Table of Contents
Embryology of the GI Tract
The GIT is derived embryologically from the foregut, midgut and hindgut, with their corresponding neurovascular supply. Developmentally, the oral cavity is formed with the loss of the buccopharyngeal membrane, while the pharynx originates from the pharyngeal arches. The next section of the foregut, after the oral cavity, is the primitive pharynx, followed by a single esophagotracheal tube, which lies dorsal to the primitive heart.
During the fourth week of embryonic life, the primitive gut develops from an out-pouching of the yolk sac, which is a structure lined by endoderm. The yolk sac is attached to the large midgut generated from the lateral embryonic fold via the vitelline duct, which regresses during fetal life. In some cases, the vitelline duct may persist beyond fetal life, and then it is known as the Meckel’s diverticulum.
The primitive gut gives rise to the foregut, midgut and hindgut. The foregut is the precursor from which develop the esophagus, the stomach and the first and second part of the duodenum, liver, gallbladder as well as the superior part of the pancreas. All these organs are supplied by the branches of the artery of the foregut, the celiac trunk.
The midgut gives rise to the third part of the duodenum, the jejunum, ileum, cecum, appendix, ascending colon and the right two-thirds of the transverse colon. These are supplied by the branches of the artery of the midgut, the superior mesenteric artery.
The hindgut gives rise to the left one-third of the transverse colon, descending colon, rectum and upper part of the anal canal. These structures are supplied by the branches of the artery of the hindgut, the inferior mesenteric artery.
This major artery begins as a continuation of the thoracic aorta at the aortic hiatus of the diaphragm at the level of the T12 vertebra. It ends by bifurcating into the common iliac arteries at the level of the L4 lumbar vertebra.
Three anterior branches arising from the abdominal aorta supply the gut. From superior to inferior, their names are the celiac trunk, the superior mesenteric artery and the inferior mesenteric artery. These three branches are individual entities. Although their terminal branches anastomose, the three arteries themselves cannot substitute each other.
The celiac trunk is an important artery from a surgical stand point. It originates from the abdominal aorta and lies in the transpyloric plane at the upper border of the L1 vertebra. It is approximately 1.25 cm long.
The artery branches out into the splenic artery, the left gastric artery and the common hepatic artery. The celiac trunk and its branches supply oxygenated blood to the abdominal esophagus, the stomach, the first and second part of the duodenum, liver, gallbladder, spleen and the superior part of the pancreas.
The celiac trunk is the only major arterial trunk which does not have a correspondingly named vein. Although the spleen is not derived embryologically from the foregut, the celiac trunk supplies blood to it.
|Branches of the celiac artery||… and their branches|
|1. Left gastric a.||Branch to stomach and esophagus|
|2. Common hepatic a.||Right gastric a., gastroduodenal a., proper hepatic a.|
|3. Splenic a.
||Short gastric a., left gastro-omental a., dorsal pancreatic and greater pancreatic a.|
Table 1: Branches of the celiac trunk
Superior Mesenteric Artery (SMA)
This midgut artery originates from the abdominal aorta, about 1 cm inferior to the origin of the celiac trunk. In adults, its origin lies anterior to the inferior border of the L1 vertebra. It then runs anteroinferiorly, behind the splenic vein and the neck of the pancreas. During this part of its journey, it is separated from the aorta by the following structures:
- Third part of the duodenum and part of the small intestines, which may be compressed by the SMA, causing the superior mesenteric artery syndrome.
- Pancreatic uncinate process, which hooks around the SMA.
- The left renal vein, which travels from the left kidney to the inferior vena cava and can be compressed here by the SMA and the abdominal aorta, causing the nutcracker syndrome.
Once the SMA reaches the neck of the pancreas, it branches into the intestinal arteries, the inferior pancreaticoduodenal artery, the ileocolic artery and the right and the middle colic arteries. These supply the organs of the midgut, namely the third part of the duodenum, the jejunum, ileum, cecum, appendix, ascending colon and the right two-thirds of the transverse colon.
Inferior Mesenteric Artery (IMA)
This artery of the hindgut arises from the abdominal aorta, inferior to the superior mesenteric artery at the level of the L3 vertebra. It is the smallest branch among the three anterior branches of the abdominal aorta.
