Small intestine

The normal intestinal epithelium is comprised of a single layer of epithelial cells linked by tight junctions and intercalated with immune cells.

 

The small intestinal epithelium is a dynamic tissue containing  a series of villi and crypts.

The proteins joining membranes between adjacent cells form the intercellular tight junctions, adherens junctions, and spot desmosomes which define the permeability of diffusible solutes and serum proteins.

Intercellular junctions are dynamic where surface epithelial cells which constantly regenerate and move along the crypt -villus axis.

Can rapidly remodel the intercellular junctions as needed for nutrient absorption, the presence of mucosal inflammation and wound repair.

Physiologic stimuli such as inflammatory cytokine tumor necrosis factor alpha and glucose absorption after meals.

Pluripotent stem cells near the crypt bases give rise to progenitors that proliferate and migrate toward the villus and the crypt while differentiating into enterocytes, globlet cells and enteroendocrine cells and Paneth cells, respectively.

Lamina propria contains large numbers of previously activated/memory CD4+ T cells that play a role in intestinal immunity and in the induction and maintenance of chronic intestinal inflammation.

Approximately 8-9 liters of fluid enter the small intestine daily.

Of small bowel fluid 1-2 liters come from oral intake and the rest from saliva, gastric, pancreatic, biliary and intestinal secretions.

Absorbs most of the fluid that flow through it with 1-1.5 liters passing on to the colon.

The small bowel is the part of the gastrointestinal tract between the stomach and the large intestine.

It is where most of the absorption of food takes place.

Its primary function is the absorption of nutrients and minerals from food, using protrusions called villi.

It has three distinct regions – the duodenum, jejunum, and ileum.

The duodenum is the shortest part of the small intestine.

The duodenum is shaped like a “C”, and surrounds the head of the pancreas.

The duodenum structure ranges from 20 cm (7.9 inches) to 25 cm (9.8 inches) in length,

The duodenum is where preparation for absorption begins.

The duodenum receives bile and pancreatic juice through the pancreatic duct, controlled by the sphincter of Oddi.

The duodenum receives gastric chyme from the stomach, digestive juices from the pancreas and the liver bile.

Small intestine length varies from 9 feet to 34.4 feet, but is generally 9.8-16.4 feet in length..

Taller people generally have a longer small intestine.

When the bowel is empty, as in death, it measures generally longer.

It is approximately 1.5 cm in diameter in newborns

It is 2.5–3 cm (1 inch) in diameter in adults, and is considered abnormally dilated when the diameter exceeds 3 cm on abdominal x-rays, and over 2.5 cm on CT abdominal scan.

The small bowel mucosal surface is about 30 square meters, due to the surface area of the human small intestinal mucosa, is 30 square meters caused by folds, villi and microvilli

The duodenum contains Brunner’s glands, which produce a mucus-rich alkaline secretion containing bicarbonate.

Duodenal secretions, with bicarbonate from the pancreas, neutralize the stomach acids contained in gastric chyme.

The division between the duodenum and the jejunum is marked by the suspensory muscle of duodenum.

The jejunum is the midsection of the small intestine.

The jejunum connects the duodenum to the ileum, and is about 2.5 m long.

The jejunum contains the plicae circulares, and villi that increase its surface area.

In the jejunum digestive enzymes break down proteins and bile emulsifies fats into micelles.

Sugars, amino acids, and fatty acids, the products of digestion, are absorbed into the bloodstream from the jejunum.

The ileum is the final section of the small intestine.

The ileum is about 3 m long, and contains villi similar to the jejunum.

The ileum absorbs mainly vitamin B12 and bile acids, and other remaining nutrients.

At the ileocecal junction, it joins the cecum of the large intestine.

The mesentery, part of the peritoneum, suspends the jejunum and ileum.

The mesentery carries the arteries, veins, lymph vessels, nerves of the small bowel.

The small intestine receives a blood supply from the celiac trunk and the superior mesenteric artery.

The duodenum receives blood from the celiac trunk via the superior pancreaticoduodenal artery and from the superior mesenteric artery via the inferior pancreaticoduodenal artery.

The jejunum and ileum receive blood from the superior mesenteric artery.

Branches of the superior mesenteric artery form a series of arches within the mesentery known as arterial arcades, and vasa recta travel from the arcades closest to the ileum and jejunum to the organs themselves.

Food from the stomach enters into the duodenum through the pylorus by the pyloric sphincter.

Chemical digestion takes place, mainly in the small intestine, as many of the digestive enzymes that act in the small intestine are secreted by the pancreas and liver and enter the small intestine via the pancreatic duct.

Pancreatic enzymes and bile from the gallbladder enter the small intestine in response to the hormone cholecystokinin.

Cholecystokinin is produced in the small intestine in response to the presence of nutrients.

Secretin, produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach.

There are 3 major classes of nutrients that undergo digestion in the small intestine are proteins, lipids and carbohydrates.

Proteins are degraded into small peptides and amino acids before absorption, with chemical breakdown beginning in the stomach and continues in the small intestine.

