How the bowels and anus work

To understand constipation, it helps to know how the lower part of the gut works.

The colon (large bowel) is the lower part of the gut. It is more than 1 metre long. Its job is to store faecal material and remove fluid from it, so that faeces are fairly solid and the body does not waste water. The colon may absorb 1 litre of fluid a day. The colon contains lots of helpful bacteria that break down food residues (turning some of them into wind) and manufacturing some vitamins. The muscles of the colon gently contract and relax all the time, rolling the waste matter about like clothes in a washing machine. Several times a day, usually after meals, the colon makes some big muscular contractions to dump the faecal material in the rectum beyond it.

The rectum and anal canal. The large bowel (colon) leads into the last part of the gut, which is called the rectum. It is about 12–15 cm long. The final 3 cm of the gut is called the anal canal.

When faeces arrive in the rectum, it sends a message to the nerve centres in the spinal cord, and these send a message to the sphincter muscles of the anal canal, making them relax to open the anus. If it is inconvenient for us to have our bowels open, the brain sends a message to the spinal cord to prevent the ‘open anus’ message being sent. We are not aware of this until the rectum becomes very full, when we have to make a conscious effort to keep the anus closed.

When we allow the anus to open, the muscles in the wall of the large bowel and rectum contract to push the faeces out. The wall of the anal canal is very muscular. The muscles keep the anus closed, except when faeces are passed.

The ring of muscle at the top of the anal canal is called the ‘internal sphincter’. This muscle is not under our conscious control.
The ring at the opening of the anus is called the ‘external sphincter’. This muscle is more like the sort of muscle that we have in our arms and legs, and we are able to control it (until the urge to pass faeces becomes overwhelming).

It is obvious that in babies the system of nerve messages that keep the anus shut is not in place – babies pass faeces as soon as the rectum fills. After about 18 months of age, the system develops, but in some children this can take a long time.

A network of small veins lies under the lining of the anal canal. These veins form a soft, spongy pad that acts as an extra seal to keep the canal closed until you go to the lavatory. The lining of the gut is very slimy (so that faeces can pass along easily); the extra seal stops the slime (mucus) from leaking out. The spongy pads can become swollen. When this happens they are called piles (haemorrhoids).

As food moves through it, the colon absorbs water while forming waste products. The waste products are the faeces. Muscle contractions in the colon push the faeces towards the rectum, which is the last section of the bowel before the anus (back passage). By the time the faeces reach the rectum, they are almost solid because most of the water has been absorbed.

The hard and dry faeces of constipation occur when the colon absorbs too much water. This happens because its muscle contractions are slow or sluggish, causing the faeces to move through it too slowly.

Written by: Dr Margaret Stearn
Edited by: Dr Margaret Stearn
Last updated: Tuesday, April 5th 2011

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Comments on this article

Posted by Optional on 24/03/2013 at 01:19

I just got out of the hospital after 5 days cause i thought i had a hernia but i was constipated really bad could not poop at all the hospital gave me everything they had for it amd nothing my wife gave me 2 womens laxative pills and man what a relief you take them at bedtime and in the morning what a relief i have always had trouble going everyday and it ended up being from 2 to 7 days my doctor told me it is not good should go everyday or every second day i hope this helps some people

Posted by Optional on 19/03/2013 at 12:31

Still need advice on going, i took two castor oil yesterday, was busy and got back home nothing, drank tea this morning half went, back home now it is eve and still haven't gone, so i took two more. i'm waiting now and drinking water. stress related.

Posted by pain on 11/03/2013 at 07:36

After complete hysterectomy i ended up with constipation,its been years now i get inpacted and have to remove stool at times when stool softeners dont work,it usually is like a hollow area with stool in there the past week have been having alot of low abdominal pain i thought it may be stool in there but as soon as i put finger in anus theres a buldge like thing that has a hole in it,could it be a protruding rectum? Any help would be appreciated!

