Small intestine

Synonyms in a broader sense

Interstitium tenue, jejunum, ileum, duodenum

English: intestinal


The small intestine is the section of the digestive tract that follows the stomach. This is divided into three sections. It begins with the duodenum, followed by the jejunum and ileum. The main task of the small intestine is to break down the food pulp (chyme) into its smallest components and to absorb these components through the intestinal mucosa.

Illustration small intestine

Figure small intestine: location of the digestive organs in the body cavity (small intestine - red)
  1. Small intestine -
    Intestine tenue
  2. Duodenum, upper part -
    Duodenum, pars superior
  3. Duodenal
    Jejunum junction -
    Duodenojejunal flexure
  4. Jejunum (1.5 m) -
  5. Ileum (2.0 m) -
  6. End part of the ileum -
    Ileum, pars terminalis
  7. Colon -
    Intestinum crassum
  8. Rectum - Rectum
  9. Stomach - Guest
  10. Liver - Hepar
  11. Gallbladder -
    Vesica biliaris
  12. Spleen - Sink
  13. Esophagus -

You can find an overview of all Dr-Gumpert images at: medical illustrations

Duodenum / duodenum


This section directly follows the exit of the stomach (pylorus). It is approx. 24 cm long, has the shape of a "C" and with this "C" it encloses the head of the pancreas. The duodenum is also divided into an upper part (pars superior), which connects directly to the gastric outlet, the descending part (pars descendens), the horizontal part (pars horizontalis) and the ascending part (pars ascendens).
The duodenum is the only part of the small intestine that is firmly attached to the posterior abdominal wall. The excretory ducts of the bile duct (ductus choledochus) and the pancreatic duct (ductus pancreaticus) end in its descending part. These mostly open together into the papilla vateri (papilla duodenalis major). If the ducts open into the duodenum separately from each other in rare cases, there is an additional pancreatic outlet into a smaller papilla (papilla duodenalis minor).

Further information on the anatomy of the abdominal cavity can be found here: Abdominal cavity

"Internal organs" illustration

  1. Thyroid cartilage / larynx
  2. Windpipe (trachea)
  3. Heart (cor)
  4. Stomach (gaster)
  5. Large intestine (colon)
  6. Rectum
  7. Small intestine (ilium, jejunum)
  8. Liver (hepar)
  9. Lungs or lungs

Empty bowel / ileum

The two longer parts of the small intestine Jejunum) and Ileum (Ileum) lie in the middle of the abdomen and are from the Large intestine framed. These two sections of the small intestine are very mobile because they rest on a special suspension structure, the so-called Mesentery are hung, which the Intestines flexibly attached to the posterior abdominal wall. This fatty structure also contains the blood vessels, nerves and Lymph nodesthat supply the small intestine. The small intestine is so hung up on the mesentery that it lies in large folds, also called that Small intestinal mesentery are designated.
The jejunum is approx. 3.5 m long, the ileum is approx. 2.5 m. No sharp boundary can be drawn with the naked eye between these two sections of the small intestine. The parts of the small intestine can only be distinguished from each other by means of the tissue (histologically). At the end of the small intestine, the ileum opens laterally into the cecum part of the large intestine, this opening from the large intestinal valve (ileozaekal valve, Bauhinschen flap) is covered. This flap acts as a functional seal between the ileum and the large intestine. The bacteria colonized in the large intestine cannot penetrate into the sterile small intestine through this valve.


The small intestine is a very active organ and therefore has no fixed length. Depending on the state of contraction, the small intestine is 3.5 to 6 meters long, with each section being of different size. The smallest part of the small intestine is the duodenum (Duodenum), which connects directly to the stomach. It measures an average of 24-30 cm. The is attached to the duodenum Jejunum (Jejunum), which measures 2.5 meters when relaxed. The last section before going into the colon is the Ileum (Ileum), this is about 3.5 meters long. This is Guide valueswhich can vary from person to person, and from a purely anatomical point of view there is no clear line between empty and ileum.

