When I was going through the basic sciences, something about understanding the GI innervation and blood flow just took forever for my study group to understand. I say that including myself. After going over it, we finally decided to try laying it out in a table (which we later learned was about half made already in First Aid). Seeing it that way made it all suddenly make sense. This week, we’re going to walk through that process and then end with the final table for you to copy.
As with most things, it is helpful to start with the embryology. The whole GI tract is really just one giant tube – from mouth to anus. Before it slips out of the abdomen to twist around during development, it’s pretty much a straight line. Start with visualizing that, then divide it into three parts – the foregut, midgut and hindgut. We can now fill in the first part of our table:
We should also clarify where those regions begin and end. After all, this tube is going to herniate, twist all around and come back in. Now, it’s going to look more like what we’re expecting. So, where does one section end and the next begin? The foregut is basically from the mouth down to the first ⅓ of the duodenum. It’s not really the mouth, but its a good way to conceptualize it. It’s more like the pharynx down. I would also think of it not so much as the first ⅓ (what does that mean, anyway?) but as the ampulla of vader. The midgut picks up there and ends at the ligament of treitz aka the splenic flexure of the large colon. Well, okay, technically it’s like the proximal ⅔ of the transverse colon is the midgut and then its hindgut, but that isn’t quite as easy to conceptualize or remember. Finally, from there, it’s all hindgut to the end.
Each section, even during preherniation, already have individual blood supplies. You know it, foregut is supplied by the celiac trunk, the midgut gets it from the superior mesenteric artery and the hindgut from the inferior mesenteric artery. It’s like that aortic and heart tube just has these three branches reaching forward to connect it’s little baby blood supply to it’s little baby GI tube. Picture it like that and it should also make sense that these are the only unpaired branches of the abdominal aorta. Of course, after its done twisting, these blood supplies are still true. We can now fill in the second column.
There isn’t a good mnemonic to use, but don’t overcomplicate the third column. Since we’re talking about the digestive system, the first thing we want to do is send all that blood, which has just picked up a bunch of nutrients, to the liver. Foregut is right there, so it can just send it through the splenic vein. While the midgut and the hindgut gets its blood supply from SMA and IMA, respectively, it sends blood back through the same names – SMV and IMV, respectively. The IMV leads to the splenic vein and the SMV leads to the hepatic portal vein.
|Foregut||Celiac A.||Splenic V.|
|Midgut||SMA||SMV (to Hepatic Portal V.)|
|Hindgut||IMA||IMV (to Splenic V.)|
We often think of the sympathetic nervous system being the driving force for the various processes that we study. It’s kind of the opposite in the GI tract. If you’re in sympathetic flight or fight mode, digesting that burger is not real high on the list. However, for parasympathetic we say rest and digest, so it’s even in the name. The vagus nerve comes down from the head and then goes sort of all over. It actually derives its name from the word vagabond, the term for homeless guys who used to ride the rails. They travelled all over, just like the vagus. Ultimately, the vagus n. will provide innervation for both the foregut and the midgut. The hindgut is taken care of by the pelvic splanchnic nerve, coming off of S2, S3 and S4. The mnemonic for that is “S2, S3, S4 – keep the a**hole off the floor.” With that little ditty, here is the next column!
|Foregut||Celiac A.||Splenic V.||Vagus n.|
|Midgut||SMA||SMV (to Hepatic Portal V.)||Vagus n.|
|Hindgut||IMA||IMV (to Splenic V.)||Pelvic splanchnic n|
Last, of course, we need the sympathetic innervation to provide inhibition and balance out the parasympathetic. There isn’t a great way to logically connect these, but the descending order can make it memorable. It all comes right in a row, in a descending fashion. Start with the greater splanchnic (T5-T9), the lesser splanchnic (T10-T11) and end with the lumbar splanchnic (L1-L2). You can couple that with the hindgut parasympathetic innervation which more or less comes next in line (pelvic splanchnic at S1, S2 and S3). Last column!
|Foregut||Celiac A.||Splenic V.||Vagus n.||Great Splanchnic (T5-T9)|
|Midgut||SMA||SMV (to Hepatic Portal V.)||Vagus n.||Lesser Splanchnic (T10-T11)|
|Hindgut||IMA||IMV (to Splenic V.)||Pelvic splanchnic n||Lumbar Splanchnic (L1-L2)|
Thinking through the embryology and organizing the information in this fashion should help. I’d recommend writing it out a handful of times and then you’ll likely have it down. There are also a number of YouTube tutorials that will walk you through drawing out the blood flow. Those are great for understanding how that one section works, but couple it with this table for a fuller picture.