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How the bowels and anus work

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

The colon

The colon (large bowel) is the lower part of the gut. It is more than 1 metre long. As food moves through the colon it absorbs water and forms waste products (faeces). This ensures 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 and pushing the faeces towards the rectum, which is the last section of the bowel before the anus (back passage). Several times a day, usually after meals, the colon makes some big muscular contractions to dump the faecal material in the rectum beyond it.  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 more slowly.

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 innermost ring of muscle of the anal canal is called the ‘internal sphincter’. This muscle, which is not under our conscious control,  maintains a resting pressure in our anal canal.
  • The outermost ring of muscle of the anus is called the ‘external sphincter’.  It also forms the pelvic floor eventually. 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 spongy pads can become swollen. When this happens they are called piles (haemorrhoids).

Mucus secretion

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. Passage of mucus or slime from the bowel can be a normal phenomenon, but the amount varies between individuals and is increased in certain conditions. The commonest reason for increased mucus is irritable bowel syndrome but it can also be increased in inflammatory bowel disease (ulcerative colitis and Crohn’s disease) as well as some bowel tumours.

Written by: Mr Andrew Ramwell
Edited by: Mr Andrew Ramwell
Last updated: Thursday, November 7th 2013

 


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Comments on this article

Posted by beth on 16/01/2014 at 08:39

Hi I have been having a traumatic two weeks . I got extremly constipated to the point I was hospitalized due to a impacted stool it had been 2 weeks without a bowel movement I had tried fybogel docusate piccasulphate microlax and glycerol suppositries nothing moved it the stool did however pass eventually and consisted of a few lumps with soft bits the pain was awfull . since passing this stool on tuesday I have not passed anything else im frightned as of what is going on ive been drinking loads of water taking laxatives still amd eating fibre ritch foods .

Posted by Gordon on 03/01/2014 at 12:22

Hi. I have had soreness problems with my anus for around 20 years on and off, where at times it gets very sore especially after going to toilet. This soreness can last a couple of hours or often all day. Sometimes so sore I can hardly walk/move at all, thus making life very depressing. I feel so much more "lively" if it doesnt hurt! My stools seem ok, and correct consistency, occasionally hard to pass. Also sometimes once having been to the toilet, hardly anything to wipe away with toilet paper, often though nearly half a roll of paper to clear, then washing etc etc. Most annoying! I have had camera up at the hospital twice, and no problems there. I am over weight, but then lots of others who are bigger than me, dont have my problem. Its a vicious circle as I need to excersise, but cant due to the soreness. Any help please?

Posted by Optional on 14/12/2013 at 07:47

My problem started a few months ago I was suffering a bad head and chest cold at the time,I was taking a lot of over the counter meds to help me and as I started feeling better I noticed I was not having stools like I use to. and I should use a laxative to be able to go but it seam to keep happening and started taking oral stool softeners to help,they helped some so I started taking prune juice 8oz a day and that got things going pretty well but started noticing I was loosing weight.this all started back in April of 2013 my starting weight was at 300lbs now I am at 234lbs can any one help me understand what could be going on I'm scared that it could be cancerplease help''

Posted by 2tight on 20/11/2013 at 12:18

I have this weird issue latelyfirst i had hemeroids,and they seem to have gone away but for like a couple of months now (not always but often) i feel like i have to go very badly,however when i go to the toilet,nothing will come out,i was feeling if my hemeroids got bigger or something but still don't feel any,however did feel that my anal muscle was shut tight,so tight even that probably even tip of pinky finger wouldn't go in,so i am wondering is it possible that my muscles and my brains aren't connected properly,or is there more chance of internal hemeroids?Would be really grateful if anyone that has had similar problem or knows anything about how it works would bother to answer...

Posted by embarrassingproblems.com on 12/09/2013 at 05:43

To Optional on 11/09/13 - for more information on the symptoms of colon cancer, please see Dr Phil's bowel cancer video: www.embarrassingproblems.com/expert-guides/Dr-Phil-bowel-cancer Get any anal bleeding, or long-term change in bowel habit, abnormal mucus secretion or abdominal pain checked out by your doctor.

Posted by Optional on 11/09/2013 at 01:54, United States

What are symptoms of colon cancer

Posted by Optional on 01/09/2013 at 08:43, United Kingdom

I suffer with 90% paralysed bowel after I had a prolapse repair on my bowel, it is horrendous , I know like on 3 dulolax a night and as u can imagine , my tummy feels like its bin in a washing machine and spun , it makes me feel very sick , I'm fed up with it , just want to be normal again ! If I know the risk I wouldn't have had the operation done in the first place

Posted by Optional on 27/07/2013 at 01:33, United States

i havent went poop for 3 months been goin to the hospitals back an fourth they told me i would b dead its impossible until one day i hit the floor my left side was hurting so bad i was crying so i was rushed to the hospital an they give me a ct-scan an seen i have a infection in my colon i took everything an nothing has worked i still havent went but now i feel worse im having nausea a migraine for 2 days

Posted by Optional on 24/07/2013 at 05:31, United Kingdom

i am still having problems with my intestine moving bowels and when ever i excrete my poo will come watery and weeks or even days i still have the same issue with my body.

Posted by Optional on 15/07/2013 at 03:49, United Kingdom

Severely constipated after chemotherapy, almost like being in labour but managed to clear. Sat backwards on the loo (face the cysten) it gives you something to lean on if nothing else. I put my thumb into my vagina and pushed back towards the bowel. It worked, bit sore but nowhere near as sore as in the past after sitting for hours pushing and splitting.

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

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, United States

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, United States

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, United States

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 Optional on 07/02/2013 at 08:00, South Africa

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, Nigeria

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, United Kingdom

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, Ireland

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, United Kingdom

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, United Kingdom

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, India

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, United States

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, United States

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, United States

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, United States

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, United States

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, United Kingdom

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, New Zealand

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, United States

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, United Kingdom

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, United States

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, Australia

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, Australia

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, United States

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 Amy on 10/04/2010 at 02:26, United Kingdom

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, United Kingdom

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 Anonymous on 04/01/2010 at 05:01, Canada

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 Shane on 27/08/2009 at 08:16, United Kingdom

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, United Kingdom

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.

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Fascinating facts

An average person on a typical Western diet passes about 150 g (5 oz) of faeces each day. Faeces consist of about one-third solids and two-thirds water

The solid matter in faeces is cellulose from vegetables, dead cells cast off from the lining of the gut, bacteria, some salts and pigment from bile (which gives the brown colour)

Most of the waste matter from food is passed out in the faeces within 72 hours, but in healthy people up to 30% may remain in the colon for a week or more

Defecation is a very efficient process, normally taking only 10 seconds. Presumably it has to be quick - animals (and primitive man) cannot run from a predator easily if they are in the middle of defecating. This is probably why it is even quicker when we are frightened

In the USA, more than $800 million is spent on laxatives each year

Constipation results in more than 2–5 million visits to doctors in the USA each year

About 4,500,000 people in the USA say they are constipated most or all of the time (National Health Interview Survey)

In a UK survey, 6% of people said they had suffered from constipation in the past year, 13% had some difficulty in passing their faeces at least once a month, and 19% took laxatives at some time

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