Wall of the small intestine

Layer structure and structures of the wall of the small intestine

  • From the inside, the wall of the small intestine is lined with mucous membrane (tunica mucosa), which is divided into three sub-layers. The top layer is a covering tissue (lamina epithelialis mucosae). In this covering tissue, special cells (goblet cells) are embedded, which are filled with mucus, which they periodically release into the intestine and thus guarantee the ability of the intestine to slide. The next lower layer is a connective tissue shifting layer (lamina propria mucosae), followed by a very narrow layer of intrinsic muscles (lamina muscularis mucosae), which can change the relief of the mucous membrane.
  • This is followed by a loose shifting layer (tela submucosa), which consists of connective tissue and in which a dense network of blood and lymph vessels runs, as well as a nerve fiber network called the submucosal plexus (Meissner's plexus). This nerve plexus represents the so-called enteric nervous system and innervates the intestine independently of the central nervous system. In this layer of the duodenum there are also the so-called Brunner's glands (Glandulae interstinales), which form various enzymes and an alkaline mucus, which the Stomach acid able to neutralize. The following intestinal muscle layer (tunica muscularis) is divided into two sub-layers, the fibers of which run in different directions: first an inner, strongly developed circular muscle layer (stratum circulare) and then an outer longitudinal muscle layer (stratum longitudinal). Between this ring and longitudinal muscle layer runs a network of nerve fibers, the myenteric plexus (Auerbach plexus), which innervates (stimulates) these muscle layers. These muscles are responsible for the wave-like movement of the intestine (peristaltic movement).
  • Another connective tissue layer (Tela subserosa) follows.
  • The conclusion is a covering of the peritoneum, which lines all organs. This coating is also called tunica serosa.

Mucous membrane from the small intestine

The small intestine needs a large area to absorb food components. By a severe wrinkling and numerous protuberances, a great increase in the surface of the mucous membrane is achieved. This is guaranteed by various structures:

  1. Kerkig folds (Plicae circulares)
    These are ring folds that form the rough relief of the small intestine and in which both the mucosa and the submucosa protrude.
  2. Small intestine villi (Villi interstinales)
    These finger-shaped protrusions, 0.5-1.5 mm in size, are found in all sections of the small intestine, in which the epithelium and lamina propria protrude.
  3. Lieberkühn crypts (Glandulae interstinales)
    In the valleys of the villi there are tubular depressions that extend to the lamina muscularis.
  4. Microvilli
    This so-called “brush border” forms the micro-relief of the small intestinal mucous membrane and enlarges it 10 times. In the case of the microvilli, the cytoplasm (filling content of the cells) of the individual small intestinal cells (enterocytes) is everted.

The fine tissue (histological) differences between the individual small intestine sections are briefly presented here:

  • Duodenum (duodenum)
    The duodenum is characterized by very high kerking folds and leaf-shaped, imposing small intestinal villi. The most important feature, however, are the Brunner's glands (Glandulae interstinales), which occur only in the duodenum, are located in the submucosa and participate in the formation of the small intestinal juice and form enzymes such as maltase and amylase.
  • Jejunum
    Here the kerking folds become smaller over time, the small intestine villi become longer and have a more finger-shaped structure
  • Ileum
    The kerking folds are particularly low in this section of the small intestine and are completely absent in the lower ileum. The villi also get shorter and shorter and the number of goblet cells increases as the intestine progresses. The large number of lymph follicles (accumulation of lymph cells) in the ileum is particularly noticeable. If many follicles are gathered in one place, this place is also called Peyer's plates. These structures are involved to a large extent in the immune defense of the intestine.

Function / tasks

As part of the digestive tract, the main role of the small intestine is the Further processing of the meal and the Absorption of the contained nutrients, electrolytes, vitamins and fluids.

In the small intestine, the previously chopped up food components are broken down into their basic components and absorbed. This is done on the one hand by the Addition of digestive enzymes to the chyme, on the other hand through contact of the basic components with the cells of the small intestinal mucosa. The small intestine uses several tricks to design the contact surface of the chyme with the mucous membrane and thus the absorption of the food as large as possible: Wrinkled protuberances protrude into the interior of the intestinal sections, from which cell assemblies such as tentacles protrude again. Every single cell of these tentacles now has so-called on its surface Microvilli, finger-like protuberances that enlarge the contact area again. Overall, the small intestine enlarges its surface so on up to 200 m².

If the chyme reaches the duodenum through the stomach passage, the secretions from the gallbladder and pancreas are emptied in its so-called "descending part". The pancreas produces daily up to 1.5l secretion. This consists largely of bicarbonate, which the neutralizes the acidic environment of the porridge.

The main work is done here, however, by those included pancreasenzymes, they further break down the food. There is a specific enzyme for every component of food: for Fats (including pancreatic lipase and phospholipase A), carbohydrates (alpha amylase), proteins (including trypsin and aminopeptidases), DNAComponents (Ribonuclease, deoxyribonuclease) etc.

The important part of the bile for digestion is the Bile acidswhich have a special property. They can bind both fat and water and thus simplify the processing of fats in food. The bile acids, which are synthesized from cholesterol, form so-called fats with food Micelles. These are small "lumps" of fat, consisting of the fat components inside and the bile acids as a protective ring to the watery external environment.

