This organ is located in the gastrointestinal tract, between the stomach and the large intestine.
It is, on average, 23 feet long and is made up of three structural parts; the duodenum , jejunum, and ileum.
Functionally, the small intestine is primarily involved in the digestion and absorption of nutrients. It receives pancreatic secretions and bile through the hepatopancreatic duct that helps with its functions.
The histological structure of the small intestine is similar to that of other organs of the digestive tract. There are four main layers:
- Mucosa (innermost layer): contains the epithelium, the lamina propria and the muscularis mucosa.
- Submucosa: layer of connective tissue that contains blood vessels, lymphatics, and the submucosal plexus.
- Muscularis externa : consists of two layers of smooth muscle; the outer longitudinal layer and the inner circular layer. The myenteric plexus is between them.
- Adventitia (outermost layer): composed of fibroblasts and collagen freely arranged, with the vessels and nerves that cross it. Most of the adventitia of the small intestine is covered by mesothelium and is commonly called serous.
The small intestine is the main absorption site in the gastrointestinal tract and therefore has several modifications to aid its function.
The mucosa and submucosa form large numbers of folds (or folds) arranged in a circular fashion in the lumen (hence called circular folds). In addition, the stems contain microvilli to further increase the surface area, which increases absorption.
The epithelium of the small intestine lines the luminal surface. There are several components of the epithelium:
- Enterocytes: tall columnar cells, which have an absorption function. They contain brush-edge enzymes on the surface that have an important digestive function.
- Goblet cells: exocrine glands that secrete mucin.
Crypts of Lieberkuhn
Crypts of Lieberkuhn are glands found in the epithelial lining.
They contain numerous cells such as stem cells to produce new cells to replace cells lost due to abrasion, as well as enteroendocrine cells to synthesize and secrete hormones.
To protect against pathogens, there are Paneth cells that secrete protective agents (such as defensins and lysosomes) and Peyer’s patches that are only found in the ilium.
Peyer’s patches contain mucosa-associated lymphatic tissue (MALT) that houses white blood cells and lymphocytes. These cells can produce antibodies to further protect the small intestine from infection.
Enteroendocrine cells are found within the crypts of Lieberkuhn. They secrete hormones in response to various stimuli. There are four main classes of enteroendocrine cells, each with a different secretory product. These are I cells, S cells, K cells, and enterochromaffin cells.
The cells secrete cholecystokinin (CCK) in response to the presence of fat in the small intestine. CCK stimulates the contraction of the gallbladder (which pushes the bile into the cystic duct) and the release of pancreatic enzymes.
Both bile and pancreatic enzymes play a key role in the digestion of lipids. S cells secrete secretin in response to the low pH of chyme in the small intestine. Secretin induces HCO 3 – secretion from the pancreas and inhibits gastric emptying.
K cells secrete gastric inhibitory peptide (GIP), in response to chyme entering the small intestine. GIP has a misleading name, as it actually stimulates the release of insulin, ready to put the freshly digested carbohydrates into cells for storage.
Finally, enterochromaffin cells are mechanically stimulated by the presence of chyme in the small intestine. They release serotonin, which acts on the enteric nervous system to activate cystic fibrosis transmembrane regulators (CFTRs).
This ion channel secretes Cl – ions into the intestinal lumen, with Na + ions and H 2 O following. Na + is required in light for the absorption of a number of nutrients.
The small intestine receives secretory products from other abdominal viscera. Here, we will look at the exact mechanism of their secretion and how they help bowel function.
There are three main substances that the small intestine receives: bile, pancreatic enzymes, and alkaline juice (HCO 3 -). Bile plays an important role in lipid digestion and is secreted from the gallbladder into the common bile duct in response to CCK.
Pancreatic enzymes and alkaline juice are secreted from the pancreas into the pancreatic duct, in response to CCK and secretin.
The common bile duct and the pancreatic duct join to form the hepatopancreatic ampulla (also known as the ampulla of Vater). This opens to the inner surface of the duodenum and is marked by the greater duodenal papilla.
The greater duodenal papilla serves as an important anatomical landmark, as it is where the embryonic foregut becomes the midgut. Secretions in the small intestine are regulated by the sphincter of Oddi.
The pancreas plays an important role in digestion, as it produces many of the enzymes necessary for digestion. These enzymes come in two forms. Some enzymes are already active, such as pancreatic lipase and pancreatic amylase, while others are secreted as inactive enzymes such as trypsinogen.
Inactive enzymes are released as zymogens (precursors to the active form); This prevents enzymes from digesting the pancreas. Trypsinogen is converted to trypsin (active form) in the small intestine upon contact with the brush border enterokinase enzyme. The trypsin then converts the remaining inactive proteases to their active form.