Posted by Optional on 21/02/2013 at 09:31

I have nerve damage at L4 L5 and cant sit or walk without crutches. When I have to poop which is difficult I have to sit on toilet and push That causes pressure on feet and glute and causes more pain Do u know how I can do this fast so I dont cause more pain?

Posted by Joy on 10/02/2013 at 12:42

My comment on it, is that my question is not answered, i said am pregnant and i have a painful boil on my anus, and i dont know what to do?

Posted by Optional on 07/02/2013 at 08:00

I have ms,stool stays in for 8daysand is very large when it finaky cames out,i have to ask someone to help me,it to big for me to get out,trihelped many thingd but it dont help i just wait until colon is full and it has to come out,its is just very big,can you help

Posted by Christoph on 02/02/2013 at 08:30

I used too lift up heavyweights & do sit ups to develop six packs. But, at a stage, it became very pain for me to sit with my stomach relaxed. And to develop muscles in lifting up heavy weights while standing, i heard a sound like something cracked at my back. Every since, i developed a lump at my left backside. Years after not seeing a doctor, i started passing out blood though i had too much of sugar while taking too much soft drinks. And not, i can barely get up from the loo, even if i do, i feel pains in my right leg & cant walk well nor sit comfortably due to enlargement in my anus. Pls help, sometimes, i go unconscious whenever i try to stretch. And i also have a bulging out scapula at my right arm that sometimes make my fingers & my arm numb

Posted by Optional on 28/01/2013 at 11:50

HELP! i am really constipated and havn't pooed in 11 days now. I am 16, and as a kid i was always contipated because i am scared to poo. This is recently getting worse again, an at the same time as really wanting to poo i am scared. My worse experience was having bleeding anal hole, which put me off pooing. I keep having really big farts and thining its a poo. Ive trie everything, ive tried coffee,Actimel,yoghurts,raisens,grapes. I went hinking earlier, and i could feel a poop coming, however the urge only comes when i'm out. Also, i cannot poo unless there s no one else in the house because i feel self conscious as alo my family know i block the loo. This is taking over my life, i just want to over come this it's been going on too long now! has anyione got some advice, how to soften my stool to make it eaier t come out? or how to make myself go for a poop more than once every fortnight.

Posted by mrs mcgarr on 28/12/2012 at 02:45

Hi ive been very constipated for 2 days now i am in agony..It feels like its way too big and hard to pass through.. I keep gettin the urge to go and nothing happens when i go toilet..I cant sit down properly or sleep to good. Ive tried Duphalac Miralax Prune Juice Orange Juice & water..None have worked.Anyone with any tips would help a lot..Thank you

Posted by stiffy on 04/10/2012 at 12:05

hi, i just came back from bodrum turkey, and afer 2 days there i was constipated. i tried the usual methods but as a regular 2 times a day guy im now thinking it may be a bit more than a bit bunged up, i feel fine in myself but am very bloated in the stomach and now am getting out of breath very quickley. any ideas?

Posted by rachael on 13/09/2012 at 11:54

i have had constipation for ages. i've always had problems with my 'movements' since i was born. but recently i've been having constipation for 4-7 days, then i have a large painful bm. but i've had constipation for about 10 days and whenever i feel the urge to go to the toilet, my rectum opens up and the feces just sticks out a little bit but not much..but i never get a good movement. any tips? i also have a giant bloated belly, i look pregnant..it's not even funny.

Posted by lHergu on 18/07/2012 at 06:26

I have constant urge for bm but i feel constipated and BM does not happen. The pressure is not in stomach or intestine but rectum.I feel constant residue left even I have BM. I have some lower back pain but I do not know if this is thje cause.

Posted by Optional on 17/06/2012 at 08:55

for the pat 4 years in order for me to move my bowels i have to stik my finger in my anal the poop is very hard i have not pain. i have an apetite, i eat a lot of fruit i am not over weight

Posted by Sydney on 20/05/2012 at 12:10

I lost the nerve sensation in the internal anal sphincter muscle, due to MS. Has anyone else experienced this problem?

