What is the small intestine?
The small intestine begins at the pylorus and ends at the Bauhin’s valve, the transition to the large intestine. Its total length is around five to six meters.
Sections of the small intestine from top to bottom are Duodenum (duodenum), jejunum (jejunum) and ileum (ileum).
Duodenum (duodenum)
The duodenum begins at the outlet of the stomach and is around 25 to 30 centimetres long. You can read more about this first section of the small intestine in the article Duodenum.
Empty intestine (jejunum)
The term jejunum comes from the fact that this section of the intestine is usually empty in cadavers. You can read more about the structure and function of the jejunum under Jejunum.
Crooked intestine (ileum)
At around three meters long, the ileum is the longest section of the small intestine. You can read more about its anatomy and functions in the article Ileum.
Wall of the small intestine
The wall of the small intestine is made up of different layers from the inside to the outside.
- The mucous membrane lies on the very inside, followed by a layer of loose connective tissue with blood and lymph vessels and a nerve plexus.
- This is followed by a two-part muscle layer (ring muscle layer, longitudinal muscle layer). Their wave-like movements and contractions ensure the mixing and further transportation of the chyme.
Mucous membrane of the small intestine
Initially, the duodenum still has a smooth inner surface. In the other areas and in the jejunum, the structure of the small intestine changes – the inner surface becomes larger due to folds (Kerck ring folds), finger-shaped protrusions (villi), depressions (crypts) and the brush border (microvilli: fine projections on the surface of the wall epithelium). This considerable enlargement of the inner surface significantly increases the absorption capacity for nutrients and water.
The Kerck ring folds in the jejunum are lower than in the duodenum and become rarer as they progress. There are hardly any folds in the ileum, but there are increasing accumulations of lymphatic tissue (Peyer’s plaques) for immune defense.
What is the function of the small intestine?
The function of the small intestine is initially the chemical digestion of food. The small molecules produced during the breakdown of carbohydrates, fats and proteins (simple sugars, fatty acids, amino acids etc.) are then absorbed into the blood via the wall of the small intestine. Vitamins are also absorbed into the blood via carriers and receptors on the inner surface of the intestine.
Bile is produced in the liver and passed into the duodenum. Among other things, it contains bile acids for the digestion of fat. At the end of the small intestine, most of the bile acids are absorbed back into the blood and transported back to the liver (enterohepatic circulation).
The mucous membrane in the duodenum contains numerous glands (Brunner’s glands). These glands secrete hydrogen carbonate, which neutralizes the acidic chyme coming from the stomach. Only then can the digestive enzymes in the small intestine become active. These enzymes are provided by the pancreas and Brunner’s glands.
What problems can the small intestine cause?
Crohn’s disease is a chronic inflammatory disease that can affect the entire digestive tract (from the oral cavity to the anus). The disease most commonly affects the last part of the small intestine (ileum).
Diverticula are protrusions of the intestinal wall that usually remain asymptomatic. However, they can become inflamed (diverticulitis) or start to bleed and cause serious complications.
In coeliac disease, the immune system reacts hypersensitively to the gluten protein gluten (contained in cereals): The mucous membrane of the small intestine is damaged, which hinders the absorption of nutrients.
