What are Crohn’s and colitis?

Inflammatory bowel disease (IBD) describes a group of conditions, the two main forms of which are Crohn's disease and ulcerative colitis. IBD also includes indeterminate colitis

Crohn's disease and ulcerative colitis are diseases that inflame the lining of the GI (gastrointestinal) tract and disrupt your body's ability to digest food, absorb nutrition, and eliminate waste in a healthy manner. These are lifelong disease and people can experience acute periods of active symptoms (active disease or flare), and other times when their symptoms are absent (remission).

Signs and symptoms can include abdominal pain and cramping; severe diarrhea; rectal bleeding; blood in stool; weight loss and diminished appetite.

The exact cause of Crohn's disease and ulcerative colitis remains unknown, and as a result, there is no cure for these diseases. Causes and risk factors of Crohn's and colitis include genetics, the environment, and microbiome.

Watch the video below to learn more about IBD and scroll down the page to find information about the anatomy and function of the gastrointestinal (GI) tract, Crohn's disease and ulcerative colitis.

Dr. Mike Evans is founder of the Health Design Lab at the Li Ka Shing Knowledge Institute, an Associate Professor of Family Medicine and Public Health at the University of Toronto, and a staff physician at St. Michael's Hospital.

Anatomy and function of the GI tract

In order to understand Crohn's disease and ulcerative colitis, it is helpful to learn the anatomy and function of the healthy gastrointestinal (GI) tract. Below is a medical illustration of the GI tract. When you eat, food travels through the GI tract in this order: 

Digestive System

Mouth [ 1 ]

Esophagus [ 2 ] (tube that connects the mouth to the stomach)

Stomach [ 3 ] (food is mixed with stomach acid and enzymes to break down the material into smaller pieces called chyme)
Small Bowel [ 4 ] (or the 'Small Intestine') is made up of three sections: Duodenum [ 7 ] (about 8 cm in length); Jejunum [ 8 ] (around 3 metres long); and Ileum [ 9 ] (about 3 metres in length).

The functions of the small bowel are to digest your food and absorb the nutrients. In particular, the jejunum and ileum are the organs responsible for absorbing nutrients from your food. Without the small bowel, we would not be able to convert food into useable nutrition.

Ileocecal Valve [ 5 ] (regulates the amount of material passed from the small bowel to the large bowel and prevents "dumping" all at once)

Large Bowel [ 6 ] (also called the Large Intestine or the Colon). The colon is much wider in diameter than the small bowel and is approximately 1.5 metres long. The different sections of the colon are identified as the:

  • Cecum [ 10 ] and appendix [ 11 ] 
  • Ascending colon 
  • Hepatic flexure (a bend in the gut at close to the location of the liver) 
  • Transverse colon 
  • Splenic flexure (another bend located near the spleen) 
  • Descending colon 
  • Sigmoid colon
  • Rectum [ 12 ]
  • Anus [ 13 ]
The main functions of the colon are to extract water and salt from stool, and store it until it can be expelled via the anus.

Stool and bowel movements

Stool is the by-product of digestion through the GI tract. When stool first enters the colon from the small bowel, it is very watery. As it traverses the large bowel, water is reabsorbed and the stool gradually becomes firmer. 

In a healthy individual, it is usually composed of water, dead and living bacteria, fiber (undigested food), intestinal mucous, and sloughed-off lining of the gut. It is not normal to have blood in feces, nor large amounts of mucous.

Stool from an individual without any gut disease is soft enough to pass comfortably from the rectum and anus, and (depending on the person) is typically expelled one or two times a day.

Bowel movements are an entirely different matter for someone with Crohn's or colitis. Individuals with these diseases face some very real challenges related to feelings of urgency, diarrhea, and bloody stool.

What is Crohn's disease

Digestive system showing inflammation in someone with Crohn's disease

Crohn’s disease is named after the doctor who first described it in 1932 (also known as 'Crohn disease'). Inflammation from Crohn’s can strike anywhere in the gastrointestinal (GI) tract, from mouth to anus, but is usually located in the lower part of the small bowel and the upper colon.

Patches of inflammation are spread out between healthy portions of the gut, and can penetrate the intestinal layers from inner to outer lining.

Crohn’s can also affect the mesentery, which is the network of tissue that holds the small bowel to the abdomen and contains the main intestinal blood vessels and lymph glands.

Crohn's disease can be controlled with medication and reduce the chance of a disease replase. In severe cases, surgery of the small or large intestine may be required to manage the disease. 


What is ulcerative colitis

Digestive system showing inflammation in someone with ulcerative colitis

Ulcerative colitis is more localized in nature than Crohn’s disease. Typically, the disease affects the colon (large intestine) including the rectum and anus, and only invades (inflames) the inner lining of bowel tissue. It almost always starts at the rectum, extending upwards in a continuous manner through the colon.

Colitis can be controlled with medication and in severe cases can even be treated through the surgical removal of the entire large intestine. 

Indeterminate colitis is a term used when it is unclear if the inflammation is due to Crohn's disease or ulcerative colitis.

Comparing Crohn's and colitis

There are similarities and differences between Crohn's disease and ulcerative colitis. We've described above how Crohn's disease and ulcerative colitis involve different areas of the gastrointestinal tract.

These diseases can also differ in their symptomssurgery and treatment options; disease complications; and the impact of smoking. Some examples are listed below: 

  • Females are more likely than males to be diagnosed with Crohn's disease; whereas the risk for being diagnosed with ulcerative colitis is the same across females and males 

  • Blood in the stool or rectal bleeding and a sense of urgency or false urges are more common in ulcerative colitis than in Crohn's disease

  • Sores in the mouth and around the anus occur more often in Crohn's disease than ulcerative colitis

  • 5-aminosalicylates and sulfasalazine medications are typically only used for ulcerative colitis; other forms of medication are used for both Crohn's and colitis (e.g., steroids, immunomodulators, biologics)

  • ​There are no existing cures for Crohn's disease, whereas a colectomy (removal of the colon or large bowel) may be considered "curative" and induce remission in ulcerative colitis. Maintenance therapy is used to reduce for both Crohn's and colitis to reduce the chance of relapse. 

