Anatomy of the abdominal viscera: Small intestine

The small intestine is a part of the digestive tract specialized in absorbing nutrients and minerals from the food we eat.It’s located in the abdominopelvic cavity, and it begins at the pylorus of the stomach and it ends at the ileocecal junction, where it continues with the large intestine.It has three major components: the duodenum, which can be divided into four parts, the jejunum and the ileum.Now, when talking about structures of the abdomen we often encounter the terms ‘retroperitoneal’ or ‘intraperitoneal’.Retroperitoneal is a description for abdominal structures that are only partially covered by peritoneum and lie posterior or behind the abdominal peritoneum, where intraperitoneal means that these structures have invaginated and are completely covered by the visceral peritoneum.So, of these components, the proximal portion of the first part of the duodenum, in addition to the jejunum and ileum are intraperitoneal, where the distal portion of the first part of the duodenum, in addition to the second, third, and fourth parts of the duodenum are retroperitoneal, and are found posteriorly in the retroperitoneal cavity.That being said, let’s have a closer look at the duodenum and its four parts.Overall, the duodenum is shaped like the letter C, curving around the head of the pancreas, and consists of the first, or superior part; second, or descending part; third, or inferior part; and fourth, or ascending part.The first part lies in the transverse plane and begins anterolaterally to the right of the L1 vertebral body, continuing the pylorus.The proximal portion of the first part has a segment of the lesser omentum called the hepatoduodenal ligament attached to its superior surface, and the greater omentum attached to the inferior surface.The proximal 2cm of the first part is also more dilated and freely mobile as it is intraperitoneal with its anterior and posterior surface covered in peritoneum, and is referred to as the ampulla or the duodenal cap.This is in contrast to the distal 3cm which is retroperitoneal, and therefore not mobile.The first part of the duodenum travels laterally to the right and has the liver and gallbladder overlying it, and it has the IVC, bile duct and gastroduodenal artery posterior.The second part, or descending part of the duodenum, is completely retroperitoneal.It curves downward around the head of the pancreas which is medial, and runs inferiorly from the L1 to the L3 vertebra, to the right of and parallel to the IVC.Overlying the second part is the transverse colon and the convolutions of the small intestine,....where posterior to it there’s the hilum of the right kidney, its vessels and ureter.If we open up the second part of the duodenum and look inside, we can see the major duodenal papilla on its posteromedial wall.That’s where the main pancreatic duct combines with the common bile duct to form a hepatopancreatic ampulla, which carries the pancreatic juices and bile to the duodenum.Of note, these can also be referred to as the papilla of vater and ampulla of vater respectively, not to be confused with the popular Darth Vader.Sometimes, above the major duodenal papilla there is also a minor duodenal papilla where the accessory pancreatic duct opens.The duodenum then turns to the left, and the third part starts at the level of L3.This part is also completely retroperitoneal.

