Answer:
a stretched rectum
Step-by-step explanation:
The urge to defecate is caused by the stretching of the rectum. The rectum is the last part of the large intestine and it is where feces are stored until a person has a bowel movement.
When the rectum is stretched by the accumulation of feces, the muscles in the rectum walls contract, creating an urge to defecate. This is known as the gastrocolic reflex, and it is the body's way of signaling that it is time to have a bowel movement.
The stretching of the rectum stimulates the nerves in the rectal walls, which then send a signal to the brain to initiate the urge to defecate. The contractions of the rectum also help to move the feces towards the anus, which makes it easier to pass them out of the body.
It is important to regularly have bowel movements to prevent the accumulation of feces in the rectum and maintain good digestive health.