Next Lesson - Anatomy and Physiology of the Hindgut
Core
Anatomical Features of the Intestines
The brush border refers to the microvilli (tiny projections) on the apical surface of enterocytes in the small intestine. The apical surface is the side that faces the lumen (the hollow centre of the gut), and enterocytes are the absorptive cells that line the small intestine. Microvilli increase the surface area for absorption. Surface area is also increased by plicae circulares and villi.
- The plicae circulares (see below) are the mucosal folds (folds of the inner lining) of the wall of the small intestine.
- These are lined with villi, finger-like projections into the lumen that increase the surface area.
- On the surface of the villi cells are microscopic projections called microvilli. Each enterocyte has many microvilli.
The lining (epithelium) of the small intestine contains several cell types. Enterocytes carry the brush border and are the cells responsible for the majority of absorption. The others include goblet cells (mucus production), enteroendocrine cells (hormone production), Paneth cells (antimicrobial peptide production) and stem cells (which provide replacement of a variety of cell types).
The plicae circulares, also known as valvulae conniventes, are the circular folds in the walls of the small intestine. Unlike rugae (the folds of the stomach lining), they do not disappear when the intestine extends.
They slow the passage of the food through the intestine and provide a larger surface area for absorption.
Plicae circulares can be differentiated from haustra (the pouches of the large bowel wall, described in the next section) because the plicae circulares cross the whole width of the bowel.
The large bowel has longitudinal ribbons of smooth muscle on the external surface called teniae coli. When these fibres contract lengthways, they shorten the wall of the bowel and bunch it into outpouchings of colon called haustra. Unlike plicae circulares, the haustra do not cross the whole diameter of the bowel, and this is how the two can be told apart.
Vasa recta are straight arterial branches in the mesenteries of the jejunum and ileum. The mesenteries are the folds that attach the bowel to the back wall of the abdomen and carry its vessels. The vasa recta branch from the arcades (loops formed where neighbouring arteries join), which are themselves anastomoses (joins between vessels) of the jejunal and ileal arteries.
In the jejunum, the vasa recta are few and long, whereas in the ileum they are shorter and more numerous.
The duodenum is the most proximal part of the small intestine. It is the section of the gut between the pylorus (the muscular outlet of the stomach) and the start of the jejunum, and it is split into 4 parts.
The function of the duodenum is to neutralise the acidic chyme (the semi-digested mixture of food and acid that leaves the stomach) with bicarbonate-rich secretion (HCO3-) before the chyme enters the jejunum. It is also where iron is absorbed from the chyme.
The Ampulla of Vater (also called the hepatopancreatic ampulla) is the small widened chamber where the common bile duct and the pancreatic duct join. It is located in the second part of the duodenum, where it opens at the major duodenal papilla. This is where bile and pancreatic enzymes enter the small intestine, from the bile duct and the pancreatic duct. Bile emulsifies fat (breaks it into tiny droplets), which makes it easier for the digestive enzymes to digest it. The pancreatic enzymes digest the carbohydrates, proteins and fats in the chyme.
The duodenum has a dual blood supply. The part nearer the stomach than the major duodenal papilla is supplied by the gastroduodenal artery. The gastroduodenal artery is a branch of the common hepatic artery. The common hepatic artery comes from the coeliac trunk (the branch of the abdominal aorta that supplies organs such as the stomach, liver and spleen). The part further from the stomach is supplied by the inferior pancreaticoduodenal artery, which also supplies the head of the pancreas. This artery is a branch of the superior mesenteric artery. The veins follow the arteries, as the pancreaticoduodenal veins. Some drain towards the portal vein and others drain into the superior mesenteric vein, which in turn drains into the portal vein. Lymphatic drainage is into the pancreaticoduodenal and superior mesenteric lymph nodes.