The third part of the duodenum passes horizontally to the left below the head of the pancreas.Posterior to it are the inferior vena cava, or IVC, the aorta, the proximal parts of the inferior mesenteric vessels and the body of the L3 vertebra.Anterior to the inferior part there are: the superior mesenteric artery, or SMA, the superior mesenteric vein, or SMV, and parts of the jejunum.Finally, the duodenum turns upwards and becomes its fourth part, called the ascending part, which ascends to the left of the L3 vertebra and aorta, up to the L2 vertebral level and the inferior border of the pancreas.Medial to the fourth part, there are the SMA, SMV and uncinate process of pancreas.Then, it turns anteriorly to connect with the jejunum, forming the duodenojejunal flexure or junction at the level of L2.This site hangs by the suspensory muscle of the duodenum, also known as the ligament of Treitz, that is made out of the skeletal muscles from the diaphragm and the smooth muscles of the duodenum.Now, the duodenum mainly gets arterial blood from the celiac trunk and the SMA.The celiac trunk gives off the common hepatic artery, which also gives rise to the supraduodenal artery supplying portions of the first and second duodenal parts, and the gastroduodenal artery, which has a branch called the superior pancreaticoduodenal artery, that supplies the first two parts of the duodenum up until the major duodenal papilla.The SMA, on the other hand, gives off a branch called the inferior pancreaticoduodenal artery, which supplies the other two parts of the duodenum, distal to the major duodenal papilla.The superior and inferior pancreaticoduodenal arteries give anterior and posterior branches that connect with each other, forming anastomoses between the celiac trunk and the SMA.The veins of the duodenum drain alongside the arteries and venous blood is collected by the splenic and superior mesenteric vein which eventually drain into the hepatic portal vein.The lymphatic vessels of the duodenum accompany arteries as well and drain via the pancreaticoduodenal lymph nodes located along the superior and inferior pancreaticoduodenal arteries, to the pyloric lymph nodes along the gastroduodenal artery and the superior mesenteric lymph nodes along the SMA.All of these ultimately drain into the celiac lymph nodes.As for the innervation of the duodenum, the sympathetic fibers derive from the greater thoracic splanchnic nerves arising from T5 to T9, the lesser thoracic splanchnic nerves from T10-T11, while the parasympathetic fibers derive from the vagus nerves.Together, these fibers of both the SNS and PNS go to the celiac and superior mesenteric plexuses that surround the roots of these arteries.Here, the sympathetic fibers synapse in prevertebral ganglia such as the celiac and superior mesenteric ganglia, and send their postsynaptic sympathetic fibers through the periarterial plexuses following the pancreaticoduodenal arteries.The parasympathetic fibers, on the other hand just pass through the celiac and superior mesenteric plexuses as well as the periarterial plexuses and eventually synapse in the intrinsic ganglia found in the walls of the duodenum, giving short postsynaptic parasympathetic fibers to their destination.Both of these nerve pathways eventually send their postsynaptic fibers to innervate the duodenum.Before we move on, let’s take a break and see if you can identify the arterial blood supply of the duodenum.Now after the duodenum, we have the jejunum which is the second part of the small intestine, beginning at the duodenojejunal flexure, where the small intestine becomes intraperitoneal again.And after the jejunum, there’s the ileum, which is the third and final part of the small intestine.There is no clear visible demarcation between the jejunum and ileum, but when we divide the abdominopelvic cavity into four quadrants, we can see that the jejunum is mostly situated in the left upper quadrant while the ileum is mostly in the right lower quadrant.Together, the jejunum and ileum are about seven meters long, with the jejunum taking the first two fifths and the ileum the last three fifths of the length.On the right side of the pelvic cavity, the ileum terminates into the medial side of the cecum at the ileocecal junction.Now, since they are intraperitoneal, both the jejunum and ileum are covered by visceral peritoneum.Where the visceral peritoneum reflects off the walls of the intestine it creates a double fold of peritoneum, called the mesentery, which attaches the jejunum and ileum to the posterior abdominal wall.The root of the mesentery extends from the duodenojejunal flexure on the left side of the L2 vertebra, across the ascending and inferior parts of the duodenum, aorta and IVC, all the way to the ileocecal junction and the right sacroiliac joint.Between the folds of the mesentery, there are the superior mesenteric vessels, lymphatic vessels and nerves that supply the small intestine, as well as fat tissue.The arterial blood supply for the jejunum and ileum is provided by the SMA, which travels within the mesentery giving off around 15-18 jejunal and ileal artery branches.On their way toward the intestines, these branches connect with each other and form loops called the arterial arcades which give rise to many straight arteries known as the vasa recta.And here’s where the difference between the jejunum and ileum comes in: the jejunum has fewer arcades with larger loops and longer vasa recta, while the ileum has more arcades with shorter loops and shorter vasa recta.Lymphatic drainage of the jejunum and ileum start in specialized lymphatic vessels in the intestinal villi called lacteals.These empty into lymphatic plexuses in the walls of the jejunum and ileum, which drain into a variety of lymphatic vessels and nodes found in the layers of mesentery.Eventually, all these collect into the superior mesenteric lymph nodes.Furthermore, lymph from the terminal ileum drains into the ileocolic lymph nodes.As for the innervation of the jejunum and ileum, the sympathetic fibers derive mainly from the T8-T10 portions of the greater and lesser thoracic splanchnic nerves, while the parasympathetic fibers derive from vagus nerves.Just like for the duodenum, they all go to the celiac and superior mesenteric plexuses, where sympathetic fibers synapse in the celiac and superior mesenteric ganglia and pass through the periarterial plexuses, and parasympathetic pass through all these plexuses to synapse in the intrinsic ganglia in the intestinal wall, where both pathways eventually continue on to innervate the jejunum and ileum.Remember, sympathetic innervation reduces peristaltic and secretory activity to stop digestion, where parasympathetic increases peristaltic and secretory activity facilitating digestion.Also don't forget about sensory information!

Visceral afferent fibers conveying pain travel with the sympathetic fibers, and visceral afferent fibers conveying reflex sensations travel with the parasympathetic fibers.Let’s take another quick break and see if you identify the jejunum and ileum, as well as their blood supply.Alright, as a quick recap… The small intestine has three major parts: the duodenum, jejunum and ileum.The duodenum has four parts.

Most of the duodenum is retroperitoneal except for the proximal portion of the first.The duodenum continues with the jejunum at the duodenojejunal junction, which is suspended by the ligament of Treitz.The jejunum and ileum are intraperitoneal structures, where the jejunum is mostly situated in the upper left quadrant and ileum in the lower right quadrant.Another difference is that the jejunum has longer vasa recta and fewer arcades with larger loops, while the ileum has shorter vasa recta and more arcades with shorter loops.The ileum continues with the large intestine at the ileocecal junction.The small intestine is supplied from the celiac trunk and the SMA, via the superior and inferior pancreaticoduodenal arteries and jejunal and ileal arteries respectively.Lymphatic drainage follows the nodes of the arteries supplying the small intestine, which eventually go on to drain into the superior mesenteric and celiac nodes.Sympathetic innervation comes from the greater and lesser splanchnic nerves, while the parasympathetic fibers derive from the vagus nerves.Visceral afferents for pain travel with sympathetic nerves, and reflex sensation with parasympathetic nerves.