Gastrointestinal Tract: Mesenteries, Blood and Nerve Supply

Following on from my post on the patterns of blood vessels, here's a more specific post about the gastrointestinal tract!

Describe the arterial supply of the GIT (gastrointestinal tract)

The gastrointestinal tract is supplied by three main arteries, as mentioned in my previous post about coelomic cavities. The coeliac artery supplies the foregut (which is the digestive tract up to where the pancreatic and hepatic ducts drain into the duodenum), the superior mesenteric artery supplies the midgut (which runs up to around 2/3 of the transverse colon of the large intestine) and the inferior mesenteric artery supplies the hindgut (which is the rest of the GI tract). From there, the arteries branch out a bit. Let's look at each of the arteries in turn, and their branches.

The coeliac artery pokes out from the aorta just under the crura of the diaphragm. It has three main branches, each of which have three branches. Some of those branches have three branches too:

1. Hepatic artery
1. Hepatic artery proper
1. Right hepatic artery
2. Left hepatic artery
3. Cystic artery (supplies gallbladder)
2. Right gastric artery
3. Gastroduodenal artery
   1. Pancreaticoduodenal artery
   2. Right gastro-epiploic/ gastro-omental artery
   3. Supraduodenal artery
2. Left gastric artery
1. Left gastric proper
2. Accessory hepatic
3. Oesophageal branches
3. Splenic artery
1. Splenic artery proper
2. Short gastric artery
3. Left gastro-epiploic/ gastro-omental artery

There are also some anastamoses here- the right and left gastric arteries join up, as do the right and left gastro-epiploic/ gastro-omental arteries.

The superior mesenteric artery emerges just above the duodenum and has five main branches:

1. Jejunal branches
2. Ileal branches
3. Ileocolic artery
4. Right colic artery
5. Middle colic artery

Essentially, the ileocolic artery supplies the area around the ileocecal (i.e. ileum - cecum) junction, the right colic artery supplies the ascending colon and the middle colic artery supplies the transverse colon. (That's just a very rough guide, it's not 100% neatly defined like that.)

The inferior mesenteric artery emerges further down the aorta and only has three main branches:

1. Left colic artery
2. Sigmoidal artery
3. Superior rectal artery

Describe pattern of organs and vessels that lie in the mesenteries of the abdomen

As I mentioned in my post about coelomic cavities, some organs are retroperitoneal (lie against the body wall and are covered by peritoneum) while others are intraperitoneal (in the middle of the abdominal cavity, anchored by mesenteries).

Arteries supplying intraperitoneal organs lie in mesenteries. Let's have a look at the mesenteries and what blood vessels lie in each one:

Mesentery	Location	Vessels
Greater omentum	Hangs over the intestines like an apron. ("Omentum" is Latin for "apron.")	Short gastric artery Right and left gastro-epiploic/ gastro-omental arteries
Lesser omentum	Between the liver and the lesser curvature of the stomach.	Hepatic artery Right and left gastric arteries
The mesentery	Anchors small intestine	Jejunal branches Ileal branches
Transverse mesocolon	Anchors transverse colon	Middle colic artery Marginal artery (connects the colic arteries)
Sigmoid mesocolon	Anchors sigmoid colon	Sigmoid arteries

Pretty much any organ that I haven't mentioned is retroperitoneal. An easy way to remember it is that pretty much every second organ is retroperitoneal: duodenum is retroperitoneal, small intestine is intraperitoneal, ascending colon is retroperitoneal, transverse colon is intraperitoneal, descending colon is retroperitoneal, sigmoid colon is intraperitoneal, rectum is retroperitoneal etc.

Understand the importance of the portal venous drainage of the GIT

As mentioned in my post about the patterns of blood vessels, a portal system goes from capillaries to veins to even more capillaries, rather than going straight back to the heart. Veins draining the GI tract go through the liver to go back to the heart.

The main veins draining the GI tract are the splenic, superior mesenteric and inferior mesenteric veins. The splenic and superior mesenteric drain straight into the hepatic vein, whereas the inferior mesenteric usually joins the splenic before draining into the hepatic vein. This allows the liver to process all the stuff digested by the GI tract before allowing it into the general circulation.

Understand the role of anastomoses in blood supply of the GIT

As I've mentioned in a previous post, anastamoses are important as they give alternative routes for blood to flow in case of blockages or whatever.

Aside from arterial anastomoses, there are plenty of places where venous anastamoses can form in the GI tract. These allow blood to bypass the liver if the liver is diseased or the blood vessels supplying it are occluded or whatever. These anastomoses are known as portocaval anastomoses, and usually form around the beginning and end of the GI tract. Here are some examples:

• Oesophageal blood can drain into the inferior vena cava via the azygos system
• Middle and inferior rectal veins drain into the inferior vena cava
• Blood from retroperitoneal organs can drain via body wall or renal veins

Understand the distribution and function of autonomic nerves in the abdomen

Nerves are important in controlling the release of stuff that helps us digest our food, and so on. I've already written about how the autonomic nervous system works- now let's have a look at how this applies to the GI tract!

The parasympathetic nerves that supply the GI tract include the vagus nerve and the pelvic splanchnic (splanchnic = visceral) nerves. The vagus nerve (which I've mentioned before) supplies the thoracic, foregut and midgut organs, by following the oesophagus into the abdomen. It also follows blood vessels to reach the target organs of the gut. The pelvic splanchnics, S234, supply the hindgut and pelvic organs. They emerge from the sacrum, join the inferior hypogastric plexus and run up to join the superior hypogastric plexus to join the inferior mesenteric artery. (As for how the parasympathetics do their job, that's a topic for a Physiology post.)

I've already spoken about the general structure of sympathetic nerves before, back when I wrote about embryology. Essentially, the preganglionic neuron goes out to the sympathetic ganglion via the white rami. In the sympathetic ganglion, it can synapse with postganglionic grey rami neurons, which innervate the body wall. However, the preganglionic neuron can also keep going out and synapse in preaortic (i.e. in front of the aorta) ganglia. These supply the viscera and include cardiac nerves, coeliac nerves, superior mesenteric nerves and so on. From here, the nerves follow the blood supply to reach target organs:

• T6-9 supplies the foregut via the coeliac plexus
• T9-11 supplies the midgut via the superior mesenteric plexus
• T11-L2 supplies the hindgut with the inferior mesenteric plexus
• T8-L1 supply the adrenals- coeliac plexus again
• T10-T12 supply the kidneys and ureters via the aorticorenal plexus
• T12-L2 supply the pelvic organs via the hypogastric plexus

Now a quick note on sensory nerves, which I don't think I've covered before! The autonomic nerves themselves are not sensory, but sensory nerves do run alongside the autonomic nerves. Hence visceral pain tends to arise from wherever the sympathetic nerve supplying that region came from, and are felt all over that entire dermatome. For example, if you have foregut pain, you might feel dull, achy pain all over T6-T9.

Somatic pain is a bit different- it's a sharp pain close to where the pain actually is.

Phrenic referred pain is a similar phenomenon to visceral pain, in which the pain is felt somewhere else. ("Referred pain" is basically pain that is felt somewhere other than the spot that's actually damaged.) Since the phrenic nerves arise from the neck and supply the diaphragm, pain around the diaphragm may be felt in the neck. As if working out what's wrong with you when you're sick had to be made even more difficult...!