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 Anal Anatomy: as it relates to Anal Sex

The anal anatomy is often this dark and mysterious place in your body. Before you considered poking around in there you probably didn't care too much what it was like inside there. As you begin to stick objects up your rectum it becomes more helpful to understand it's shape, size and how it works.

My goal here is to give you a simplified understanding of your Anal Anatomy.


Your anus (asshole) is the opening to your anal canal. It has the ability to open quite wide without much difficulty, and is loaded with sensitive nerve endings. A great deal of the sensations that come from anal sex come from objects (penis/dildo/finger/tongue) touching or moving by this area. The external sphincter muscle is what keeps this closed.

Photo of anus (external)
1. outside view of anus (asshole)

Anal Canal

The anal canal is about a 1-2" long channel that is controlled by two muscles, one at either end. On the outside is your anus (asshole) which is kept closed by your external sphincter muscle. This is the muscle that you can tighten and loosen by pulling in or pushing out like you would when you take a shit.

Just beyond that, is your internal spincter muscle. This is a muscle that you cannot consciously control, your nervous system controls this one. You can however effect it by controlling how relaxed you are. If you are uptight, this muscle is very tight. If you are relaxed, the muscle relaxes.
This is the muscle that can cause you pain if you try to force it open.

In order for something to enter your anal canal both muscles must relax in order to permit the object (penis/dildo) inside. Most people live a very uptight life, and both these muscles stay VERY tight (leading to hemorrhoids and such). In order to allow objects up your anal canal without pain, you need to be relaxed so these muscles can relax as well.

The anal canal is where the majority of the sensation occurs that leads to pleasure during anal sex. This is also the part that most needs lube to reduce the friction so it doesn't get sore.

Once you are past these two muscles, you are in the rectum (rectal cavity).

2. Rectal Canal Photo
2. rectal canal

Rectal Canal
3. side view of rectal canal

Rectum (rectal cavity)

Your rectum is a delicate pouch lined with many muscles but few nerve endings. It's about the size of a small human fist. However, the pouch can get smaller (shorter) if the person is nervous or tense (thus tensing their muscles). The rectum is normally large enough to accommodate an object that is 5-6 inches long (measured from the asshole to the back of the rectum). Like a pouch, the rectum can stretch a little longer by getting less wide.

Usually the only sensation you can feel in the rectum a sense of fullness and/or pressure against the walls of the rectum (which initially feels like you have to take a shit). With experience you begin to feel these as pleasant sensations.

In a standing person, the rectum goes up at an angle from the asshole. Inflexible objects that follow the canal straight in often hit the back wall of the rectum without getting very far in. (yet this can be useful, see prostate gland) Pushed too hard, this can cause pain. Objects that are flexible such as a penis will bend and go up into the full length of the rectum to it's constriction at the rectal valves.

Photo of rectum wall
4. Endoscopy of rectum wall *

dildo angle
5. Rectum goes up at an angle
Rectal Valves

The other end of the rectum is closed off by the rectal valves. They are not really "valves" but simply folds where the pubo-rectal sling muscles pull the rectum shut. Most people have 3 or 4 of these. When you are relaxed these can open to an object pressing on them, when you are tense, they remain tight and will resist allowing an object to pass.

People who explore with longer dildos or have partners with longer penis' will find that these objects will continue up past the sling muscles/rectal valves when you are relaxed. Objects moving past these valves creates yet another pleasant sensation.

Under controlled tension, the rectum becomes compressed and even shorter objects (5-6 inches) go up into this area where the rectum joins the large colon at the bend. Veterans of anal sex will recognize these valves/muscles as the ones they use to "clamp down" and hold onto the penis or object they have inside.

The length of this area varies greatly from person to person. For some people this can be short (6" or so from the anus to the bend) for others it can be quite long (15"+ from anus to bend). This accounts in the variations between the size objects that people can put into their rectum before they hit the limit (at the bend in the colon).

Some people enjoy objects pressing on their colon wall at the bend, for many people however, this creates a painful experience.

Rectal valve closed
Rectal valve opening
6. Rectal valve- closed, 7.opening *

Rectal valve open
Rectal valve open
8. Rectal valve- open, 9.open*

pubo-rectal sling muscle contracted
10. Rectal valves closed

ppubo-rectal sling muscle relaxed
11. Rectal valves open
Bend in Large Colon

At some point the colon makes a 90 degree turn and heads horizontally across the body before turning again and heading back down. For a majority of anal exploration, this first bend is the limit of travel for anything but extremely flexible and soft objects. Rigid objects won't "turn" at the bend and simply poke into the wall of the colon (sometimes causing pain).

Only people exploring with enemas or deep probing will generally go past the first bend in the large colon.

Prostate Gland ("P -Spot")- Males Only

Object that enter the male anal canal and are aimed lower, can be quite pleasurable because they hit the prostate gland or "P-spot" . (Only males have a prostate gland, it's for the production of seminal fluids.)

Repeated pushing on the prostate causes a very pleasurable and intense sensation as well as manually forcing seminal fluids (ejaculate) out of the penis. Prostate massages in men are often done by inserting the lubed index finger into the anus and pressing on the wall of the rectum (towards the penis). Inside the rectum you can feel a small bulge (bump) that sticks up when the male is aroused. Massaging this bump can be a very pleasurable sensation.

In male-male anal sex, when the top (penetrator) aims his penis down more and hits the prostate gland, it causes a noticeable response in the bottom (person being penetrated.) It is extremely pleasurable and intense with repeated contact.

12. Angle for prostate massage

Further Information:

If you are trying to understand more about the anal anatomy an invaluable site I found is gastrointestinalatlas.com They have tons of endoscopy (micro camera inserted into the rectum) videos that help you to "see" inside the rectum and colon. Realize that most of the endoscopies were taken because of growths or diseases that would not be present in a healthy person. Yet they do a good job of showing the general structure of the anal anatomy.

Two short videos that really helped me to understand the anal anatomy were these:
Video of rectum
Look at inside of rectum
Video of rectal valves
Look at the rectal valves

*A special thanks to gastrolab.net for the rectal/colon photography.

Todd Perkins - Jan 2005
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