What Does the Small Intestine Do?

NewsGuard 100/100 Score

The small intestine is a tubular structure within the abdominal cavity that carries the food in continuation with the stomach up to the colon from where the large intestine carries it to the rectum and out of the body via the anus. The main function of this organ is to aid in digestion.

How big is the small intestine?

As a person grows the small intestine increases 20 times in length from about 200 cm in a newborn to almost 6 m in an adult. The length of the small intestine is approximated by three times the length of the infant, or height of the child or adult.

The duodenum is about 25 cm (10 inches) long; the jejunum is about 2.5 m (8 feet) long and the ileum is about 3.6 m (12 feet) long.

Anatomy of the small intestine

The small intestine begins with the duodenum. The duodenum begins at the duodenal bulb and goes around the head of the pancreas and ends as it returns to the peritoneal cavity at the ligament of Treitz. The peritoneal cavity is a thin membrane cavity that covers the organs within the abdomen with some exceptions.

The remainder of the small intestine is suspended within the peritoneal cavity by a thin, broad-based mesentery that is attached to the posterior abdominal wall. This allows free movement of the small intestine within the abdominal cavity.

After the duodenum comes the next 40% of the mobile small intestine called the jejunum. The remaining 60% is the ileum.

The jejunum occupies the left upper portion of the abdomen while the ileum is positioned in the right side and upper part of the pelvis.

The inner walls of the small intestine show mucosal folds. These are called the plicae circulares. The plicae are more numerous in the early jejunum and reduce in numbers in the later part and are completely absent in the ileum.

Absorption takes place via primary cell type of the epithelial layer. Goblet cells, located throughout the epithelial layer, secrete mucus that helps protect the epithelial layer from digestion.

Enteroendocrine cells secrete hormones into blood vessels that penetrate each villus. Paneth cells, located in the epithelial layer facing the intestinal crypts, secrete lysozyme, an enzyme that destroys bacteria. An inner core of lamina propria (connective tissues) contains blood capillaries and small lymphatic capillaries called lacteals.

Enteroendocrine cell

Enteroendocrine cell

The submucosa under the mucosa contains Brunner's (duodenal) glands, found only in the submucosa of the duodenum. It secretes an alkaline mucus that neutralizes the gastric acid in the incoming chyme.

Aggregates of lymphoid follicles are scattered throughout the small intestine but are found in highest concentration within the ileum, where they are designated Peyer's patches. These are more prominent among children and infants. The small intestine ends at the ileocecal valve that leads it to the colon. The ileocecal valve provides a barrier to the back flow of the colonic contents into the small intestine.

The wall of the small intestine and colon is composed of four layers: mucosa (or mucous membrane), submucosa, muscularis (or muscularis propria), and adventitia (or serosa).

Functions of the small intestine

The small intestine is the part of the intestines where 90% of the digestion and absorption of food occurs, the other 10% taking place in the stomach and large intestine. The main function of the small intestine is absorption of nutrients and minerals from food.

Digestion involves two distinct parts. The first is mechanical digestion by chewing, grinding, churning and mixing that takes place in the mouth and the stomach. The second part of digestion is the chemical digestion that uses enzymes, bile acids etc. in order to break down food material into a form that can then be absorbed, then assimilated into the tissues of the body. Chemical digestion occurs in the small intestine (and, to a lesser extent, also in some other part of the gastrointestinal tract.

Digestion of proteins

Proteins, peptides and amino acids are acted upon by enzymes such as trypsin and chymotrypsin, secreted by the pancreas. This breaks them down to smaller peptides. Chemical breakdown begins in the stomach and continues until the large intestine.

Digestion of lipids

Enzymes, like lipases secreted from the pancreas, act on fats and lipids in diet. This breaks the triglycerides into free fatty acids and monoglycerides. It is helped by bile salts secreted by the liver and the gall bladder. The lipase is soluble in water but the fatty triglycerides are not. The bile salts hold the triglycerides in the watery environment until the lipase can break them into the smaller parts that can enter the intestinal villi for absorption.

Digestion of carbohydrates

Carbohydrates are broken down to simple sugars and monosaccharides like glucose. Pancreatic amylase breaks down some carbohydrates to oligosaccharides as well. Some carbohydrates and fibers pass undigested to the large intestine where they may, depending on their type, be broken-down by intestinal bacteria.

Absorption in the small intestines

Once broken down the nutrients are absorbed by the inner walls of the small intestine into the blood stream. The nutrients are rendered small enough so that they may pass, or "be transported", across the epithelial cells of the gastrointestinal tract. The nutrients are absorbed by processes of simple/passive diffusion, facilitated diffusion, primary active transport, or secondary active transport.

The small intestine is good for absorption since it has a large inner surface area. This is formed due to the plicae circulares which project many tiny finger-like structures of tissue called villi. The individual epithelial cells also have finger-like projections, which are called known as microvilli.

For transport, nutrients commonly rely upon:

  • Lipids – undergo passive or simple diffusion
  • Short-chain fatty acids – diffusion
  • Amino acids – primary active transport
  • Glucose – secondary active transport
  • Fructose – facilitated diffusion

Other absorbed substances in the small intestines include:


Most of the water in ingested food and beverages is absorbed by osmosis.