Diagram - The anatomy of the duodenum
Creative commons source by Luke Guthmann [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The jejunum is the second part of the small intestine. It begins at the duodenojejunal flexure (the sharp bend where the duodenum becomes the jejunum). There is no clear anatomical landmark that marks where the jejunum ends and the ileum begins. The jejunum is located in the left upper quadrant of the abdomen. It has a thick intestinal wall (due to an increased number of plicae circulares). It also has long vasa recta and few arcades. These features differentiate the jejunum from the ileum.
The function of the jejunum is to absorb the majority of nutrients such as sugars, amino acids, and fatty acids. The jejunum is specifically designed for this function as the combination of plicae circulares, villi and microvilli (brush border) provides a very large surface area ideal for absorption.
The majority of absorption in the small intestine occurs across the brush border (the microvilli on the surface of the enterocytes) into the enterocytes.
Monosaccharides are transported into enterocytes by transporters: SGLT1 (sodium-glucose linked transporter 1) absorbs glucose or galactose (a sugar released, along with glucose, when lactose is digested) and GLUT5 (glucose transporter 5) absorbs fructose. These sugars are then transported across the basolateral membrane (the side of the cell facing the blood, opposite the apical surface) into the blood by GLUT2 (glucose transporter 2), which carries all of these monosaccharides. Enterocytes also absorb short peptides (chains of two or three amino acids that have not been completely digested prior to absorption) via the PepT1 (peptide transporter 1). Free amino acids are absorbed by other transporters.
The jejunum’s blood supply is from the jejunal arteries which branch off the superior mesenteric artery (SMA). Its venous drainage is into the jejunal veins which then drain into the superior mesenteric vein (SMV). Lymphatic drainage is into the superior mesenteric nodes.
The ileum is the third part of the small intestine. As stated in the jejunum section, there is no clear demarcation of where it begins. It ends at the ileocaecal junction (where the ileum meets the caecum, joining the small bowel to the large bowel). The ileocaecal valve is located here to prevent reflux from the caecum back into the small intestine.
The ileum is mainly located in the right lower quadrant of the abdomen. It has thinner intestinal walls, shorter vasa recta, and more arcades than the jejunum.
The function of the ileum is to absorb the remaining nutrients not absorbed in the jejunum, including vitamin B12 and bile acids. Water and electrolytes are absorbed throughout the bowel, not only in the ileum. About 8 to 9 litres of fluid enter the intestine each day (from food, drink and the body's own secretions). Most of it is absorbed in the small bowel, and the large bowel absorbs almost all of the rest, so less than 100 ml (about 1%) is lost in the faeces.
Water is absorbed in the small intestine through an osmotic gradient. Sodium is co-transported (carried across the membrane together) with glucose and amino acids into the enterocytes. This creates an osmotic gradient that water follows. Secretion of water into the intestine is predominantly driven by the movement of chloride ions.
The ileum’s blood supply is from the ileal arteries which branch off the SMA, and its venous drainage is into the ileal veins which then drain into the SMV. Lymphatic drainage is into the superior mesenteric nodes.
Diagram - The three divisions of the small intestine
Creative commons source by Vedzitux, edited by Dr. Laura Hansell [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The appendix is a narrow blind-ended tube that usually originates from the posteromedial aspect of the caecum. It is suspended from the terminal ileum by the mesoappendix (a section of mesentery). The position of the free end of the appendix can vary greatly, but its most common position is retrocaecal (behind the caecum).
The arterial blood supply of the appendix is from the appendicular artery which is a branch of the ileocolic artery. Its venous drainage is via the appendicular vein which drains into the SMV. Lymphatic drainage is into the ileocolic lymph nodes. The appendicular artery, vein, and lymphatics lie within the mesoappendix.
The appendix has no essential functions in the body, although it contains lymphoid tissue (tissue that makes immune cells) and is thought to have a role in immune function. Appendicitis is acute inflammation of the appendix. In about half of cases, it is thought to be caused by a blockage of the lumen (the hollow inside) of the appendix. Causes of blockage include a faecolith (a hard lump of faeces) and swelling of lymphoid tissue during an infection. Because the appendix is a blind-ended tube, a blockage leaves its contents trapped, and the appendix can become inflamed or infected.
