Small intestine (intestinal tenue) – structure, functions and diseases

Picture: “Small Intestine” by philschatz. License: CC BY 4.0

Location and sections of the small intestine

The small intestine closes orally in the digestive tract to the Magenpförtner (pylorus) and discharges aboral at the ostium ileale (Ileocecal valve) in the colon. The small intestine lies convoluted in the abdomen, is over the mesentery, in which vessels also run, connected to the abdominal wall and lies in its course largely intraperitoneal. The small intestine anatomically divides into the following sections:

The duodenum (Duodenum): This short part connects to the pylorus of the stomach. Here, the bile duct (ductus choledochus) and the pancreatic duct (pancreatic duct) open to the papilla vateri.
The jejunum (Jejunum): The mean proportion is about 2/5 of the length of the small intestine.
The ileum (Ileum): The terminal portion opens into the right iliac fossa in the colon. The ileocecal junction forms the ostium ileale (Bauhin flap).

Picture: “Small Intestine” by philschatz. License: CC BY 4.0

Structure of the small intestine wall

The structure of the small intestine adheres as far as possible to the blueprint of the rest of the gastrointestinal tract, but has some important anatomical features, which should be seen in connection with its function. Here are the individual layers in the order from outside to inside:

Image: “Layers of the Alimentary Canal” by Goran tek-en. License: CC BY-SA 3.0

The Tunica serosa (Serosa) and Tunica subserosa (Subserosa): These layers correspond to the peritoneum viscerale (serosa) and loosely arranged connective tissue (subserosa) between this and the muscularis. If the small intestine in parts of the duodenum is not completely surrounded by peritoneum, ie, if there is a secondary retroperitoneal condition, this is called the connective tissue layer of the Tunica adventitia.
The Tunica muscularis (Muscularis): The muscularis consists of the outer longitudinal tunica muscularis (longitudinal muscle layer) and the inner tunica muscularis circulare (circular muscle layer). The latter is more pronounced in the small intestine. Between the layers is the Myenteric plexus (Auerbach’s plexus), which belongs to the Enteric Nervous System (ENS) and controls bowel movement. The muscularis provides peristaltic contractions for the transport of food.
The Tela submucosa (Submucosa): The submucosa consists of loose connective tissue and many elastic fibers and contains, in addition to blood vessels (especially small arteries and veins) and lymph vessels, the nerve cells of the ganglion Plexus submucosus (Meissner plexus). This is also part of the enteric nervous system (ENS) and responsible for the movement of the intestinal mucosa and secretion regulation.
The Tunica mucosa (Mucosa): The mucosa is the inner (luminal) layer of the small intestine wall and is subdivided into 3 laminae. The lamina muscularis mucosae is a thin layer of smooth muscle cells that allows the mucosa to move independently. Inside it lies the lamina propria mucosae, a connective tissue layer containing capillaries, nerve endings and lymphatic vessels. The innermost layer kle >Tip: The small intestinal mucosa is characterized by particularly fast cell division. Every 24-72 h the epithelium is replaced.

The mucosa is the most variable wall layer of the gastrointestinal tract is well adapted to the tasks of each organ. For optimum water and nutrient exchange, the small intestinal mucosa must provide the largest possible surface area. Visible with the naked eye mucosal folds, the Plicae circulares (Kerckring wrinkle) extend about 1 cm into the lumen and involve the submucosa. These folds, in turn, are 0.2-1 mm high from the mucosa Villi (Villi intestinales), which significantly increase the surface area and thereby improve the absorption of food components. There are smaller ones between the villi crypts (Dear crypts) enter the gland ducts and from which the cell division of the small intestinal epithelium proceeds.

The duodenum (duodenum)

Image: “duodenum anatomy” by Luke Guthmann. License: CC BY-SA 3.0

The duodenum (duodenum) is the first 30 cm long section of the small intestine. It runs in a C-shaped curve around the pancreatic head and connects the stomach outlet and the jejunum. In duodenum one differentiates 4 parts:

Pars superior: The initially large lumen of this 5 cm long section is called Ampulla duodeni and lies directly anterior to the A. pancreaticoduodenalis. A duodenal ulcer perforating the duodenal wall may therefore cause severe bleeding at this site. The pars superior is located intraperitoneally.
Pars descendens: From here the duodenum runs secondarily retroperitoneal. At the Papilla duodeni major (Vateri) is the mouth of the common duct of the ductus choledochus (bile duct) and ductus pancraticus (pancreatic duct). As anatomical variation is the Papilla duodeni minor (Santorini), an additional muzzle of the pancreatic duct.
Pars horizontalis: This short section is horizontal and is sometimes referred to as pars inferior.
Pars ascendens: Here, the duodenum flows into the again intraperitoneal jejunum. The transition is called flexure duodenojejunalis and forms the end of the upper gastrointestinal tract.

Jejunum and ileum

Vascular supply and innervation of the small intestine

The arterial supply of the small intestine sections is divided as follows:

duodenum: The duodenum becomes arterial over the river areas of the Truncus celiacus and the mesenteric superior provided. Those originating from the A. gastroduodenalis (a branch of the A. hepatica of the celiac trunk) A. pancreaticoduodenalis superior posterior and anterior supply the top Parts of the duodenum. The A. retroduodenales originating from the A. gastroduodenalis supplies the dorsal parts of the duodenum. Those originating from the superior mesenteric artery A. pancreaticoduodenalis inferior supplies with her Rami anterior and posterior the lower Sections of the duodenum.
jejunum: The jejunum is from the Aa. supplied jejunales, the mesenteric superior come. These run in the mesentery and run in Aa. rectae perpendicular to the intestinal wall
ileum: The ileum becomes arterial via the Aa. ileales from the mesenteric superior provided. These run like the arteries of the jejunum in the mesentery.

