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.
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.
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.