The Exit Point: A Guide to Anus Function and Anatomy
Updated: 3 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD | Last reviewed: October 7, 2026
Quick Answer: What is the anus?
The anus is the last three to four centimeters of the digestive tract [1] and enables bowel movements. It uses two ring-like sphincter muscles, the involuntary internal sphincter and the voluntary external sphincter [1], which work with nerves to signal the urge to poop and allow conscious control. The anal canal also has mucus glands and the pectinate line.
What the research says
Anal fissure has several evidence-based non-surgical options. A network meta-analysis of 22 randomized trials with 1,770 participants found nifedipine had the highest healing rate, diltiazem had the lowest recurrence and the best pain relief, and nitroglycerin (GTN) had the highest rate of adverse effects (Wang et al., International Journal of Surgery, 2025) [2].
Surgery on the internal sphincter heals most chronic fissures. In a meta-analysis of 8 randomized trials with 1,035 participants, lateral internal sphincterotomy led to healing in 90.2% of patients, with recurrence in 3.7% and postoperative incontinence in 8.9% (Asefa and Awedew, Scientific Reports, 2023) [3].
What is the anatomy of the anal canal?
The anus is not just a simple opening; it is a highly specialized canal with a complex internal structure. One of its most defining features is the pectinate line (also known as the dentate line), which serves as a major anatomical boundary. Above this line, the canal is lined with tender mucosa and supplied by involuntary nerves. Below the line, the tissue transitions into normal skin and is supplied by voluntary nerves, making the lower portion significantly more sensitive to pain and touch [1].
The internal lining also features vertical folds known as anal columns, separated by grooves called anal sinuses. These sinuses house small glands that secrete mucus to lubricate the passage of stool [1]. At the base of these sinuses are the anal valves, which mark the transition point of the tissue lining [1]. For more on how sphincters work throughout the body, see our guide to sphincters.
Feature | Location | Primary Function |
Internal Sphincter | Upper Anal Canal | Involuntary muscle that stays closed until the rectum is full. |
External Sphincter | Lower Anal Canal | Voluntary muscle under conscious control for bowel movements. |
Anal Sinuses | Between Columns | Contain glands that secrete mucus for lubrication [1]. |
Pectinate Line | Middle Canal | Dividing line for nerve sensitivity and vascular supply [1]. |
How do the anus and anal canal control bowel movements?
The primary function of the anus is to facilitate controlled bowel movements. This process requires precise coordination between the rectum, the anus, and the nervous system. When the rectum—the reservoir just above the anus—becomes full, it sends signals to the brain, triggering the urge to have a bowel movement. This causes the internal anal sphincter to relax automatically [1].
However, the external anal sphincter remains under your conscious control [1]. This allows you to hold in waste until you reach a restroom [1]. When you are ready, the nerves and muscles coordinate to push the stool through the lubricated canal. Beyond waste elimination, the anus also acts as a barrier, keeping the digestive tract closed to prevent external infections from entering the body [1].
What are common anal conditions, and how are they diagnosed?
Because the anus is a sensitive and high-traffic area, it is susceptible to various disorders. Hemorrhoids are perhaps the most common, involving swollen blood vessels that can occur above or below the pectinate line [1] (see hemorrhoid banding for one treatment option). Anal fissures, or small tears in the lining, can cause significant pain and bleeding during bowel movements [1]; read more in our guide to anal fissures. Other issues include anal stenosis (narrowing due to scar tissue) [1] and functional disorders like anismus, where muscles fail to coordinate correctly [1].
Healthcare providers use several methods to diagnose these issues, ranging from a simple digital rectal exam to more advanced tests like anoscopy (using a scope to view the canal [1]; see proctoscopy) or anorectal manometry, which measures muscle pressure and nerve response.
“Muscles, nerves and mucous membranes in your anus work together to make healthy bowel movements that you can control. They also work to keep your anus closed to infections coming in from the outside.” — Cleveland Clinic [1]
What symptoms of anal health problems mean you should see a provider?
It is important to monitor for signs of anal health issues, as early intervention can prevent more serious complications. Common symptoms that warrant a visit to a healthcare provider include persistent anal pain, itching, or unusual discharge [1]. Bleeding during bowel movements should always be evaluated, even if it is painless, as it can be a sign of anything from a minor fissure to more serious conditions like anal cancer (see also anal dysplasia).
Frequently Asked Questions
What is the anus?
The anus is the final three to four centimeters of the large intestine where waste exits the body.
What is the difference between the anus and the rectum?
The rectum is a reservoir where poop collects; the anus is the exit canal.
What are anal sphincters?
Ring-like muscles that keep the anus closed and control bowel movements.
Is the internal sphincter voluntary?
No, it is involuntary and relaxes automatically when the rectum is full.
What does the external sphincter do?
The external sphincter is a voluntary muscle that allows you to consciously control when you poop.
What is the pectinate line?
A dividing line in the anal canal that separates different nerve and blood supply zones.
Why is the lower anus more sensitive to pain?
Because it is supplied by voluntary nerves, unlike the upper portion.
What are anal columns?
Vertical folds in the inner lining of the anal canal.
What is the function of anal mucus?
Anal mucus lubricates the canal to help stool pass smoothly.
What are hemorrhoids?
Swollen blood vessels in the anus or rectum.
What causes an anal fissure?
A tear in the anal lining, often caused by passing hard stool.
What is an anal fistula?
An abnormal tunnel leading from an infected gland to the outer skin.
What is anismus?
A condition where muscles don’t coordinate correctly to allow a bowel movement.
What is an anoscopy?
A diagnostic test using a small scope to look inside the anal canal.
Can the anus develop cancer?
Yes, anal cancer is a distinct disease, usually squamous cell carcinoma.
Related reading on Rinnit
Sources
Cleveland Clinic. Anus. Cleveland Clinic. Last updated March 3, 2023. https://my.clevelandclinic.org/health/body/24784-anus-function
Wang C, Ni J, Xiong Y, et al. The efficacy of diltiazem, glyceryl trinitrate, nifedipine, minoxidil, and lidocaine for the medical management of anal fissure: a systematic review and network meta-analysis of randomized controlled trials. Int J Surg. 2025;111(4):3020-3029. PMID 39878173. https://doi.org/10.1097/JS9.0000000000002263
Asefa Z, Awedew AF. Comparing closed versus open lateral internal sphincterotomy for management of chronic anal fissure: systematic review and meta-analysis of randomised control trials. Sci Rep. 2023;13(1):20957. PMID 38017243. https://doi.org/10.1038/s41598-023-48286-z
Medical disclaimer
This content is for informational purposes only and does not constitute medical advice. Always seek the advice of a qualified healthcare provider with any questions regarding a medical condition.
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