Initially, it descends anterior to the abdominal aorta, and then crosses to the left as it continues to descend. It gives off the following branches: the left colic, two to three sigmoid branches and the superior rectal artery, which is a terminal branch. These supply oxygenated blood to the left one-third of the transverse colon, descending colon, rectum and superior part of the anal canal.
Branches of the SMA and IMA anastomose to form the marginal arteries of Drummond and the Riolan’s arcade (anastomoses between the left colic and medial colic artery). Thus, they provide abundant oxygenated blood to the colon, except for certain watershed areas with poor blood supply, e.g., splenic flexure of the colon.
The marginal artery runs distal, while the arc of Riolan (also known as the meandering mesenteric artery) runs proximal to the root of the mesentery. The terminal branches of the right colic, middle colic, left colic and sigmoid branches enter the colonic wall and are called the vasa recta.
Due to this significant blood supply, the IMA does not have to be re-attached to the abdominal aorta after an abdominal aortic aneurysm repair.
Clinical Relevance of the Abdominal Arteries
This artery is an important source of oxygenated blood to the foregut structures as the various anastomotic networks with other major arteries in the region do not provide adequate perfusion. Therefore, in a living individual, the artery cannot be ligated safely as its occlusion leads to necrosis of the various tissues that it supplies. This is essential to remember during surgical procedures of the foregut.
Variations in the celiac trunk
As mentioned, the celiac trunk is an important artery, and its anatomy has surgical significance. Variations in its pattern of branching are seen in approximately 15% of the population. Knowledge about these variations is important for diagnostic and surgical abdominal procedures. Otherwise, there is a high risk of surgical errors, which may lead to possible lethal complications.
Celiac artery compression syndrome
This syndrome, also called Dunbar syndrome, is a condition characterized by abdominal pain associated with meals, weight loss and abdominal bruit. It is a result of the external compression of the celiac artery and celiac plexus by the median arcuate ligament.
It is diagnosed clinically after excluding other causes of abdominal pain. The diagnosis can be confirmed with ultrasound, CT scan or MRI. Treatment is usually surgical excision of the constricting median arcuate ligament and the celiac ganglion.
Superior mesenteric artery syndrome
This is a rare syndrome and is caused by the compression of the last part of the duodenum by the SMA and the abdominal aorta. It has to be differentiated from celiac artery compression syndrome as well as nutcracker syndrome.
It is characterized by severe, stabbing, post-meal, abdominal pain, early satiety, nausea and vomiting. Early diagnosis is difficult but essential as delay can lead to severe dehydration, malnutrition, electrolyte abnormalities, etc. Treatment consists of medical management and, in severe cases, surgical bypass.
This syndrome is characterized by hematuria and left flank pain due to compression of the left renal vein between the abdominal aorta and the SMA.
In the saggital or transverse plane, the abdominal aorta and the SMA appear like a nutcracker with a nut (the left renal vein) in its jaws, hence the name. The diagnosis of the syndrome is usually confirmed with left renal venography. Treatment consists of endovascular stenting/renal vein re-implantation depending on the severity of the symptoms.
Superior mesenteric artery thrombosis
The SMA is rarely affected by atherosclerosis. However, in rare cases, especially following atrial fibrillation or myocardial infarction, the artery may get occluded by a thrombus leading to ischemic colitis.
This usually occurs in the watershed areas like the splenic flexure and is characterized by acute onset abdominal pain and blood in the stools. In most cases, there is a history of recent myocardial infarction or atrial fibrillation. The condition may occur acutely or over a period of time (chronic ischemia). Diagnosis is confirmed with mesenteric angiography and definitive treatment consists of emergency laparotomy with bowel resection–anastomoses.
Chronic ischemic colitis
When blood flow to parts of the colon is reduced due to arterial blocks or thrombus, the colon may become inflamed and eventually gangrenous. In such situations, we may find the marginal artery and the arc of Riolan significantly enlarged in an attempt to perfuse the ischemic colonic segment.
This surgical procedure involves the removal of the left side of the large bowel, usually necessary in case of tumors. The IMA and its branches have to be resected during this surgery as the IMA supplies the left colon.