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Small intestine

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The small intestine is shaded above

The small intestine is a very important part of the digestive system as being the site where most of the nutrient absorption takes place. Nutrient absorption is not the only function of the small intestine. The small intestine also aids in the absorption of water and electrolytes. The small intestine is broken down into 3 segments, the duodenum, the jejunum, and the ileum. Each of these segments perform the function of absorbing nutrients but also have unique functions that they must perform in order for the digestive process to work correctly. The small intestine all functions as a medium for food as it carries from from the stomach to the large intestine. After the food has been processed in the stomach it enters the small intestine through the duodenum and from there it is carried all the way through the small intestine until it reaches the end of the ileum where it is then taken to the large intestine. Although it is called the small intestine it is much longer than the large intestine. If one were to uncoil the small intestine it would end up being around 6 meters long. The small intestine is located inside the abdomen.[1]

Anatomy

Villi on the lining of the small intestine help absorb nutrients.
Micrograph of the small intestine mucosa showing the intestinal villi and crypts of Lieberkühn.

The small intestine is a long tube that connects the pylorus to the large intestine. It is about 6 meters(20feet) long and also has a rather large surface area because of the tiny projections called villi that are located on small intestine. The small intestine consists of 3 parts and those parts are:[2]

  • Duodenum: The duodenum is a minute part of the small intestine. It is connected to the stomach at the pyloric sphincter and being around 10 inches long, the duodenum is relatively small in comparison to the other parts of the small intestine. The duodenum consists of four sections, the superior duodenum, the descending duodenum, the inferior duodenum, and the ascending duodenum. The last segment of the duodenum, the ascending duodenum, connects the duodenum to the Jejunum, the middle segment of the small intestine. The majority of food breakdown takes place in the duodenum. Brunner's glands, glands that produce and alkaline mucus, is found lining the walls of the duodenum. The mucus produced by the Brunner's glands helps intestinal enzymes and also plays a role in absorbing nutrients. The duodenum also provides a way of getting bile and pancreatic juice into the small intestine. Pancreatic juice and bile finds it way into the small intestine by way of the pancreatic duct. The pancreatic duct is attached to the "descending" duodenum. The duodenum produces hormones that cause the pancreatic duct to produce pancreatic juice which helps in the breakdown of food, and to produce bile, which helps in the absorption and digestion of fats. Another function of the duodenum is to counteract the acidic chyme that makes it way into the small intestine. Alkaline fluids produced by the Brunner's glands and the pancreatic duct help in neutralizing the acidic chyme. The mucus that Brunner's glands produce not only helps in the absorption of nutrients but also protects the walls of the duodenum from the acidic chyme.[3]
  • Jejunum The jejunum is the largest part of the small intestine. It comes after the duodenum and connects the duodenum to the ilium. The jejunum makes up around half the length of the small intestine. The jejunum receives its blood supply from the superior mesenteric artery. The jejunum is not tightly situated but rather it is in a suspended. The suspended state that the jejunum is in lets it move as digestion takes place. The surface area of the jejunum is quite massive due to the folds of tissue that make up the jejunum and tiny projections located on the surface of the jejunum known as villi. Besides increasing the surface area of the jejunum the other function of the villi is to absorb nutrients. A lot of nutrient absorption takes place in the jejunum. Excess bile that is still around also gets recycled in the jejunum. Digestive enzymes found in the jejunum break down large pieces of food into smaller ones. This process does not happen once chyme has entered the jejunum but actually begins in the stomach. The function of the jejunum is a very important part of the digestion process. If these enzymes were not produced then many of the nutrients found in the food would not get absorbed. Anything that stops these enzymes from accomplishing their jobs can lead to malnutrition, as no nutrients are being absorbed, and many other serious diseases.[4]
  • Ileum The ileum is the segment at the end of the small intestine. It begins after the jejunum and connects the small intestine to the large intestine. The ileum is the site where many bile starts and Vitamin B12 is absorbed. The ileum is around 12 feet long. It is connected to the large intestine by the ileocecal valve. It is held firmly in place by the mesentary and like the jejunum, also receives its supply of blood from the superior mesenteric artery. The walls of the ileum are lined with special cells that produce specific enzymes that help breakdown proteins and carbohydrates, but that is not the only function of the ileum. The ileum also absorbs Vitamin B12 and helps maintain a normal level of bile salts in the small intestine to help in the digestion and absorption of fats. The walls of the ileum are a lot thinner than the walls of the jejunum and are formed of smooth muscle. The ileum also has folds, villi, and microvilli to further aid it in the absorption of nutrients and to increase its surface area. Not only that but the villi and microvillia take the absorbed nutrients and send them into the bloodstream and the liver. Lymphatic cells known as Peyer's patches are also found in the walls of the ileum and these cells place a role in the immune mechanisms of the body. After the ileum absorbs all the nutrients it can from the chyme, it then sends whatever is left of the chyme onward towards the large intestine .[5]

