The anus is the opening at the very end of the digestive tract where solid waste leaves the body. It sits at the bottom of a short passageway called the anal canal, which connects the rectum (the final storage chamber for stool) to the outside. Though small, the anus is a surprisingly sophisticated structure, with layered muscles, specialized nerve endings, and a built-in reflex system that lets you tell the difference between gas and something more solid before anything exits.
Basic Structure and Size
The anal canal is the tube that ends at the anus. In adults, the full surgical length averages about 4.2 centimeters (roughly 1.7 inches), with a range of 3 to 5.3 cm. Men tend to have a slightly longer canal than women. The lower portion, measured from a key internal landmark called the dentate line down to the outer opening, averages about 2.1 cm. That’s shorter than a thumb joint, yet it packs in two distinct muscle rings, multiple types of tissue lining, and two separate nerve systems.
The dentate line (also called the pectinate line) is worth knowing about because it divides the canal into two halves that behave very differently. Above it, the lining resembles the moist, column-shaped tissue found in the intestines. Below it, the tissue transitions to the tougher, flat-cell skin type that covers the outside of your body. This transition matters for sensation, blood supply, and the types of problems that can develop in each zone.
How the Sphincters Work
Two rings of muscle wrap around the anal canal, and each one does a different job. The internal anal sphincter is a smooth-muscle ring that stays contracted on its own without you thinking about it. It provides the baseline “seal” that keeps the canal closed at rest. You have no conscious control over this muscle.
The external anal sphincter, by contrast, is made of skeletal muscle and is fully under your voluntary control. When your rectum fills and nerves trigger the urge to go, the internal sphincter relaxes automatically. The external sphincter lets you override that signal and hold on until you reach a bathroom. This two-layer system is the reason you can sense the urge to have a bowel movement but choose to delay it.
The two sphincters also run on different wiring. The internal sphincter is controlled by the autonomic nervous system (the same system that manages heart rate and digestion without your input). The external sphincter is powered by the pudendal nerve, which carries voluntary motor signals from the spinal cord at the S2 through S4 level.
Nerve Supply and Sensation
The anus has a split personality when it comes to sensation, divided right at that dentate line. Below the line, the tissue is packed with sharp, precise nerve endings supplied by the inferior rectal nerve, a branch of the pudendal nerve. This area can detect touch, pain, and temperature with the same clarity as your fingertip skin. That’s why problems in the lower canal, like a fissure, tend to be intensely painful.
Above the dentate line, sensation shifts to a dull, vague quality carried by the autonomic nervous system. You can sense pressure and fullness up there, but not sharp pain. This is also why internal hemorrhoids, which sit above the line, often cause painless bleeding rather than the sharp sting of an external hemorrhoid.
The Sampling Reflex
One of the most underappreciated things the anus does is distinguish between gas, liquid, and solid stool before you decide what to do about it. This process, called the sampling reflex, happens dozens of times a day without you noticing. When material moves into the upper rectum, the internal sphincter briefly relaxes while the external sphincter tightens. This lets a tiny amount of rectal contents contact the nerve-rich lining of the lower canal, which “samples” what’s there. Based on that feedback, your brain decides whether it’s safe to pass gas or whether you need a toilet. When this reflex stops working properly, it can contribute to incontinence.
How a Bowel Movement Happens
Defecation is a coordinated chain of events. Large-scale muscle contractions in the colon push waste into the rectum. As the rectal walls stretch, stretch receptors fire signals through a local nerve network called the myenteric plexus. Those signals do two things at once: they intensify the wave of muscle contraction pushing stool downward, and they tell the internal anal sphincter to relax. At this point, you feel the urge to go.
If the timing is right, you consciously relax your external sphincter, the pelvic floor muscles shift to straighten the anorectal angle, and increased abdominal pressure helps push stool through the canal and out. If the timing isn’t right, the external sphincter stays contracted, the rectum gradually relaxes to accommodate the stool, and the urge fades until the next wave of contractions.
Blood Supply
The dentate line also splits the blood supply into two systems. Above the line, arteries branch from the superior rectal artery, and veins drain into the portal system (the network that routes blood through the liver). Below the line, blood comes from the middle and inferior rectal arteries, and veins drain into the systemic circulation that flows directly back to the heart.
This dual drainage is the reason hemorrhoids are classified as internal or external. Internal hemorrhoids involve the upper venous cushions draining into the portal system. External hemorrhoids involve the lower veins draining into the systemic system. The two types look, feel, and behave differently because they arise from fundamentally separate vascular networks, even though they sit just centimeters apart.
Common Conditions
Because the anus handles mechanical stress every day and sits in a warm, moist environment, it’s prone to several common problems:
- Hemorrhoids are swollen blood vessels inside or around the anal canal. Internal hemorrhoids typically cause painless bleeding; external ones are more likely to hurt, itch, or swell visibly.
- Anal fissures are small tears in the lining of the lower canal. They cause sharp, burning pain during bowel movements and sometimes light bleeding. Most heal on their own with softer stools and time.
- Abscesses are pockets of infection in the tissue around the anus, often causing throbbing pain and swelling.
- Fistulas are abnormal tunnels that form between the anal canal and the skin surface, usually as a complication of a previous abscess.
- Anal itching (pruritus ani) is extremely common and often caused by moisture, irritation, or over-cleaning rather than a serious underlying condition.
- Anal cancer is relatively rare but can develop in the cells lining the canal.
Diagnosis for most anal conditions starts with a visual exam of the skin around the anus, followed by a digital rectal exam where a provider uses a gloved finger to feel for abnormalities. If more detail is needed, an anoscopy (a short scope inserted into the canal) provides a direct view of the lining.