Posted by lindsay on 09/05/2012 at 06:38

To Amy: I too am having the exact problems. I have pretty severe IBS and I've tried everything. The only thing I've found in the last 2 years that seems to help is taking an "Align" tablet once every night without missing a day and drink about 2 pints/1 1iter a day. My stomach pain and occasional bleeding make me worry that it could be something serious..

Posted by jen on 12/03/2012 at 10:32

I am having this horrible time with constipation and a horrible feces-butt Body Odor, my anus itches and when i do go to the restroom I have to use my finger to get the remainder of my stool out because it just sits there in my anus. I have several questions I read above that hard adn dry feces is because the colon absorbs to much water at one time all I did was drink water adn I'm talking 16 oz. bottles at least 8-9 a day if not more..i was really thirsty.... never checked for diabetes but my odor began after i had my 2nd child who was 8lbs, my first was 6lbs could that second birth caused my fecal body odor to began or could it have been already in my genes-TMAU- and I just never knew until then, my odor doesn't only have a fecal smell but at times it can have a garbage, trash, skunk, unrine, sweet s**t smell. I know that is no where near normal please help me I have been to doctor after doctore with no success what so ever. Thanking you in advance, Jen

Posted by Michael on 24/02/2012 at 01:40

I just had surgery and found I had a 3 day no bowel movement. This is not usual for me. The surgery was 3 weeks ago and I take norco and tramadol and I also take a stool softener but this am I feel the urge to go and when I tried is what concerned me. I felt like my entire end was trying to come out, almost like a 5-6 inch circle coming out. Very concerned and again unable to go I try to feel what's going on. This is when I realize I have a huge rock hard ball of feces that won't budge. I'm starting to worry since I had been Pushing and not have a large amount of prolapsing! I thinking what do I do? So I decided to dig it out. I began bleeding but got it all out. Is the prolapse due to straining and why would this build up like this if there is a stool softener on board? Just wanted to share since many are turned off about sharing since it's about poop!

Posted by Kath on 20/09/2011 at 10:47

Hi, The problem i was looking for was a clear discharge from the anus. I googled it and it brought me to this page but still couldnt find the answer.

Posted by steve on 14/09/2011 at 02:04

My job is industrial abseiler so I can only go to the loo when it's possible as I am normally 'trapped' in my work area and it takes too long to get to the loo. I have done this job for 8 years now and have no problem with it. My question relates to the fact that people think I am strange for taking so long on the loo when I'm not at work. I have always visited the loo and had a 'double poo' as I call it and this to me is normal. I am also an engineer so when I have my first poo, and then wait for the second, I think. After the first I have an incomplete feeling so I naturally remain on the loo. It's like my body is telling me there is still some bad waste that the body doesn't want and it wants me to sit and wait, but for what? Untill the second round comes. This suggests to me that the passage is a rotating chamber that can only hold so much, then it off loads, spins around, collects another load, and then comes back around to drop it off. I have read about the workings on the net but nothing has mentioned this circulating chamber thing. If you could tell me why this happens I would be very greatful, and probably amazed! I know this may sound like a joke but I genuinely am intersted! Hope you can help, thanks, steve

Posted by cxty on 09/09/2011 at 06:48

I have a problem when I have bowel movements. I can use the bathroom just fine but it always seems that right when the last bit of fecal matter goes to exit my anus, it gets stuck right at the end. Its like I don't have the muscle at the very end of my anus to push the last bit out. Is this possible, if so, what is is called? Also, what can I do about it?

Posted by Tom wallace on 06/05/2011 at 06:01

I have spina bifida and have a majorly embarrising problem my problem is every time I have an ejaculation my bowels open . No matter if I have just been to the loo it has happened many time any suggestions to help me please.

Posted by Mike Johnson on 18/03/2011 at 07:25

Hi,I. Have a problem where its very very difficult to have a Bowell movement.At first I thought it was constipation but soon I found out that it was not that.I have a lot of pressure in my butt and it feels like my anus is closed shut.I hardly go to the bathroom and when I do its watery like never solid.Iam so depressed I don't know what's wrong with me or can I be fixed.can anybody help?

