There are many possible causes of rectal bleeding. Below is a brief overview of the more common causes:
Piles (haemorrhoids)
Haemorrhoids are swellings that can occur in the anus and lower back passage (lower rectum). There is a network of small blood vessels (veins) within the inside lining of the anus and lower rectum. These veins sometimes filled with more blood than usual. These swollen veins and the overlying tissue may then form into one or more small swellings called haemorrhoids. The most common symptom is bleeding after going to the toilet. Larger haemorrhoids may cause a mucous discharge, some pain, irritation and itch.
Anal fissure
An anal fissure is a small tear of the skin of the anus. Although the tear of an anal fissure is usually small (usually less than a centimetre), it can be very painful. Often an anal fissure will bleed a little. You may notice blood after you pass stools (faeces). The blood is usually bright red and stains the toilet tissue but soon stops.
Anal fistula
Anal fistula is a small channel that develops between the end of the bowel, known as the anal canal, and the skin near the anus. They’re usually painful and can cause bleeding when you go to the toilet.
Diverticula
Small bulges in the lining of your lower bowel. These contain weakened blood vessels that can burst and cause sudden, painless bleeding (you may pass quite a lot of blood in your stools).
Crohn’s disease
Crohn’s disease is a condition which causes inflammation in the gut. Symptoms vary, depending on the part of the gut affected and the severity of the condition. Common symptoms include bloody diarrhoea, abdominal pain and feeling unwell.
Ulcerative colitis and other forms of colitis
Ulcerative colitis is a disease where inflammation develops in the colon and rectum. A common symptom when the disease flares up is diarrhoea mixed with blood. There are other rare causes of inflammation of the colon (colitis) or inflammation of the rectum (proctitis) that can cause rectal bleeding.
Polyps
A bowel polyp is a small growth that sometimes forms on the inside lining of the colon or rectum. Most develop in older people. Polyps are non-cancerous and usually cause no problems. However, sometimes a polyp bleeds and sometimes a polyp can turn cancerous.
Bowel cancer (colon or rectal cancer)
You should always get checked by your GP if you have rectal bleeding. The only symptom of bowel cancer may be rectal bleeding in the early stages, so don’t ignore it. Bowel cancer can be treated more easily if it’s diagnosed at an early stage.
Angiodysplasia
Angiodysplasia is a condition where you develop a number of enlarged blood vessels within the inner lining of the colon. The cause is unknown but they occur most commonly in older people. Bleeding from an angiodysplasia is painless.
Stomach and duodenal ulcers
An ulcer in the stomach or duodenum may bleed. This can cause melaena – where your faeces turn black- or plum-coloured as described earlier.
Some gut infections
These may cause bloody diarrhoea due to inflammation of the gut, caused by some infections.
Anal cancer
Radiation Therapy
References
http://patient.info/health/rectal-bleeding-blood-in-faeces
http://www.nhs.uk/Conditions/rectal-bleeding/Pages/Introduction.aspx
