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Intestine - Anatomy, Function, and Common Conditions

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Intestines comprise a significant part of the digestive tract and play an essential role in digestion. The article explains the small and large intestines.

Medically reviewed by

Dr. Jagdish Singh

Published At January 3, 2023
Reviewed AtAugust 16, 2023


The intestine (bowel) is a muscular tube that extends from the stomach to the anus. An essential function of the intestine is the digestion of food and absorption of nutrients. The intestines are divided into the small intestine and the large intestine. Common conditions affecting the intestine are inflammation, microbial infections, or ulcers. These can lead to colitis, ulcerative colitis, small bowel disease, and many more.

What Is the Anatomy of the Small Intestine?

  • The small intestine is the most extended segment of the gastrointestinal tract. It is directly connected to the stomach and is approximately 3 to 5 meters long. The small bowel is divided into three sections- duodenum, jejunum, and ileum.

  • The small intestine is coiled inside the lower abdominal cavity below the stomach. It is surrounded by a large intestine which frames the edges of abdominal walls.

  • The small intestine is a pink or red tube with multiple folds with a width of approximately the pointer finger. In width, it is smaller than a large intestine, but in length, it is longer, with a dimension of 22 feet. The small intestine comprises layers of tissues and muscles that contain blood vessels, nerves, immune cells, and lymphatic cells.

What Are the Different Parts of the Small Intestine?

  • Duodenum: It is the first part of the small intestine where the stomach ends. It is a short ‘c’ shaped descending chute which curves around the pancreas.

  • Jejunum: The middle section of the intestine lays in coils in the lower abdominal cavity. It comprises many blood vessels, which makes it look redder.

  • Ileum: It is the longest section of the small intestine. The walls of the small intestine begin to thin and narrow at this section, and the blood supply is reduced. Most of the water and nutrients are absorbed from the food in the ileum.

What Functions Does the Small Intestine Perform?

The longest duration of digestion takes place in the small intestine.

The general functions of the intestine are:

  • Breaking down food with the help of digestive enzymes and bile.

  • Absorption of nutrients from the digested food.

  • Absorption of water from the food.

  • Movement of food in the digestive tract towards the large intestine.

What Is the Function of Different Parts of the Small Intestine?

  • Duodenum: It receives digestive juices from different organs in the digestive system, such as the liver, gallbladder, and pancreas. When the food reaches the duodenum, hormones in the duodenum lining stimulate the organs to secrete digestive juices and enzymes, which start the digestion of food in the small intestine.

  • Jejunum: After digestion in the duodenum, the food passes to the jejunum. In the jejunum, the muscular digestion of food starts. Nerves in the jejunal wall trigger the muscles to churn the food back and forth. This process is known as segmentation. After segmentation, the food is mixed with digestive juices, and other muscles help the food to move forward. This movement is known as peristalsis.

  • Mucosa: It is the lining of the intestinal walls comprising many glands that help in both absorption and secretion of a small amount of digestive enzymes. The thick mucosa has many folds and projections with a surface area about 100 times as broad as the surface area of the skin. Thus almost 95 % of carbohydrates and proteins are absorbed in the intestine. Mucosa also absorbs almost 90 % water during digestion.

  • Ileum: In the ileum, food segmentation gradually slows down, and peristalsis takes over to move the food toward the large intestine. Nerves and hormones signal the valve to open, let the food pass to the large intestine, and eventually close up to keep the bacteria out. Special immune cells in the ileum protect it from bacteria and their infections.

What Are the Common Conditions Which Affect the Small Intestine?

  • Celiac Disease: It is an autoimmune disorder that damages the small intestine. The disease is caused due to gluten intolerance in the body. Symptoms of the disease include gas, bloating, anemia, or growth retardation.

  • Crohn’s Disease: Inflammation of the bowel leading to swelling and irritation of the small intestine. It is a life-long disorder that cannot be cured. It includes symptoms such as diarrhea, rectal bleeding, and weight loss.

  • Irritable Bowel Syndrome: Characterized by uncomfortable or painful abdominal symptoms such as diarrhea, constipation, bloating, or gas. Irritable bowel syndrome was not reported to damage the digestive tract.

  • Intestinal Pseudo-Obstruction: It is an intestinal disorder similar to intestinal blockage. The cause of pseudo-obstruction is generally congenital.

  • Meckel’s Diverticulum: It is a congenital bulge or pouch present in the lower part of the small intestine. The bulge is a leftover of the umbilical cord. It can cause symptoms in some cases such as gastrointestinal bleeding ,abdominal pain and cramping.

Large Intestine:

It is the last organ of the gastrointestinal tract. It is located in the lower abdominal cavity and surrounds the small intestine in a square question mark outline shape. It looks like a segmented semi-flat tube that lies loosely around the edges of the abdominal cavity. The length of the large intestine can be approximately six feet and is wider than the small intestine. It is made up of tissue layers and muscles, which allow the contraction of the large intestine.

What Are the Parts of the Large Intestine?

  • Cecum: In the first section of the large intestine, which is attached to the small intestine. The small intestine passes the food into the cecum through a small channel known as an ileocecal valve. Thus, the end of the cecum is closed like a pouch, the first 6 inches of the large intestine. It is the widest portion of the large intestine when the food arrives from the small intestine. When the cecum is full, it stimulates the movement of muscles of the large intestine to start functioning.

  • Colon: It is divided into three parts: ascending, transverse, and descending. Any remaining water or electrolytes in the food are absorbed in the ascending colon and transverse colon, so the food waste entering the descending colon is generally solid. The colon secretes mucus which binds and lubricates the food wastes and helps them to pass through smoothly as it is stiff and dehydrated. The friendly gut bacteria in the large intestine break down the remaining carbohydrates into key vitamins B and K, which are absorbed through the mucosa.

  • Rectum: The food waste delivered by the sigmoid colon to the rectum is feces. It consists of indigestible matter, dead cells shed by the intestinal mucosa, and some amount of mucus and water. When feces enter the rectum, and the rectum is full, it triggers the urge to defecate.

  • Anus: It is the ending part of the large intestine which is closed on each side by muscular sphincters. The internal muscular sphincter on the inside of the anus involuntarily passes the feces through. However, the outer sphincter is voluntary, which means it can be controlled.

What Are the Conditions Which Affect the Large Intestine?

  • Ulcerative Colitis: It is a condition that causes irritation and ulcers (open sores) in the large intestine. It can cause bloody diarrhea, cramps, and abdominal pain.

  • Diverticulum: Inflammation in the bulging or pouch-like structure attached to the intestine. The pouch is called diverticula.

  • Appendicitis: Appendix is a small tube-like organ that is attached to the large intestine. Inflammation in the appendix is known as appendicitis. It can be a life-threatening condition if not treated.

  • Proctitis: Inflammation in the lining of the rectum. It can lead to rectal bleeding, anal or rectal pain, and a feeling of rectal fullness.


Intestines are a very important part of our digestive system. A large amount of food digestion and absorption takes place in the intestines. It is important to maintain a disease-free bowel by eating a proper diet and consuming a good amount of water. The presence of any kind of inflammation or infection in the intestines can alter the process of digestion. So it is important to diagnose the disorder and treat it.

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Dr. Jagdish Singh
Dr. Jagdish Singh

Medical Gastroenterology


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