Posted by Kerrie on 12/12/2010 at 01:52

To Amy: I had the same problems. Treatment for Giardia fixed it. My constipation is caused by chronic pain medication. Nothing worked till I found Duphalac (a lactase syrup) which softens the stools. (No, I don't have anything to do with the sale of it.)Drinking at least a litre (2 pints) of water a day helps. Good luck. I am now having clear leakage, especially at night. I'd appreciate some advice.

Posted by Optional on 05/11/2010 at 09:09

Went to the toilet just to wee. Then felt the need to poo. It felt stuck half in and half out so I just sat there waiting. After about half an hour I didn't know what to do. I couldn.t get up because I was pushing (bowel open) so I eventually had to use my finger. It was the most disgusting and traumatic experience I have ever had. Why does this happen?

Posted by Optional on 04/11/2010 at 01:44

i recently had a partial hysterectomy and now my anal canal in having spasms when i try to have a bowel movement what can i do

Posted by Utpala on 16/09/2010 at 10:55

I am getting fluid out from my anus. what could be the reason?

Posted by Amy on 10/04/2010 at 02:26

Hi there, I'm having a lot of problems with my digestive system at the moment which is causing me to be very anxious and stressed. To relief the pain from my stomach and to enable to poo most days I have to eat 3 weatabix a day without fail! My pain is mostly on a morning when I wake up.. I have a lot of pain in my kidneys and also in my stomach (I can only describe the pain as an ulcer feeling)I get A LOT of wind too which is very horrible and embarrassing have been to the doctors/referred to the hospital and they diagnosed me with IBS and examined my anus and told me it looked healthy. They also told me to not eat too much fibre which can also cause IBS, which I found this only made my problem even worse to the point I didn't poo for 10 days! The pain got so bad I could hardly move and any kind of pressure on my stomach area was soooo painful. I took it upon myself to start eating weatabix everyday which I found this really helped me to go to the toilet nearly everyday and relief the pain and discomfort a little from my stomach. Now I can poo everyday I get this other problem which is even worse for me! I noticed I was getting a lot of itching around my anus which was so irritating for me. I went to the doctors thinking I had some sort of Ring/Tread worms (because I have a dog and a cat and thought I may have caught worms from them. I told the doctors my symptoms so she described me with treatment for worms. This problem did not go away with the treatment. I then went back to see the doctor and told them it had not worked. She told me to give them a stool sample for them to take away and test. The sample came back negative (I didn't even have worms!!!) NOW WHAT! She told me I might be suffering with piles which maybe me causing the irritation and itchiness. She gave me some thyroid cream to treat inside and around my anus to see if that helped. The treatment was a cream that had a pointy end to it which enables you to squirt the cream inside your anus. I found the application quite difficult to use and felt as though not enough of the cream was been inserted inside me. I took it upon myself to apply the cream inside me with my finger rather than using the application. Whilst applying this inside my anus I felt something! It was poo inside me! I couldn't belive after all this time, after all the treatment and discomfort I had gone through I suddenly new what the problem was! Forget the pile treatment, when I was going for a poo I must not have been finishing properly (although I thought I had done) and poo was getting stuck and staying inside me. The more I've noticed this problem the more I find myself removing the excess poo from inside me with a baby wipe and my finger because I know if I don't the itchiness and discomfort will start again. I have to do this EVERYDAY now and it really must be effecting the tissues inside of me. I see that there is bleeding on the baby wipe whilst I’m doing this. I've also noticed bleeding in my poo too. This is also definitely effecting my personal hygiene by having to do this. This problem is really causing me severe depression and anxiety along with all of my stomach pains throughout the day and first thing on the morning/All the wind I get throughout the day/Having to eat manky weatabix every morning WITHOUT FAIL and sometime even before bed on a night/I've also started suffering from chest pains now too(This maybe related to my anxiety and depression I don’t really know?? I'm just so worried that I’ve got some kind of Cancer/Tumour somewhere within my digestive system. My specialist told me he wasn't going to take it any further and that he was going to refer me to a dietician. I'm still waiting for the letter after two months! This is really ruining my life and getting me so down I don't even want to leave the house most days, it's also really effecting my love life too. I don't know if the pain in my stomach and me not been able to poo properly is related??? Please if anyone have any kind of answers or has any similar symptoms please can you talk about this with me. I hope I'm not the only one going through this as I am so worried. To the point I’m starting with really severe Physiological problems that I’m slowly dying! Please help. Thank you Amy

