
| Control | Involuntary | Involuntary reflex with voluntary override |
| Trigger | Rectal filling (often after feeding) | Rectal distension |
| External Sphincter | Relaxes automatically | Under conscious/voluntary control |
| Initiation | Automatic | Can be consciously initiated |