Sampling

Figure 5: To see if the rectum contains flatus or faeces, the recto-anal inhibitory reflex results in relaxation of the internal anal sphincter and tightening of the external anal sphincter. The top end of the anal canal opens into the rectum and the sensitive epithelium above the dentate line checks to see if the cause of the distension was flatus or faeces.

As the rectum distends there is a reflex relaxation of the internal anal sphincter. This is termed the recto-anal inhibitory reflex. As part of this reflex the external sphincter is tightened to maintain continence. Relaxation of the internal sphincter permits opening of the top of the anal canal and `sampling' of the rectal content by the mucosa above the dentate line. This permits descrimination of flatus from faeces.