Functions

The small intestine performs many important functions, all of which are very important in making sure that the process of digestion goes down smoothly. The small intestine plays an important role in the digestive system by absorbing nutrients from the chyme that enters into it and by bringing both electrolytes and water to the different parts of the body. Not only does it absorb nutrients from food but it also functions in moving the food from the stomach to the large intestine where further digestion and excretion place. Food from the stomach moves through the small intestine by a process called peristalsis. Peristalsis is when the smooth muscles of the intestines contract to produce a rolling movement that guide the food in a specific direction. The 3 sections of the small intestine aid in the absorption of nutrients from food. This is not the only function of these sections but it is a very important one. The functions of each of these sections of the small intestine provide some sort of aid to the digestive process. The duodenum helps by neutralizing the acidity of the chyme that enters the small intestine so that it does not damage the walls of the intestine, the jejunum further breaks down the chyme and absorbs nutrients from it, and the ileum helps by absorbing Vitamin B12. Besides absorbing nutrients the small intestine has another function. The small intestine aids in the intake of water and electrolytes. This is very important because without these things the body would not be able to survive so it is incredibly important that the body maintain regular levels of water and electrolytes. The walls of the small intestine of full of sodium pumps that form a gradient. This gradient allows for the absorption of many things such as water, nutrients and electrolytes. With the help of the mesentry the small intestine is able to send these nutrients it has absorbed throughout the body to various systems.[6]

Diseases & Disorders

Picture of intestine afflicted by Crohn's disease.

There are many disorders and diseases that can effect the small intestine. Some of the diseases closely related to the small intestine are Celiac disease, Crohn's disease, Intestinal cancer, Intestinal obstruction, Irritable bowel syndrome, and ulcers.[7]

  • Celiac disease:

Celiac disease hinders the small intestine's ability to absorb nutrients and also damages the small intestine itself. This disease causes the person's immune system to respond negatively towards gluten and anything that has gluten in it such as wheat, rye barley, medicines, vitamins, and certain lip balms. The immune system reacts towards the gluten by either damaging the villi found in the small intestine or by completely destroying them. The villi allow the small intestine to absorb nutrients that are vital to body and without the villi the small intestine will not be able to absorb the nutrients that the body needs. This will cause the person to become malnourished and more susceptible to other diseases. Symptoms very between persons and are more evident in young children than in adults. Symptoms associated with Celiac disease found in young people are: abdominal bloating and pain, chronic diarrhea, vomiting, constipation, pale,foul-smelling, or fatty stool, and weight loss. Symptoms found in adults are: unexplained iron-deficiency anemia, fatigue, bone or joint pain, arthritis, bone loss or osteoporosis, depression or anxiety, tingling numbness in the hands and feet, seizures, missed menstrual periods, infertility or recurrent miscarriage, canker sores inside the mouth, and an itchy skin rash called dermatitis herpetiformis. Only one form of treatment is available to those suffering from Celiac disease and that is a gluten-free diet.[8]

Crohn's disease is a chronic disease that effects the gastrointestinal tract, otherwise known as the digestive tract. It causes inflammation to any part of the digestive part but normally takes place in the lower part of the digestive system. The swelling that Crohn's disease brings intense pain and causes frequent diarrhea. Diagnosing Crohn's disease can be rather difficult as it shares similar symptoms to other intestinal diseases such as irritable bowel syndrome and ulcerative colitis. Symptoms associated with Crohn's disease are abdominal pains, diarrhea, rectal bleeding, weight loss, arthritis, skin problems, and fevers. There is currently no cure for Crohn's disease but there are treatments that help reduce the number of times one experiences the inflammation. These treatments come in the form of drugs, nutrition, supplements, or surgery.[9]

  • Irritable bowel syndrome:

Irritable bowel syndrome causes cramping, abdominal pain, bloating, constipation, and diarrhea. Although a person with this disease will experience incredible discomfort and pain they do not have to worry about any serious problems as irritable bowel syndrome does not permanently damage the intestines or induce any other harmful diseases. Irritable bowel syndrome does effect the social life of a person as it is a very disabling disease in that the pain it produces may make people unable to work or travel. People afflicted with irritable bowel syndrome wait a while before looking for medical assistance. There is currently no cure for irritable bowel syndrome but there are treatments available for those suffering from this disease that reduces the intensity of the pain which for some people is more than enough.[10]

References

  1. The Small Intestine Date Accessed may 12,2011.
  2. Gross and Microscopic Anatomy of the Small Intestine R. Bowen, Last updated on April 18, 2000.
  3. What is the Duodenum Niki Foster, Last Modified 28 March 2011.
  4. What is the Jejunum S.E.Smith, Accessed May 12, 2011.
  5. What is the Ileum Archana Khambekar, Accessed May 12, 2011.
  6. The Small Intestine Essortment, Accessed May 12,2011.
  7. Small Intestine Disorders Date Accessed May 12,2011.
  8. Celiac Disease September 2008.
  9. Crohn's Disease February 2006.
  10. Irritable Bowel Syndrome September 2007.