The mixture of chyme and digestive enzymes is now through the Small bowel peristalsis transported further towards the colon. The walls of the small intestine contract the more slowly the further they move away from the stomach. The Duodenum contracted 12 times per minutewhile that Ileum only 8 contractions per minute having.

The sections of the small intestine differ not only in the number of contractions per minute, but in particular in their Wall construction and the absorbed food components. In the duodenum, calcium, iron, Magnesium, Single and double sugar absorbed.

The following are now in descending order fat soluble Vitamins, Egg whites, water-soluble vitamins and Fats reabsorbed until the bile acids in particular are reabsorbed in the terminal ileum and vitamin B12 is absorbed.

The further towards the colon you move, the more accumulations of Lymph follicles can also be found in the intestinal wall. The intestine not only serves as a digestive organ, but also as a Immune defense station against germs and bacteria ingested with food.

The final part of the small intestine forms the Bauhin'sche flap. It defines the transition from the small to the large intestine and prevents the stool from flowing back from the large to the small intestine. From the Bauhin'schen flap the number of Intestinal bacteria rapidly and the species that occur change.

Movement / peristalsis

After inclusion in the Small intestinal mucosa the nutrients are carried into the bloodstream. Through the vascular network (capillaries) in the small intestinal villi, the sugars, amino acids (from peptides) and short to medium-chain fatty acids are absorbed into the blood vessels and passed on to the liver via the portal vein. The long-chain fatty acids, the cholesterol esters and phosphilipids, are broken down into large protein-fat molecules (Chylomicrons) installed and via the lymphatic vessel in the small intestinal villi, past the liver and into the blood circulation funneled.

The intestines are also important for them Absorption of water. Approx. A total of 9 liters of fluid is absorbed in one day. About 1.5 liters of this comes from the liquid drunk and the rest are the liquids (secretions) that the Gastrointestinal tract forms. This includes saliva, Gastric juice, small intestinal juice, pancreatic and biliary juice.


Intake of food

After being absorbed into the mucous membrane of the small intestine, the nutrients are transferred into the bloodstream. Through the vascular network (capillaries) in the small intestinal villi, the sugars, amino acids (from peptides) and short to medium-chain fatty acids are absorbed into the blood vessels and passed on to the liver via the portal vein. The long-chain fatty acids, the cholesterol esters and phosphilipids, are broken down into large protein-fat molecules (Chylomicrons) built in and channeled through the lymphatic vessel in the small intestinal villi, past the liver and into the bloodstream.

The intestine is also important for the absorption of water. Approx. A total of 9 liters of fluid is absorbed in one day. About 1.5 liters of this comes from the liquid drunk and the rest are the liquids (secretions) that the gastrointestinal tract forms. These include saliva, gastric juice, small intestinal juice, pancreatic and biliary juice.

Figure digestive tract

Figure digestive tract: (digestive organs in the head, neck and body cavity)

Digestive tract
A. - Food route
a - digestive organs
in the head and neck
(upper part of the digestive tract)
b - digestive organs
in the body cavity
(lower part of the digestive tract)

  1. Oral cavity - Cavitas oris
  2. Tongue - Lingua
  3. Sublingual salivary gland -
    Sublingual gland
  4. Trachea - Trachea
  5. Parotid gland -
    Parotid gland
  6. Throat - Pharynx
  7. Mandibular salivary gland -
    Submandibular gland
  8. Esophagus - Esophagus
  9. Liver - Hepar
  10. Gallbladder - Vesica biliaris
  11. Pancreas - Pancreas
  12. Colon, ascending part -
    Ascending colon
  13. Appendix - Caecum
  14. Appendix -
    Appendix vermiformis
  15. Stomach - Guest
  16. Large intestine, transverse part -
    Transverse colon
  17. Small intestine - Intestine tenue
  18. Large intestine, descending part -
    Descending colon
  19. Rectum - Rectum
  20. Nach - anus

You can find an overview of all Dr-Gumpert images at: medical illustrations

Small intestine pain

Pain in the small intestine is not easy to pinpoint. There are many different conditions that can cause pain in the small intestine. The spectrum here ranges from simple blockages or Gastrointestinal inflammation up to heavier ones chronic inflammation up to Intestinal ulcers or Mesenteric infarcts.

Many of these diseases also cause relatively unspecific pain in the lower abdomen, which on the one hand cannot be easily distinguished from one another and on the other hand also pain symptoms in other diseased organs such as resemble the pancreas, gallbladder, peritoneum or large intestine.

