Intestines

• The small intestines begins in the duodenum which is largely retroperitoneal. Then it turns forwards to become the jejunum, and then ileum. Both the jejunum and ileum are intraperitoneal.

  The jejunum occupies the upper part of the abdomen and the ileum the lower part. The jejunum is about two-fifths of the length of the small intestine, and the ileum three-fifths of the length.
• The large intestines is partly retroperitoneal and partly intraperitoneal. It starts with the caecum and ascending colon which are retroperitoneal. The transverse colon is intraperitoneal, but the descending colon is retroperitoneal. The sigmoid colon is intraperitoneal. The distal end of the alimentary tract is the rectum and finally, the anal canal and anal sphincter.
• The embryological division of the gut are:

  • The foregut ends halfway down the 2nd part of the duodenum.
  • The midgut ends two-thirds of the way along the transverse colon.
  • The hindgut ends halfway down the anal canal.

  The embryological divisions of the gut are important in terms of vascular supply, and venous and lymphatic drainage.
• Given that parts of the gut are intraperitoneal, this means that they have great mobility, and have a mesentery.

  • The transverse colon is supported by the transverse mesocolon.
  • The jejunum and ileum are supported by the mesentery.
  • The sigmoid colon is supported by the sigmoid mesentery.
• The mesentery stems from the posterior abdominal wall. It starts at the duodenojejunal flexure in the upper left side of the abdomen and ends at the ileocaecal junction in the lower right side. This root is 15cm in length.
• The mesentery is able to support 6m of intestines by folding upon itself many times.

  This folding does make it easy for a loop of small intestine and its segment of mesentery to become twisted, producing a volvulus. This can result in the vessels in this segment of mesentery, and the gut itself, becoming obstructed.
• The small intestines is comprised of the duodenum, jejunum and ileum. The duodenum is largely retroperitoneal and makes up about 5% of the total small intestines. The jejunum is the proximal two-fifths of the intraperitoneal part of the small intestine, and the ileum makes up the distal three-fifths. There is no clear delineation between the jejunum and ileum. The jejunum comprises just under 40% of the total length and the ileum just under 60%.
• The lining of the small intestinal mucosa is very highly specialized for maximising digestion and absorption of nutrients, salt and water. For example, the lining is highly folded to form villi to increase the surface area for absorption.
• Specific regions of the small intestine are adapted to perform specific functions (alongside the adaptions that aid in the absorption of nutrients):

  • The duodenum coordinates how the stomach empties, and controls the rate of emptying of bile into the intestines. The duodenum is also a major site for iron absorption.
  • The jejunum is a major site for the absorption of folic acid (vitamin B9).
  • The end of the ileum is the most important site for absorption of vitamin B12 and bile salts.
• It takes about 3-6 hours for the chyme to pass through the small bowel. The movement is generated by peristaltic and segmentation. Peristalsis is a wave-like movement. Segmentation is a series of more randomised contractions of small sections of the bowel, and this serves to churn the chyme to make it more accessible for absorption.
• The lining of the small intestine possess a series of large folds called plicae circulares. Plicae circulares are more prominent in the duodenum and jejunum. The plicae are further folded by the presence of villi. Each of the surface epithelial cells have a further increase in surface area by the presence of microvilli on their surface. Increased surface area increases the rate of absorption.
• The epithelial cells secrete enzymes to help digest the chyme. Peptidases break down peptides into amino acids. Sucrase, maltase, and lactase break down the disaccharides sucrose, maltose, and lactose respectively. These will give monomers of glucose, fructose, and galactose. Intestinal lipase breaks down fatty acids into glycerol.

  At the base of the villi, the crypts of Lieberkühn release a watery mixture containing mucus that serves as a carrier fluid for absorbing nutrients from chyme.
• There are duodenal glands in the submucosa of the small intestine which secrete a thick, alkaline solution containing bicarbonate ions, that helps neutralize chyme. This is regulated by distention or irritation of intestinal mucosa by acidic chyme.
• The duodenum is the first part of the small intestine whose main function is absorption. It lies slightly to the right of vertebrae L1-L3, at a position above and to the right of the level of the umbilicus.

