The anal canal is the area between the rectum and the anus, and is where the large intestine, and technically the digestive system, ends. It is approximately 1 inch (2.5 cm) to 1.5 inches (4 cm) long and is surrounded by a sphincter, a muscle that controls the opening and closing of the canal. The sphincter is what controls bowel movements.
Technically, the canal has three distinct parts: zona columnaris, zona hemorrhagica, and zona cutanea. The zona columnaris takes up the upper half of the anal canal and is lined with the same tissue as the small intestine, stomach and large intestine, whereas the lower half of the canal contains the zona hemorrhagica and zona cutanea. Both lower parts contain different tissues than the zona columnaris, including tissue typically seen in more exposed skin, such as the lips.
The sphincter of the anal canal controls the evacuation of feces. It surrounds the lower half of the canal and has two aspects: an inner sphincter and an outer sphincter. The inner sphincter, the part close to the large intestine, is involuntary and can’t be consciously controlled, and the outer sphincter is voluntary, meaning it can be willfully opened or closed. A sphincter can both hold feces in or let it out through the anus.
One problem that is commonly seen in the anal canal is hemorrhoids. Hemorrhoids are enlarged veins that can cause itching and discomfort. In some cases they can fall out, or prolapse, out of the canal into the anus. They often bleed in more severe cases. When feces is in the canal, this can cause more discomfort, even with small hemorrhoids. Fortunately, hemorrhoids can be treated with either medication or surgery.
Other problems can arise with the anal canal, including fissures, abscesses, and fistulas. Fissures are small tears in the canal and can be very painful, and the contracting and expansion of the sphincter exacerbates the fissure. Abscesses are infected growths and can easily arise from a fistula. A fistula is an abnormal tract or channel that forms in the skin as the result of infection.
Cancer of the anus and anal canal is relatively rare, accounting for roughly 2% of all cancers of the gastrointestinal tract. It is much less common than colon cancer, which has a roughly 100 times higher rate of fatality. Anal cancer occurs equally in men and women.
Fortunately, the anal canal is easily accessible as it is so near the outside of the body. As a result, changes or problems in the area are readily noticeable. Since the anal canal is readily accessible, treatment, particularly surgery, is relatively easy. Even so, approximately 600 people a year die of anal cancer in the US.