Trypsin and chymotrypsin, and other proteolytic enzymes are secreted by the pancreas and cleave proteins into smaller peptides.

The pancreatic brush border enzyme Carboxypeptidase splits one amino acid at a time, while aminopeptidase and dipeptidase free the end amino acid products.

Fats are degraded into fatty acids and glycerol in the small intestine.

Pancreatic lipase with the help of bile salts breaks down triglycerides into free fatty acids and monoglycerides.

Bile salts attach to triglycerides, helping to emulsify them, which allows access by pancreatic lipase.

Bile salts emulsify the triglycerides, until the lipase can break them into the smaller components that are able to enter the villi for absorption.

In the small intestine some carbohydrates are degraded into simple sugars, or monosaccharides.

Pancreatic amylase breaks down some carbohydrates, particularly, starch into oligosaccharides.

Brush border enzymes dextrinase and glucoamylase, further break down oligosaccharides.

Some carbohydrates pass undigested into the large intestine and further handling by intestinal bacteria.

Other brush border enzymes include maltase, sucrase and lactase.

Digested food to passes into the blood vessels in the wall of the intestine through either diffusion or active transport.

The small intestine is the site where

Most of the nutrients from ingested food are absorbed in the small intestine.

The mucosa of the small intestine is lined with simple columnar epithelial tissue.

The mucosa is covered in folds called plicae circulares, which are permanent.

Rugae are non-permanent or temporary allowing for distention and contraction.

The plicae circulares project microscopic villi, and the individual epithelial cells also have finger-like projections known as microvilli.

The functions of the plicae circulares, the villi, and the microvilli are to increase the amount of surface area available for the absorption of nutrients.

The villi limit the loss of said nutrients to intestinal fauna.

Each villus has a network of capillaries and fine lymphatic vessels called lacteals.

The epithelial cells of the villi transport nutrients- amino acids, carbohydrates, and lipids from the lumen of the intestine into these capillaries.

Undigested and unabsorbed material remaining passes into the large intestine.

The jejunum is the site of absorption of the majority of nutrients.

Iron is absorbed in the duodenum.

Vitamin B12 and bile salts are absorbed in the terminal ileum.

Water is absorbed by osmosis and lipids by passive diffusion throughout the small intestine.

Sodium absorption occurs in two stages in the GI tract: The first is via intestinal epithelial cells, whereby Na passes into enterocyte cells by co-transport with glucose, via the SGLT1 protein.

Sodium in the intestinal epithelial cells is pumped by active transport via the sodium-potassium pump through the basolateral cell membrane into the extracellular space.

Sodium bicarbonate is absorbed by active transport, and co-transported by glucose and amino acids.

Fructose is absorbed in the small intestine by facilitated diffusion.

The small intestine supports the body’s immune system by gut flora, and Peyer’s patches within the ileum of the small intestine.

Peyer’s patch are part of the lymphatic system, and provide a site for antigens from potentially harmful bacteria or other microorganisms in the digestive tract to be sampled, and subsequently presented to the immune system.

About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal duodenum.

Conditions that may affect the function of the small bowel:

Small intestine obstruction or obstructive disorders

Meconium ileus

Paralytic ileus

Volvulus

Hernia

Intussusception

Adhesions

Obstruction from external pressure

Obstruction by masses in the lumen- foreign bodies, bezoar, gallstones.

Infectious diseases: Giardiasis, Ascariasis

Tropical sprue

Tape worm

Hookworm

Nematodes

Protozoa

Bacterial infections

Enterotoxigenic Escherichia coli

Salmonella enterica

Campylobacter

Shigella

Yersinia

Clostridium difficile

Mycobacterium

Whipple’s disease

Vibrio (cholera)

Enteric (typhoid) fever and paratyphoid fever

Bacillus cereus

Clostridium perfringens

Viral infections

Rotavirus

Norovirus

Astrovirus

Adenovirus

Calicivirus

Neoplasms

Adenocarcinoma

Carcinoid

Gastrointestinal stromal tumor (GIST)

Lymphoma

Sarcoma

Leiomyoma

Metastatic tumors, especially Small cell lung cancer or melanoma

Developmental, congenital or genetic conditions

Duodenal atresia

Hirschsprung’s disease

Meckel’s diverticulum

Pyloric stenosis

Ectopic pancreas

Enteric duplication cyst

Cystic fibrosis

Malrotation

Persistent urachus

Omphalocele

Gastroschisis

Lsctase deficiency

Gardner syndrome

Familial adenomatous polyposis syndrome (FAP)

Crohn’s disease

Inflammatory bowel disease

Typhlitis

Celiac disease

Mesenteric ischemia

Embolus or thrombus of the superior mesenteric artery or the superior mesenteric vein

Arteriovenous malformation

Gastric dumping syndrome

Irritable bowel syndrome

Duodenal ulcers

Gastrointestinal perforation

Hyperthyroidism

Diverticulitis

Radiation enterocolitis

Mesenteric cysts

Peritoneal Infection

Sclerosing retroperitonitis

Small intestinal bacterial overgrowth

Endometriosis