  The 1 st part of the duodenum lies at the level of the pylorus of the stomach hence it is positioned at the transpyloric plane. The duodenum is C-shaped and the C then curves down into the umbilical region.
• The duodenum is divided into 4 parts:

  • The 1st part is ascending backwards from the pyloric sphincter, and is known as the duodenal cap. There are no plicae here and it’s smooth internally.
  • The 2nd part is the descending portion, and together with the remainder of the duodenum, is retroperitoneal. This part receives the common bile and pancreatic ducts.
  • The 3rd part is the horizontal portion.
  • The 4th part of the duodenum is the ascending portion.

  Because the majority of the duodenum is retroperitoneal, it's fairly immobile.
• The end of the fourth part of the duodenum turns forwards at the duodenojejunal flexure and acquires a mesentery to become the jejunum.

  The duodenojejunal flexure is held in position by the ligament of Treitz, or suspensory ligament, of the duodenum. This ligament derived via muscle slips from the right crus of the diaphragm. The ligament is also partly fibrous.
• A contrast medium is required to view the duodenum in an X-ray. On a X-ray, the first part of the duodenum is seen as a roughly triangular duodenal cap, perched on top of the pyloric end of the stomach. The rest of the duodenum takes in a floccular appearance due to the presence of numerous plicae circularis.
• The duodenum receives bile from the liver, and pancreatic exocrine secretions (protease and amylase).

  Pancreatic secretions are alkaline, since they contain bicarbonate ions, but they enter the duodenum around half-way along its length. The epithelium in the first half of the duodenum is protected from the acidic chyme by mucus secreted from large mucous glands of Brunner.
• The common bile duct and pancreatic duct open into a single duct, which terminates on the posteromedial wall of the 2nd part of the duodenum. The entry of this duct into the duodenal wall creases a papilla on the mucosal wall, and this papilla is called the major duodenal papilla. It's also known as the hepatopancreatic ampulla (of Vater). This papilla is protected by the sphincter of Oddi.
• The Sphincter or Oddi is particularly evident around the lower end of the common bile duct. The sphincter of Oddi is under the control of cholecystokinin (CCK), which is a hormone produced by the duodenum via a vasopepide (vasoactive intestinal polypeptide (VIP)). In this way, the duodenum acts as a signal to the gall bladder and pancreas to release their content into the gut to aid digestion.
• The duodenum begins to the right of the the vertebra L1. The 2nd part of the duodenum descends to the level of L3, and the 3rd part of the duodenum crosses anterior to L3. The 4th part of the duodenum ascends and turns forward to join the jejunum, ending at L2.

  The duodenum ends up defining the perimeter of the head of the pancreas, encircling it on all three sides.
• The upper half of the second part of the duodenum is sandwiched between the hilum of the right kidney and ureter (behind), and the beginning of the transverse colon (in front). It is crossed by the transverse mesocolon.

  The third part of the duodenum crosses in front of the aorta and inferior vena cava, and has the superior mesenteric vessels crossing it anteriorly.

  The fourth part of the duodenum ascends and turns forwards, towards the duodenojejunal junction.
• The jejunum and ileum lie surrounded by the greater sac of the peritoneal cavity. They are covered anteriorly by the greater omentum, which is attached to the transverse colon.
• The jejunum is intraperitoneal so it has a serosa on its surface. It has a muscularis mucosa with an outer longitudinal and inner circular muscle layer. There is a glandular submucosa and a mucosa thrown into a large number of plicae circulares.

  The jejunal walls are thicker than the ileum. The walls are also pinker is colour because the mesentery is thinner due to having less fat and a richer blood supply. The mesentery is almost devoid of fat near the gut wall.
• The plicae circulares of the ileum are small in its superior part and absent in its terminal part. Otherwise, the layers of the ileum are the same as that in the jejunum.

  One of the characteristic features of the small intestine is the presence of aggregated lymph follicles in the mucous membrane. In the ileum however, these are large enough to be seen with the naked eye where they are known as Peyer's patches.

  The other key difference between the ileum and jejunum is that the ileum has a thicker mesentery which is full of fat.
• The jejunal arteries have less complex vascular arcades compared to those of the ileum, and there are long vasa recta (straight arteries) that reach the gut wall. These vasa recta are anatomical end-arteries. Hence if one becomes blocked, the portion of gut wall that it supplies will necrose and ulcerate.
• The ileal branches of the superior mesenteric artery divide and reunite to form a complex series of arches and arcades, much more so than the jejunum. The vasa recta of the ileal arteries are hence shorter, but like those of the jejunum, they are anatomical end-arteries.

