Small intestine

It is attached to the stomach as a tube about 3-4 cm in diameter, 3-5 m long when alive and just after death. The actual length of the intestine cannot be determined because it depends on the degree of contraction or relaxation of the wall musculature, and in relation to this, the time since death and the technique of measurement. After death (after relaxation of muscular tension), the small intestine may extend up to 7 m, with the stretching of the intestine during removal during dissection playing a role. The main and final stage of enzymatic digestion of food into absorbable components and absorption of the resulting components takes place here.

Duodenum
The duodenum is the horseshoe-shaped initial part of the small intestine connected to the stomach. The length of the duodenum is between 20 and 28 cm and its diameter is 3.5 to 4.5 cm. With the exception of its initial part, the duodenum is connected to the posterior abdominal wall and secondarily covered by the peritoneum parietale.

The segments of duodenum include:


 * 1) pars superior - starts from the pylorus as the ampulla (bulbus) of the duodenum, at the level of the L1 vertebra, touching the liver (often also the gallbladder) from below, behind it the v. portae, the flexura duodeni superior - the bend between the pars superior and the pars descendens.
 * 2) pars descendens - is descending along the right side of the L2 and L3 vertebrae, in front of the hilum of the right kidney, behind the pars descendens is the ductus choledochus, flexura duodeni inferior - bend at the right side of L3, through which the pars descendens passes into the pars horizontalis.
 * 3) pars horizontalis (inferior) - passes from right to left in front of the body of L3.
 * 4) pars ascendens - along the left flank of the L2 vertebra obliquely upwards to the left, flexura duodenojejunalis - duodenum passes anteriorly into jejunum.

The pancreas is inserted into the tail of the duodenum. Through the pars horizontalis of the duodenum pass the a. et v. mesenterica superior. Horizontally the mesocolon transversum passes through the duodenum. Anteriorly the duodenum is covered by the margin of the liver, the colon transversum and the villi of the jejunum.

Jejunum and ileum
These two parts flow into each other without a clear boundary. The differences between them are listed in the attached table below.

The ostium ileocaecale is the outlet of the ileum into the appendix located in the right iliac fossa. On the mucosal side of the caecum, a valve is formed, which determines the direction of passage from the ileum in one direction only.

Small intestine wall
It consists of four basic layers.

The mucosa layer
The light pink mucosa is covered by a single-layered cylindrical epithelium capable of resorption. Between the epithelial cells are cup cells, which form a protective layer of mucus. The surface is formed by plicae intestinales - transverse cilia 6-8 mm high. In the duodenum, they are formed the most and distally they decrease.

The entire mucosal surface is covered with slender finger-like projections called villi intestinales or intestinal villi. They are 0.3-1 mm high (gradually decreasing in height), with 10-40 villi per 1 mm2. Thanks to the villi, the mucosal surface is enlarged to 7 m2 (more surface area for nutrient resorption). The villi are formed by the epithelium and lamina propria of the mucosa.

Glandulae intestinales (Lieberkühn's crypts) are present in all parts of the small intestine. They descend as simple, slender tubular glands to the lamina muscularis mucosae.

The mucosal ligament is sparse and extends into the villi. In places it contains lymphoid tissue, which increases distally. It can be found in two forms:

folliculi lymphatici solitarii - tiny nodules everywhere in the mucosa;

folliculi lymphatici aggregati - clusters of nodules that form plaques (called Peyer's plaques);


 * most often found in the sites of the mesentery tendons;
 * only in the ileum and distally they increase.

The lamina muscularis mucosae separates the mucosal connective tissue from the submucosal connective tissue.

Epithelium
Single-layered cylindrical epithelium is composed of several cell types:

enterocytes - most of the cells;


 * slender, cylindrical with numerous microtubules on the surface;
 * constantly renewed, detached at the apex of the villi;
 * in the light microscope the microclusters form a so-called brindle rim;
 * main function - absorption of substances from the contents of the small intestine;

cup cells - individually among enterocytes;


 * their contents consist of mucin droplets which push the nucleus and other organelles towards the basal part;
 * secreted mucin forms a protective layer on the surface of the intestine;
 * the mucus layer is a layer of mucin;

M-cells - in places where there is lymphoid tissue under the epithelium;


 * mediate communication between lymphocytes and the small intestine environment;
 * are actively involved in mucosal defence mechanisms;

tuft cells - sporadically among enterocytes;


 * thicker tufts of microtubules on the surface;
 * function unknown.

Submucosal connective tissue

Sparse connective tissue rich in blood and lymphatic vessels and nerve plexuses. Plexus submucosus Meissneri is a nerve plexus in this connective tissue.

Muscle

A layer composed of smooth muscle. It forms two basic layers:

stratum circulare - the inner circular layer;

stratum longitudinale - the outer longitudinal layer.

Tunica serosa

A peritoneal translucent covering formed by the mesothelium (single-layered flat epithelium), under which is a thin layer of subserosal connective tissue.

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