Diagram - Anatomy of the large intestine, with the appendix highlighted in red
Public Domain Source by Mikael Häggström and B jonas, edited by Dr. Laura Hansell [Public domain]
The caecum, ascending colon and proximal two-thirds of the transverse colon are derived from the midgut. The distal third of the transverse colon, descending colon and sigmoid colon are derived from the hindgut.
The caecum is the most proximal part of the large intestine and lies within the peritoneum (the lining of the abdominal cavity). It is the site of storage for chyme that is received from the small intestine and contains the ileocaecal valve to prevent reflux of this chyme back into the ileum. The caecum’s arterial supply is from the ileocolic artery which branches off the SMA and its venous drainage is into the ileocolic vein which drains into the SMV. Lymphatic drainage is into the ileocolic lymph nodes.
The ascending colon is retroperitoneal (behind the peritoneum, against the back wall of the abdomen). Its arterial supply is from branches of the SMA: the ileocolic artery and the right colic artery. The right colic artery is variable: it often arises from the middle colic artery instead, and it is sometimes absent. Its venous drainage is into the SMV. Lymphatic drainage is into the superior mesenteric lymph nodes.
The transverse colon lies within the peritoneum and has its own mesentery (the transverse mesocolon). Its arterial supply is mainly from the middle colic artery, which branches off the SMA. The distal third is supplied by the left colic artery, which comes from the inferior mesenteric artery (IMA). Its venous drainage follows the arterial territories. The middle colic vein drains towards the SMV. The distal territory also drains through the left colic vein towards the inferior mesenteric vein (IMV). Lymphatic drainage similarly follows the superior and inferior mesenteric territories.
The colon is much wider and shorter than the small intestine, and it has crypts (small pits in the lining) rather than villi, and these increase its surface area. The external longitudinal muscle of the colon is incomplete: it is made up of three bands of muscle called teniae coli, which cause the formation of haustra (sacculations) when they contract.
The main functions of the colon are final water and electrolyte absorption (facilitated by epithelial sodium channels, ENaC), and the formation and temporary storage of faeces prior to defaecation.
Diagram - The teniae coli and haustra of the large intestine
Public Domain Source by Cunningham's Textbook of Anatomy [Public domain]
Plain abdominal x-rays are of limited use because they cannot provide very much information about the soft tissue structures that make up most of the abdominal cavity. Their added value in acute abdominal pain is too limited to justify routine use. One of the few indications for a plain abdominal x-ray is obstruction of the bowel (a blockage that stops the contents passing). A CT scan of the abdomen and pelvis with contrast (a dye that makes the organs and blood vessels show up more clearly) is now the imaging test of choice for suspected bowel obstruction. A plain x-ray is still used in some settings as a quick first test, and it can show whether the obstruction is in the small or large bowel.
There are a number of features that can help differentiate between the small and large intestine on plain film x-ray:
- The small bowel is narrower than the large bowel. The diameter of the small bowel should not exceed 3 cm. The diameter of the large bowel should not exceed 6 cm, or 9 cm in the caecum. Bowel wider than these limits is dilated, as seen in obstruction.
- The small bowel is often more central in the abdominal cavity, with the large bowel peripherally around the abdomen. This may be less reliable with very dilated bowel loops as can be seen in obstructions (the main indication for plain film abdominal x-rays).
- The plicae circulares traverse the whole diameter of the small bowel, whereas the haustra in the large bowel do not traverse the whole diameter.
While the interpretation of abdominal x-rays is outside the scope of this article, it is important to understand the clinical applications for the anatomical knowledge explained in this article.
Connect this region to the integrated physiology: use Gastrointestinal Motility and the Enteric Nervous System for segmentation (mixing movements). It also covers peristalsis and the migrating motor complex (repeating waves of contraction that sweep through the gut between meals). Use Gastrointestinal Secretion, Digestion and Absorption for transport across jejunal and ileal enterocytes. Use Liver, Biliary and Exocrine Pancreatic Physiology for the bile salts and pancreatic enzymes that make small-intestinal absorption possible.
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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