The anus

The anus is the opening at the end of the large bowel. Stools pass through the anus when leaving the body.

The anus is the opening at the end of the large bowel where we pass poo (stools) out of the body. The bowel (colon and rectum) and anus are part of the digestive system.

The digestive system Long description: The diagram shows the stomach. Below the stomach is the small bowel. This is a long tube that is in the abdomen and leads to the large bowel. The large bowel is a wider tube. It starts on the lower right side, goes up, then across the top of the abdomen and down the left side of the body. At this point,it becomes wider to form the rectum. At the lower end, it joins the narrow tube that makes the anus where the bowel opens to the outside of the body. 

When you swallow food, it passes down the food pipe (oesophagus) to the stomach. This is where digestion begins.

The food then enters the small bowel, where nutrients and minerals from food are absorbed. The digested food then moves into the colon. This is where water is absorbed.

The remaining waste matter (poo) is held in the rectum (back passage). Nerves and muscles in the rectum help to hold on to the poo until you are ready to pass it through the anus.

The anal canal is about 3 to 4cm (1 to 1½ in) long. It connects the anus to the rectum. The area where the anus opens at the lower end is called the anal margin or anal verge.

The anus has a ring of muscle called the external sphincter. This muscle helps to control when you empty your bowels.

This illustration shows a cross section of the anal canal. It shows the lower end of the rectum leading into the transitional zone, then into the anal margin and finally the opening - the anus. The illustrations also shows the internal and external sphincters.

The walls of the anal canal are lined with cells called squamous cells.

Where the anal canal meets the rectum (transitional zone), the walls are lined with squamous cells and glandular cells. Glandular cells make mucus. This helps poo pass through the anus.