# 1.8 Defaecation 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. - **Defaecation:** The final act of digestion, where organisms eliminate solid, semisolid, or liquid waste material from the digestive tract via the anus. Also known as *defecation*. - **Faeces (or Feces):** Waste matter discharged from the bowels after food has been digested. - **Anal Sphincters:** Two muscles controlling the exit of faeces. - **Internal Anal Sphincter:** Composed of smooth muscle and is under involuntary control. - **External Anal Sphincter:** Composed of skeletal muscle and is under voluntary control. ## Defaecation Reflex in Infants - The reflex is entirely **involuntary**. - The internal and external anal sphincters relax automatically when the rectum is filled with stool. - This reflex is particularly active shortly after feeding, especially in the first week of life. ## Defaecation Reflex in Adults The reflex in adults involves both involuntary and voluntary components, requiring the integration of and . ### Voluntary Initiation A person can initiate the reflex by: - Taking a deep breath. - Contracting the abdominal muscles. 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. ### Involuntary Reflex Trigger When the rectum fills and its walls are stretched (distended), sensory nerve receptors are stimulated. This triggers the **defaecation reflex**. ### Reflex Action - Peristaltic waves are stimulated in the descending colon and sigmoid colon. - The **parasympathetic nervous system** sends signals to the sacral region of the spinal cord. - The *internal anal sphincter* relaxes (involuntary action). - A signal is sent to the brain, creating the urge to defaecate. ### Voluntary Control 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. ## Comparison: Infants vs Adults | 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 |