What are the bowels?

The bowels are the intestines which are the longest part of the human gut. There are two sections – the small intestine and the large intestine. Each section is in turn divided into separate areas depending on its anatomical location, difference in structure or function. The small intestines is mainly involved in the digestion of food and absorption of nutrients. The large intestine absorbs most of the water in the gut thereby changing the semi-fluid waste material and undigested food into solid feces.

Small intestine

  • Duodenum
  • Jejunum
  • Ileum

Large intestine

  • Cecum
  • Colon :
    1. Ascending colon
    2. Right colonic flexure (hepatic flexure)
    3. Transverse colon
    4. Left colonic flexure (splenic flexure)
    5. Descending colon
    6. Sigmoid colon

Bowel Emptying Process

Movement through the bowels

The bowels are constantly processing food, fluids and waste. These substances have to travel at through the entire gut until it is evacuated. The time it takes for any substance to travel from the mouth till it is passed out with stool is known as the bowel transit time. It can vary from 18 to 48 hours although it may take as little as 12 hours or as long as 72 hours. In a person who is severely constipated, it can take longer. Gastrointestinal motility is the term for the moving the contents through the gut. It is mainly carried out by waves in the wall of the gut when its muscles contract and relax in a coordinated manner. These waves are known as peristalsis and is discussed in greater detail under :

In order for the bowels to empty, there has to be :

  • strong and coordinated peristaltic waves
  • sufficient fluid in the gut
  • adequate bulk of the bowel contents

The lower parts of the gut comprising the jejunum and ileum of the small intestine and the entire large intestine is never completely empty at any time. Peristalsis is slower in these regions meaning that even though the upper gut is empty, usually within a few hours after eating, the lower bowels are processing all the food that has been consumed. Eventually a person will eat again within a few hours thereby ensuring that the lower bowel is once again processing food, fluid and waste. It is an ongoing process.

Emptying of the bowels

The emptying of the large colon by passing out stool into the environment is known as defecation. It is more likely to occur at certain times of the day, with certain stimuli and when the colon reaches its capacity to store feces. The following process occurs :

  • Various triggers known as defecation reflexes start the first part of the defecation process.
  • Peristaltic activity within the latter parts of the colon increases.
  • Feces enter the rectum and a person feels the urge to defecate.
  • Internal anal sphincter relaxes.

All these processes thus far are involuntary. A person seeks appropriate facilities for having a bowel movement. At this point the emptying of the rectum is facilitated by voluntary processes.

  • Conscious (and sometimes subconscious) factors can stimulate the external anal sphincter to relax.
  • Contracting the abdominal muscles increases pressure within the abdomen which helps to stimulate greater peristaltic activity within the colon.
  • Stool passes out of the rectum through the anus and into the environment.

Normally a person has a bowel movement anywhere between twice a day to three times a week.

Bowel Emptying Problems

 The main problems with bowel emptying is constipation and diarrhea. These are symptoms of conditions that affect the normal bowel habit.

  • Constipation is the passing of stool less than three times a week, often with straining and the evacuation of hard stool.
  • Diarrhea is the passing of stool more than three times a day, which is usually of a loose or watery consistency, with a total stool output in 24 hours exceeding 200g or 200ml.

Another symptom that may also be associated with bowel emptying is tenesmus which is a feeling of incomplete stool evacuation after a bowel movement or a defecation urge despite there being no need to have a bowel movement. It is commonly seen with both constipation and diarrhea.

Some of the conditions that may affect bowel emptying thereby giving rise to constipation or diarrhea includes :

  • Irritable bowel syndrome (IBS) which may be marked by diarrhea or constipation. It is a functional disorder meaning that it is not caused by any disease process but arises most likely arises as a result of rapid or slow gastrointestinal motility.
  • Inflammatory bowel disease (IBD) is a chronic disorder where the bowels, particularly the colon and rectum, become inflamed. There are two types – Crohn’s disease and ulcerative colitis.
  • Intestinal obstruction which may arise with a mass or constriction in the small intestine or large intestine. It usually causes constipation.
  • Bowel infections like gastroenteritis and infectious colitis where viruses, bacteria or other parasites irritate the lining of the bowel thereby affecting normal bowel habit. It usually leads to diarrhea.
  • Food intolerance is a result of impaired digestion or absorption of nutrients from the gut. The more commonly seen conditions include lactose intolerance, gluten intolerance, sorbitol intolerance and fructose malabsorption.
  • Stress, either psychological or physical, may affect normal bowel habit to varying degrees. This includes either constipation or diarrhea.

Other causes of constipation and diarrhea may include :

  • Inadequate water or fiber intake.
  • Sedentary lifestyle.
  • Obesity.
  • Hormonal disturbances.
  • Neurological diseases.
  • Spinal cord injury.
  • Side effects of medication.
  • Substance abuse.
  • Any number of chronic diseases.

Article reviewed by Dr. Greg. Last updated on January 7, 2012