Pain in the small intestine shows up depending on the clinical picture different "pain qualities". These range from colic-like (severe, rippling) pain when the small intestine is blocked (Ileus) from dull, long-lasting pain to acute, stabbing pain in an ulcer or an ulcer acute inflammation.

In principle, the motto here is that the more acute and stronger the pain, the more serious the disease is. It should also be noted whether a so-called in addition to the pain Defense tension occurs here, which is reflective and can only be triggered to a limited extent arbitrarily Abdominal wall hardening means when touched.

Pain in the small intestine area must always be seen in the context of known previous illnesses. For example, pain in acute small intestinal inflammation following gastrointestinal viruses or food poisoning can be "normal" as long as it does not last longer than four days; on the other hand, e.g. a Mesenteric artery infarction with following Reduced blood supply of the affected section of the small intestine with short, severe pain, which then improves again and almost disappears, while the disease takes on threatening proportions.

Inflamed small intestine

stomach pain

The inflammatory disease of the small intestine is called Enteritis designated. Because of the close positional relationship, the stomach and large intestine can also be inflamed, and these forms of disease then become gastroenteritis (Stomach) or Enterocolitis (Colon) called.

Enteritis is classified according to various criteria: 1. Is the enteritis infectious or non-infectious 2. Is the inflammation acute or chronic? 3. What caused the inflammation?

Infectious enteritis can be caused by bacteria (including salmonella, shigella, E. coli, clostridia), viruses (including rotaviruses, noroviruses, adenoviruses) or parasites (including amoebas, worms, fungi).

Non-infectious enteritis refers to inflammations of the small intestine that are of medicinal origin (cyclosporine, cytostatics), are triggered by radiation therapy, are a result of insufficient blood supply in the corresponding section, are caused by toxins, by allergies, e.g. Food allergies or after surgery or are idiopathic (without a known cause) such as ulcerative colitis or Crohn's disease.

Enteritis mainly manifests itself in diarrhea, which is often accompanied by nausea and vomiting. Other, more unspecific symptoms are intestinal cramps, abdominal pain and fever. In the course of the disease, the increased water excretion and decreased intake lead to signs of dehydration and disturbances in the electrolyte balance such as dizziness, fatigue, listlessness and leg cramps.

The therapy of enteritis depends on its triggers. Most enteritis heals spontaneously, with diarrhea subsiding within 3-7 days and nausea and vomiting subsiding within 1-3 days. In these cases, treatment is symptom-oriented and depending on the degree of severity, with treatment of nausea, diarrhea and electrolyte imbalance if necessary with medication. In the case of more stubborn inflammation, a detailed discussion with the patient is important in order to clarify the above-mentioned triggers; the pathogen is also detected via a stool sample. Therapy is then adapted to the results of the examinations. Bacterial and parasitic enteritis e.g. are treated with antibiotics if the symptoms persist.

Major illnesses

Ulcerative colitis

Ulcerative colitis is also a disease from the group of inflammatory bowel disease (IBD). Ulcerative colitis is particularly characterized by involvement of the large intestine, but can sometimes also affect the small intestine. One then speaks of an "ingrown" inflammation of the small intestine ("Backwash ileitis"). This disease is also triggered autoimmunologically and causes abdominal pain and bloody diarrhea (Diarrhea) noticeable.

Further information on this topic can be found at: Ulcerative colitis

This chronic inflammatory bowel disease (IBD) can theoretically affect the entire gastrointestinal tract from the oral cavity to the anus. However, the disease preferentially affects the lower small intestine (terminal ileum) and often appears with symptoms such as cramping abdominal pain and slimy diarrhea (diarrhea). The characteristic of this autoimmune disease, however, is the segmental infestation of the intestinal mucosa.

Further information on this topic is available at: Crohn's disease

Duodenal ulcer

The so-called duodenal ulcer refers to an ulcer in the duodenum. The two main causes of this very common disease are bacteria Helicobacter pylori and pain medication like aspirin or Not-S.teroidalA.nti-R.heumatica (NSAIDs). A dangerous complication of ulcer disease occurs when the ulcer reaches a larger vessel causing life-threatening bleeding (Gastrointestinal bleeding) is coming.

Celiac disease

This condition is commonly known as gluten-sensitive enteropathy, or native sprue. This is an intolerance of the small intestinal mucous membrane to the adhesive protein (gluten) found in many types of grain. Those affected complain of diarrhea and weight loss. The therapy for this disease is lifelong gluten free diet.

Further information on this topic can be found at: Celiacia