Posted by Charlie on 07/02/2010 at 06:20

Life long sufferer from IBS.For peace of mind,always have to be within 2 minutes of a toilet. My lifesaver is Imodium,(Loperamide Hydrochloride,2mg capsules)Two of these taken on the evening before an outing,ensure reasonable safety. Note. This medicine is used for social purposes and is not a cure. Good luck.

Posted by kaz on 06/02/2010 at 11:21

i had an hysterctomy 3 years ago and then further surgery to correct and during this op my bowel was perforated and then i had further surgery and had small bowel removed and since have been in constant need of opening my bowels after every thing i eat or drink - have you got any advice or tips please - iv'e no life can't do anything or go anywhere unless i take immodium and i am in constant pelvic pain.

Posted by Anonymous on 04/01/2010 at 05:01

So many of the comments I see here relate to improper digestion, and elimination, of the foods we eat, and the inability for nutrients to be absorbed into the blood. Helene Silver has several GREAT books about all this, one is called "The Body-Smart System", another is called "Rejuvenation". Even if you don't have the stamina to do her 21 day detoxification program, the info (in the Body-Smart System in particular) is critical reading. Due to the type of eating habits we all have become accustomed to, our bodies are very toxic, resulting in the kinds of "symptoms" noted here. We are sorely in need of detoxifying our bodies and bloodstream and changing what and how we eat. Hope this helps. (I come from a family with several members who died of colon cancer, and have been making changes all my adult life. Keeping the colon clean, nourished, and eliminating efficiently is paramount.

Posted by Anonymous on 25/12/2009 at 03:22

I have a problem with my head. Whenever am reading through the comments i feel great headache because i see only querries wit no replies or solutiom

Posted by Tom on 22/12/2009 at 09:45

Hi, i have a problem every now and then with my anus sweating and stinging, also when i wipe its as if i havent wiped properly as its always faecas coloured, its as if my anal area is weeping, but it is getting more regular mainly when im working, hope you have some good advise for me Thanks Tom

Posted by mona on 06/11/2009 at 10:28

I have aproblem w/ my vowel movements..i don't have the muscle around it so i always have to carry an extra pair of pants because i might have an accident...do you know any doctors that perform this type of surgery. I was told that in the US we don't have these services but that there are doctors in other countries.

Posted by Anonymous on 22/10/2009 at 02:51

I am passing bright red blood but I am also experiencing real back pain in the pit of my stomach and sometimes my back but find this topic very embarrassing

Posted by kumari on 07/10/2009 at 12:41

Because of medication I get constipated and sometimes the stools get stuck half in and out as they are so hard, I have had to use my fingers to remove the stools, but then get blood, is this just small broken veins.

Posted by Shane on 27/08/2009 at 08:16

I have been emitting a clear fluid from my anus these last few weeks and it seems to be on the increase ...? It seems particularly bad when I'm asleep.

Posted by Andy on 25/08/2009 at 02:39

Posted by Anonymous on 07/08/2009 at 01:06

I've had anal bleeding for a few years now, and I've been checked about 5 times in the last few years by my GP, but they still have no idea what causes my bleeding. Does anyone have any info on this?

Posted by Anonymous on 07/08/2009 at 12:46

I have great pain this the muscle area of the anus, do you or anyone know of a cure?