The anus

The anus is the name for the muscular opening at the very end of the large bowel, which is part of our digestive system.

The digestive system
The digestive system

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When food has been swallowed, it passes down the gullet (oesophagus) to the stomach, where digestion begins. From here, it enters the small bowel, where essential nutrients are taken into the body. The digested food then moves into the colon, where water is absorbed. The remaining waste matter (stool or faeces), is held in the rectum (back passage) until it’s ready to be passed from the body through the anus as a bowel motion (stool).

The area that connects the anus to the rectum is called the anal canal and is around 3–4cm (1–1½in) long. The lower end of the anal canal, where the anus opens, is called the anal margin. The anus is opened and closed by a ring of muscle known as the external sphincter.

A cross-section of the anal canal
A cross-section of the anal canal

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The walls of the anal canal are lined with cells called squamous cells. Nearly all anal cancers develop in these cells. We have more information about different types of anal cancer.

Where the anal canal meets the rectum (transitional zone), the walls are lined with a mix of squamous cells and glandular cells. Glandular cells produce mucus that helps the bowel motions to pass through the anus.

The lymphatic system

The lymphatic system helps to protect us from infection and disease. It also drains lymph fluid from the tissues of the body before returning it to the blood. The lymphatic system is made up of fine tubes called lymphatic vessels that connect to groups of lymph nodes throughout the body.

Lymph nodes (sometimes called lymph glands) are small and bean shaped. They filter bacteria (germs) and disease from the lymph fluid. When you have an infection lymph nodes often swell as they fight the infection.

Sometimes cancer can spread through the lymphatic system to nearby lymph nodes. You may have tests on the lymph nodes to look for cancer cells. We have more information about these tests.

Lymph nodes close to the bowel
Lymph nodes close to the bowel

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Back to Understanding anal cancer

Types of anal cancer

There are different types of anal cancer. The most common type of anal cancer is squamous cell carcinoma.