  • Growth failure in children and adolescents is more common in Crohn's disease and ulcerative colitis 

  • Osteoporosis is more common in Crohn's disease and ulcerative colitis

More of these characteristics are summarized in the table below:

Table. Characteristics of Crohn's disease and ulcerative colitis
  Crohn's Disease Ulcerative Colitis
Occurrence More females than males

All ages, peak onset 15-35 years
Similar for females and males

All ages, usual onset 15-45 years
Symptoms Diarrhea, fever, sores in the mouth and around the anus, abdominal pain and cramps, anemia, fatigue, loss of appetite, weight loss Bloody diarrhea, mild fever, abdominal pain and cramps, anemia, fatigue, loss of appetite, weight loss
Distribution of disease Patchy areas of inflammation Continuous areas of inflammation but can be patchy once treated
Depth of inflammation May be transmural, extending through the entire thickness of the wall of an organ or cavity deep into tissues Shallow, mucosal
Fistulas between organs Common Never
Effect of surgery Often return following removal of affected parts. Decreased likelihood of pregnancy. Usually cured by removal of colon (colectomy). Decreased likelihood of pregnancy after ileoanal pouch.
Treatment options Drug treatment (corticosteroids, immune modifiers, biologic therapies). Exclusive formula diet in children.

Surgery (repair fistulas, remove obstruction, resection, and anastomosis).
Drug treatment (5-aminosalicylates, sulfasalazine, corticosteroids, immune modifiers, biologic therapies).

Surgery (rectum/colon removal) with creation of an internal pouch (ileoanal pouch).
Cure No existing cures. Maintenance therapy is used to reduce the chance of relapse. Through colectomy only. Maintenance therapy is used to reduce the chance of relapse.
Bowel complications Blockage of intestine due to swelling or formation of scar tissue. Abscesses, sores, or fistulas. Malnutrition. Colon cancer. Bleeding from ulcerations. Perforation (rupture) of the bowel. Malnutrition. Colon cancer.
Extra-intestinal disease Osteoporosis. Liver inflammation (primary sclerosing cholangitis). Blood clots. Pain and swelling in the joints (arthritis). Growth failure (in children). Mental Illness. Liver inflammation (primary sclerosing cholangitis). Blood clots. Eye inflammation (iritis). Pain and swelling in the joints (arthritis). Mental illness.
Smoking Higher risk of acquiring for smokers Higher risk of acquiring for ex-smokers
Cancer and Mortality risk Increased risk of colorectal cancer and overall mortality.

Increased risk of lymphoma and skin cancer (due to treatments).
Increased risk of colorectal cancer. Uncertain change in mortality risk.

Increased risk of lymphoma and skin cancer (due to treatments).

Table reference. Impact of Inflammatory Bowel Disease in Canada Report (2018).

​Have you been recently diagnosed with Crohn's or colitis?

You or someone you care about has been diagnosed with Crohn’s disease or ulcerative colitis, the two main forms of inflammatory bowel disease (IBD). You likely have a lot of questions, coupled with a fair degree of concern and uncertainty. You are not alone. One in 140 Canadians are living with Crohn’s and colitis. You have started a journey of discovery that will help you to cope with your disease and not just survive, but thrive.

Watch our 4-Part Newly Diagnosed Webinar Series below to get an overview of what you should know about living with IBD from a nurse and expert in the field of IBD. Topics include treatment options, diet and nutrition, symptom management, disease complications, and more. 

If you're an older adult that has been diagnosed with Crohn's or colitis, click here watch our Living with IBD in Older Adulthood webinar to learn more about the impact of these diseases as you age. 


Strengthen your knowledge of living with Crohn's and colitis by visiting the IBD Journey section of our website.

Visit the IBD Journey section of our website to learn about living with Crohn's and colitis including testing and getting diagnosed, diet and nutrition, medication and treatment options, pain and symptom management, mental health and wellness, insurance and government support, and other essential information.  

Find a community near you

Your local chapter of Crohn’s and Colitis Canada is a powerhouse of health information and support. Through education, presentation, and discussion, people can find resources to help them cope with their Crohn’s, and meet others who are living with Crohn’s or colitis.

Visit our Find My Community page to look up the chapter closest to you. It’s in your best interest to get active and informed!

Peer support

Do you want to talk to someone that has been affected by IBD? Our Gutsy Peer Support Mentors are ready to answer your questions, provide advice, and share their experiences with you.

Click here to find out more about our Gutsy Support program to connect with others online via email, video, and/or audio chat.

Child and youth programs  

Are you a young person living with IBD, or a caregiver of a child with IBD? Crohn's and Colitis Canada offers programs for children and youth that have been diagnosed with Crohn's or colitis, including camp for kids and teens, and scholarships for post-secondary students

Contact us

If you can't find your answer here or need more support, our Crohn's and Colitis support staff are here to help you!

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Crohn’s and Colitis Canada cannot offer personal medical advice. Please contact your primary healthcare provider or IBD specialist if you require medical assistance. 

  • Canada has among the highest incidence rates of Crohn's and colitis in the world.
  • 1 in 140 Canadians lives with Crohn’s or colitis.
  • Families new to Canada are developing these diseases for the first time.
  • Incidence of Crohn’s in Canadian kids under 10 has doubled since 1995.
  • People are most commonly diagnosed before age 30.