The venous outflow is parallel to the arteries. The superior mesenteric vein joins the splenic vein V. portae (Portal vein). The blood, like that of all unpaired abdominal organs, thus enters the liver. The important lymph drainage of the small intestine first flows into numerous, located in the mesentery lymph nodes and passes from there via the Nodi lymphoidei mesenterici superiores into the truncus intestinalis and from there into the Cisterna chyli. Here begins the Thoracic duct, the largest lymphatic vessel of the human.

The motility and secretion of the small intestine is controlled by the enteric nervous system (ENS) and basically works autonomously. Nonetheless, the sympathetic and parasympathetic nervous system affects the ENS. The sympathetic nerve innervates the small intestine via the Nervus splanchnic major, wherein fibers that pass to the small intestine are switched in the ganglion ceoliacum. Fibers that go to the jejunum and ileum are switched in the ganglion mesentericum superius.

The sympathicus has an inhibitory effect on the secretion of the glands and movement of the intestinal musculature. Parasympathisch becomes the small intestine of nerve fibers of the Truncus vagalis posterior (Vagus nerve), which are switched in the intestinal wall, innervated. The parasympathetic nervous system has a secretory and motility-promoting effect on the intestine.

Image: “English: Anatomical dissections” by Anatomist90. License: CC BY-SA 3.0

Function of the small intestine

The small intestine is the central organ of the nutrient recycling, because here are both important enzymatic processes of the decomposition of the chyme, as well as the absorption of the ultimately utilizable nutrients from. Slow peristaltic movements of the small intestine allow a long contact time of the food with the mucous membrane before further transport into the large intestine (colon).

About the Papilla duodeni major in the descending part of the duodenum reaches daily up to 2 l pancreatic juice in the duodenum and meets there on the chyme. When entering the duodenum, the chymus has an acidic acidity of about 2 due to gastric acid and is alkalized there by the bicarbonate-containing secretion of the pancreas. The neutral pH is necessary for the activity of the digestive enzymes.

These digestive enzymes are secreted by the pancreas in the form of inactivated precursors, which physiologically pass into the active form on contact with the small intestinal mucous membrane. Activated pancreatic enzymes such as amylase, trypsin and lipase break down the diet into useful nutrients such as monosaccharides and amino acids, which can then be absorbed by the small intestinal epithelium.

In addition will bile Controlled hormonally via the ductus choledochus into the lumen of the duodenum and mixed there with the chyme. Here, the bile acids unfold their important function in fat digestion. These form with the fat-soluble components of food so-called micelles, which can be absorbed by the small intestinal mucosa. Thereafter, the triglycerides are transported away in chylomicrons via the lymphatic vessels. In the terminal ileum, a large part of the bile acids is reabsorbed by the mucous membrane and returns to the liver via the portal vein. This circulation, which minimizes the need for re-synthesis, is called enterohepatic circulation.

The small intestine is essential in the human body for the absorption of water responsible. It is estimated that up to 80% of the water is absorbed in the small intestine, predominantly in the jejunum. Conversely, the small intestinal epithelium is also able to secrete water to balance hypertonic food pulp. This is also the reason why diarrheal diseases of the small intestine represent a great danger in the form of exsiccosis (dehydration).

In addition, the small intestine, especially the ileum, plays an important role in the immune defense. Lymphatic tissue, v.a. The Peyer plaques of the ileum, can absorb antigens from the intestinal lumen, triggers targeted immune responses and selects between beneficial and alien intestinal bacteria. The entirety of the lymphatic tissue in the intestine is called gut-associated lymphatic tissue (GALT = well-associated lymphatic tissue).

Important small intestinal diseases

Ulcer duodeni

It is estimated that 1% of the western population lives on one Ulcer duodeni (Duodenal ulcer) suffers. This is more common than the gastric ulcer and describes an erosion of the duodenal wall, which penetrates the muscularis mucosae. Ulcers are often due to a chronic infection with H. pylori or the frequent use of non-steroidal anti-inflammatory drugs such as ibuprofen. They cause severe pain, typically alleviated by food intake, and can lead to complications such as bleeding and perforation.

Malasimilation syndrome (food intolerance)

Common diseases of the small intestine are also food intolerances (malassimilation). One differentiates between a disturbance of the food splitting (Maldigestion) and a disturbance of the absorption (Malabsorption).

A serious malabsorption syndrome is the celiac Disease, a chronic intolerance of the small intestinal mucosa against gluten. The disease is associated with severe inflammation and villi atrophy of the small intestine, but normalizes in a strictly gluten-free diet again.

A less severe form of malabsorption is the lactose malabsorption (Lactose intolerance), which is believed to affect up to 75% of the world’s adult population and is due to a deficiency in the enzyme lactase. Patients suffer from meteorism and diarrhea when taking lactose, as the lactose passes undigested through the small intestine and is fermented in the large intestine by intestinal bacteria.

Crohn’s disease

The chronic inflammatory bowel disease (CED), which also affects the small intestine is the Crohn’s disease. This chronic inflammatory inflammation of the intestinal wall, in particular, affects non-contiguous (segmental) sections of the ileum and the colon (“skip lesions”) and manifests itself in a cobblestone-like image of the mucosa. Patients with age between 15 and 35 years suffer from diarrhea, fatigue, fever and pain and are at high risk for complications such as strictures and fistulas, which necessitate surgical partial resections.