  Thus, one or more of the main branches may become obstructed, without affecting the viability of the gut. However, blockage of a vasa recta will result in necrosis of that segment of the ileum.
• Although there are no obvious features externally to distinguish jejunum from ileum, their location is characteristic. Most of the jejunum lies in the upper left side of the abdomen, whereas the ileum occupies much of the lower right side of the abdomen.
• The mucosal features of the jejunum and ileum differ from one another. The plicae circulares of the jejunum give that region a floccular appearance on an X-ray, following a contrast medium introduced into the tract. The ileum however, has few plicae and hence, fills more completely with contrast medium. As a result, the ileum has a more solid appearance on an X-ray than the jejunum.
• The gut is a place for potential infection due to ingested bacteria and viruses that have survived the acid bath of the stomach. To combat this, the mucosa of the small intestines is full of mucosal lymphoid follicles, and there are numerous lymph nodes contained within the mesentery, called mesenteric nodes.
• From the small intestines, the lymphatic vessels travel back to reach the superior mesenteric nodes, which are situated around the origin of the superior mesenteric artery at the front of the aorta. The foregut drains to the coeliac nodes and the hindgut drains to the inferior mesenteric nodes.

  The coeliac nodes, superior mesenteric nodes, and inferior mesenteric nodes are also referred to (collectively) as pre-aortic nodes. Enlargement of these nodes hence points to infection or cancer of the gut.
• Lymph from the pre-aortic nodes drain into the cisterna chyli which lies on the right-hand side of the aorta. This lymphatic sac is then drained by the thoracic duct which enters the aortic hiatus of the diaphragm. At the level of the sternal angle, the thoracic duct deviates to the left. It ultimately terminates by joining the junction of the left internal jugular and left subclavian veins.
• The large intestine consists of caecum (with its vermiform appendix), ascending colon, transverse colon, descending colon, and sigmoid colon.

  The transverse colon and sigmoid colon are intra-peritoneal and therefore mobile, but the remainder is fixed, although the caecum sometimes has a mesentery. The appendix always has its own mesentery, known as the mesoappendix.
• By the time the chyme has made it to the large intestines, it's a solution of mainly unabsorbable material. Most of the nutrients have been taken up by the small intestines, so the material that enters the large intestine is mainly liquid and fibre. The large intestine has to absorb any remaining fluid and electrolytes in a process called resorption.
• The first half of the colon absorbs fluids and recycles them into the bloodstream. The second half of the colon compacts the waste into faeces, secretes mucus to bind the substances together, and lubricates the material to ease its passage through the colon and to protect the colon itself.

  By taking all the fluids and useful material back out of the chyme, the large intestine concentrates the useless material into faeces to be removed from the body.
• There is minimal digestion that takes place within the large intestines, done by gut bacteria. Undigested sugars and fibres are broken down into fatty acids. These fatty acids will lower the pH of the colon, but the colon produces an alkaline solution to maintain its pH. The colon also produces come vitamins (vitamin K) that are then reabsorbed.

  The colon is also involved in raising immunity to the viruses that bacteria encountered in the gut.
• The colon is emptied by mass contraction of a large section of the bowel. There are 2-3 such contractions in a day which serves to propel the faecal matter towards the rectum.

  Faeces are comprised mostly of water and any unabsorbed or undigested chyme. It also contains the cells that are shed from the epithelial surface together with bacteria and viruses.
• The colon can be recognised by:

  • size
  • its three taeniae coli (longitudinal bands of muscle) that cause it to be sacculated (haustrations).
  • its appendices epipolicae, which are fatty tags scattered over the surface of the colon (usually along the positions of the taeniae coli between the haustrae).

  These features aren't found in the small intestine.
• The colon lacks a complete longitudinal layer of muscle, but still has a complete layer of circular muscle. Hence, movements in the colon consist mostly of peristalsis. These can be either minor peristaltic waves, haustral churning, or mass peristalsis.

  Haustral churning is produced by segmentation contractions which serve to mix the contents to enhance absorption. Mass peristalsis consists of large movements which occur at intervals, usually associated with meals.
• Haustral churning and mass peristalsis are often initiated by the gastrocolic reflex. This reflex stimulates the colon in response to food entering the stomach. The gastrocolic reflex is especially active after fasting, and when the food is hot or cold. It causes mas peristalsis in 15mins, and continues for about 30mins.

  These movements cause the chyme to move in several large steps through the colon, stopping at each step to be further concentrated and converted into faeces.