Approximately 80% is absorbed by the small intestine, 10% by the large intestine and the remaining 10% excreted in the faeces.


Of these sodium is absorbed by diffusion and active transport. Chloride (Cl-) is absorbed passively along with sodium or actively transported. Iodine (I-) and  Nitrate (NO3-) can passively follow Na+ ions or are absorbed actively. Calcium ions (Ca2+) are absorbed actively in a process stimulated by calcitriol (active form of Vitamin D). Iron ions (Fe2+ and Fe3+), Potassium ions (K+), Magnesium ions (Mg2+) and Phospate ions (PO43-) are absorbed by active transport mechanisms.

Vitamins and minerals

Vitamins including fat soluble ones (Vitamins A, D, E and K) are absorbed together with dietary fats. Water soluble vitamins like vitamins B and C are absorbed by diffusion. Vitamin B12 combined with intrinsic factor (from the stomach) is absorbed by active transport.

Of these iron is absorbed in the duodenum, most are absorbed in the jejunum and Vitamin B12 and bile salts are absorbed in the later part of the ileum.

Disorders of the small intestine

Some of the disorders of the small intestine include:

  • Obstruction of the small intestine. This may occur due to external pressure, masses in the lumen (foreign bodies, bezoar, gallstones), paralytic ileus, Crohn's disease, Celiac disease, Carcinoid, Meckel's Diverticulum, Gastric dumping syndrome, inguinal hernia, intussuseption, mesenteric ischemia etc.
  • Infections including Giardiasis, Ascariasis, Tropical sprue, Tape worm infestation etc.
  • Small intestine cancer

Further Reading

Last Updated: Jul 7, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Mandal, Ananya. (2023, July 07). What Does the Small Intestine Do?. News-Medical. Retrieved on April 12, 2024 from https://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspx.

  • MLA

    Mandal, Ananya. "What Does the Small Intestine Do?". News-Medical. 12 April 2024. <https://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspx>.

  • Chicago

    Mandal, Ananya. "What Does the Small Intestine Do?". News-Medical. https://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspx. (accessed April 12, 2024).

  • Harvard

    Mandal, Ananya. 2023. What Does the Small Intestine Do?. News-Medical, viewed 12 April 2024, https://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspx.


  1. Bernie McGuire Bernie McGuire United States says:

    I always thought the large intestine absorbs the majority of the water but here you say it absorbs only 10% and the small intestine absorbs 80%.  Am I misunderstanding the other sites?

  2. Bethany Morgan Bethany Morgan United States says:

    I have acne due to to my small intestine what can I do to Fix it?

  3. William Painter William Painter United States says:

    I have had Caron's disease since I was 27 years old. ( now 54 years old ) I have had three small intestine bowel re sections and an illiostomy that was reversed after 8 months. My last surgeon told me I  have very little small intestine left. With that in mind how well does medications absorbed into my system. I also have peripheral neuropathy for which I take oxycodone. Sometime it feels like the pain Meds aren't absorbed into my system. As though they pass through my small intestine without being absorbed into my system. Is there something I can do to remedy this situation?

    • wow
      Jessica Fowkes Jessica Fowkes United Kingdom says:


    • :O
      Jennifer L. Jennifer L. United States says:

      Omg, I fell so bad-I hop you are doing better! does it feel any different?

    • Janet Smith Janet Smith United States says:

      I also had my small intestine removed in 2009 due to being "dead gut" caused by blood clots.  I was on life support for 9 days with blood clots removed throughout my circulatory system.  I was dying and called out to GI and Internal medicine Dr.s who just gave me laxatives and vomiting medicine.  I have a regimen of drinking filtered water and fluids, vitamins, biotin (hair and nails fell out), B 12 shots for my iron and anemia. My medicine that I take do not show up in my urine either, because I have to take a lot of medicine for it to work.  The Doctor should look at your chart and realize that 80% of nutrients are from your small intestine not processing the food you eat!  Hang in there and get a specialist to follow your food intake and what is lacking.  There is help out there, dont give up.

  4. Jessica Fowkes Jessica Fowkes United Kingdom says:

    I love Google, because if there is anything you want to ask you can always use Google.
    Google just helped me with my boring science homework.

  5. Marguerite Astea Marguerite Astea United States says:

    There was blood found in my small intestine can this be taken care of without surgery

  6. Akshaya C P Akshaya C P India says:

    why the tapeworms do not get digested from the small intestine ?????

  7. Michael Cross Michael Cross United States says:

    I swear fiber is digestible with sufficient water

  8. tim darcy tim darcy Ireland says:

    from 200cm to 6m is only an increase x3, isn't it?

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
Post a new comment

While we only use edited and approved content for Azthena answers, it may on occasions provide incorrect responses. Please confirm any data provided with the related suppliers or authors. We do not provide medical advice, if you search for medical information you must always consult a medical professional before acting on any information provided.

Your questions, but not your email details will be shared with OpenAI and retained for 30 days in accordance with their privacy principles.

Please do not ask questions that use sensitive or confidential information.

Read the full Terms & Conditions.

You might also like...
New ultrasound technology shows promise for preventing brain damage in premature and sick infants