Small intestine

Diagram showing the small intestine and surrounding structures
Micrograph of the small intestine mucosa showing the intestinal villi and crypts of Lieberkühn.
This cross section diagram shows the 4 layers of the small intestine wall.
Absorption of glucose in the small intestine
First stage of the development of the intestinal canal and the peritoneum, seen from the side (diagrammatic). From colon 1 the ascending and transverse colon will be formed and from colon 2 the descending and sigmoid colons and the rectum.
Second stage of development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The liver has been removed and the two layers of the ventral mesogastrium (lesser omentum) have been cut. The vessels are represented in black and the peritoneum in the reddish tint.
Third state of the development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The mode of preparation is the same as in [[:Image:Sobo 1906 400.png|Fig 400]]
Small intestine in situ, greater omentum folded upwards.
Tissue layers (mucosa, submucosa & muscularis)

Organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place.

- Small intestine

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Gastrointestinal tract

Tract or passageway of the digestive system that leads from the mouth to the anus.

Diagram of stomach, intestines and rectum in the average human
Illustration of the small intestine

The GI tract includes all structures between the mouth and the anus, forming a continuous passageway that includes the main organs of digestion, namely, the stomach, small intestine, and large intestine.

Enterocyte

Schematic drawing of an enterocyte: the intestinal lumen is above the brush border.

Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells which line the inner surface of the small and large intestines.

Intestinal villus

Micrograph of the small intestine mucosa showing villi – top half of image. H&E stain
Different stages of coeliac disease
Vertical section of a villus from the dog's small intestine. X 80. (Simple columnar epithelium labeled at right, third from top.)
Transverse section of a villus, from the human intestine. X 350.
thumb|Cross-section histology of small intestinal villi of the human terminal ileum.
MicroCT-based volume projection of the jejunal mucosa of a chicken. Virtual volume block with vertically truncated villi in oblique view. Scalebar = 0.2 mm.
MicroCT-based volume projection of the jejunal mucosa of a chicken. Virtual horizontal cut through villi. Scalebar = 0.2 mm.
Structure of a villus (see reference quoted in text)
Microvilli (shaggy hair) show electron dense plaques (open arrow) at their apices.

Intestinal villi (singular: villus) are small, finger-like projections that extend into the lumen of the small intestine.

Abdominal cavity

Large body cavity in humans and many other animals that contains many organs.

The abdominal cavity is labeled 3 in this image, and together with the pelvic cavity (4) it makes up the abdominopelvic cavity 6.

Organs of the abdominal cavity include the stomach, liver, gallbladder, spleen, pancreas, small intestine, kidneys, large intestine, and adrenal glands.

Mesentery

Organ that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum.

Mesentery extending from the duodenojejunal flexure to the ileocecal junction
Two of the stages in the development of the digestive tube and its mesentery
Mesentery in red. Dorsal mesentery is the lower part of the circuit. The upper part is ventral mesentery.
Abdominal part of digestive tube and its attachment to the primitive or common mesentery. Human embryo of six weeks.
Schematic figure of the bursa omentalis, etc. Human embryo of eight weeks.
Mesenteric relation of intestines. Deep dissection. Anterior view.

The mesentery of the small intestine arises from the root of the mesentery (or mesenteric root) and is the part connected with the structures in front of the vertebral column.

Cecum

Pouch within the peritoneum that is considered to be the beginning of the large intestine.

The cecum, here in red, lies at the start of the large intestines, which are shown with the rest of the human gastrointestinal tract in this image.
Inner diameters of different sections of the large intestine, with cecum (at bottom left) measuring on average 8.7 cm (range 8.0-10.5 cm).
Gastric cecum of dissected cockroach. Scale bar, 2 mm.
Illustration of the large intestine
Cecum and ileum
Ileo-cecal valve
Cecum
Arteries of cecum and vermiform process
Inferior ileocecal fossa
Endoscopic image of cecum with arrow pointing to ileocecal valve in foreground

In dissections by the Greek philosophers, the connection between the ileum of the small intestine and the cecum was not fully understood.

Digestive enzyme

Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body.

Diagram of the digestive enzymes in the small intestine and pancreas

Secretory glands in the small intestine

Trypsin

Enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting these long chains of amino acids into smaller pieces.

The enzyme glucosidase converts the sugar maltose into two glucose sugars. Active site residues in red, maltose substrate in black, and NAD cofactor in yellow.

Trypsin is formed in the small intestine when its proenzyme form, the trypsinogen produced by the pancreas, is activated.

Gallbladder

Micrograph of a normal gallbladder wall. H&E stain.
Abdominal ultrasonography showing gallbladder and common bile duct
3D still showing gallstones
Abdominal ultrasonography showing biliary sludge and gallstones

In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ where bile is stored and concentrated before it is released into the small intestine.

Pancreas

Organ of the digestive system and endocrine system of vertebrates.

Anatomy of the pancreas
The pancreas (shown here in pink) sits behind the stomach, with the body near the curvature of the duodenum, and the tail stretching to touch the spleen.
Diagram showing different functional parts of the pancreas
This image shows a pancreatic islet when pancreatic tissue is stained and viewed under a microscope. Parts of the digestive ("exocrine") pancreas can be seen around the islet, more darkly. These contain hazy dark purple granules of inactive digestive enzymes (zymogens).
A pancreatic islet that uses fluorescent antibodies to show the location of different cell types in the pancreatic islet. Antibodies against glucagon, secreted by alpha cells, show their peripheral position. Antibodies against insulin, secreted by beta cells, show the more widespread and central position that these cells tend to have.
The pancreas originates from the foregut, a precursor tube to part of the digestive tract, as a dorsal and ventral bud. As it develops, the ventral bud rotates to the other side and the two buds fuse together.
The pancreas maintains constant blood glucose levels (shown as the waving line). When the blood glucose level is too high, the pancreas secretes insulin and when the level is too low, the pancreas secretes glucagon.
The pancreas has a role in digestion, highlighted here. Ducts in the pancreas (green) conduct digestive enzymes into the duodenum. This image also shows a pancreatic islet, part of the endocrine pancreas, which contains cells responsible for secretion of insulin and glucagon.
Pancreatic cancer, shown here, most commonly occurs as an adenocarcinoma in the head of the pancreas. Because symptoms (such as skin yellowing, pain, or itch) do not occur until later in the disease, it often presents at a later stage and has limited treatment options.
thumb|A normal pancreas on ultrasound.
thumb|Identifying pancreas on abdominal ultrasonography when it is partly obscured by bowel gas.
Pancreas of a human embryo at end of sixth week
The pancreas and its surrounding structures
Duodenum and pancreas. Deep dissection.

Below the body of the pancreas sits some of the small intestine, specifically the last part of the duodenum and the jejunum to which it connects, as well as the suspensory ligament of the duodenum which falls between these two.