Small intestine

Eva Rudolf-Müller is a freelance writer in the medical team. She studied human medicine and newspaper sciences and has repeatedly worked in both areas - as a doctor in the clinic, as a reviewer, and as a medical journalist for various specialist journals. She is currently working in online journalism, where a wide range of medicine is offered to everyone.

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The small intestine is the longest part of the digestive tract. It extends from the gastric outlet (pylorus) to the appendix (cecum), is very tortuous and five to six meters long. In the small intestine, the final breakdown of food into small molecules and their absorption through the intestinal mucosa into the blood takes place. Read everything you need to know about the small intestine!

What is the small intestine?

The small intestine begins at the stomach gate (pylorus) and ends at the Bauhin's valve, the transition to the large intestine. Its entire length is about five to six meters.

Sections of the small intestine from top to bottom are: the duodenum, the jejunum, and the ileum.

Duodenum

The duodenum begins at the exit of the stomach and is about 25 to 30 centimeters long. You can read more about this first section of the small intestine in the article Duodenum.

Jejunum

The term empty intestine comes from the fact that this section of the intestine is usually empty in corpses. Read more about the structure and function of the jejunum under Jejunum.

Ileum

At around three meters, the ileum is the longest section of the small intestine. You can read more about its anatomy and tasks in the article Ileum.

Small intestine wall

The wall of the small intestine is made up of different layers from the inside out.

The mucous membrane lies right on the inside, followed by a shifting layer of loose connective tissue with blood and lymph vessels and a plexus of nerves.

This is followed by a two-part muscle layer (circular muscle layer, longitudinal muscle layer). Their wave-like movements and contractions ensure that the food is mixed and transported further.

The muscle layer is followed by a shifting layer to the outside and then a covering made of peritoneum.

Small intestinal mucosa

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 is enlarged by folds (Kerckring folds), finger-shaped protuberances (villi), depressions (crypts) and the brush border (microvilli: fine processes on the surface of the wall epithelium ). This strong enlargement of the inner surface significantly increases the absorption capacity for nutrients and water.

The Kerckring folds in the jejunum are lower than in the duodenum and become rarer as the patient progresses. There are hardly any wrinkles 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 consists primarily in the chemical digestion of the food pulp. The small molecules (simple sugars, fatty acids, amino acids, etc.) produced when carbohydrates, fats and proteins are broken down are then absorbed into the blood through the wall of the small intestine. Vitamins are also absorbed via carriers and receptors on the inner surface of the intestine.

In addition, around eight to nine liters of water with 50 to 100 grams of electrolytes are absorbed in the small intestine every day. Only about 1.5 liters of it comes from food. Swallowed saliva makes up about one liter. 1.5 liters consist of gastric juice, three liters of glandular secretions from the small intestine, two liters of pancreatic juice and about 0.6 liters of bile.

The bile is made in the liver and passed into the duodenum. Among other things, it contains bile acids for fat digestion. At the end of the small intestine, most of the bile acids are reabsorbed into the blood and transported back to the liver (enterohepatic circulation).

The mucous membrane in the duodenum contains numerous glands (Brunner's glands). These secrete hydrogen carbonate, which neutralizes the acidic food pulp coming from the stomach. Only then can the digestive enzymes become active in the small intestine. In addition, the secretion of the Brunner glands contains digestive enzymes.

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). Most often the disease affects the last part of the small intestine (ileum).

Diverticula are protrusions of the intestinal wall that usually remain without symptoms. However, they can become infected (diverticulitis) or start bleeding and cause serious complications.

In a duodenal ulcer (duodenal ulcer), an area of ​​the mucous membrane in the duodenum is permanently inflamed and damaged. Possible causes for this are, for example, an infection with Helicobacter pylori or the use of certain painkillers and rheumatoid drugs (acetylsalicylic acid, ibuprofen, diclofenac, etc.).

In celiac disease, the immune system is hypersensitive to the gluten protein gluten (contained in cereals): The mucous membrane of the small intestine is damaged, which hinders the absorption of nutrients.

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