Okay, we're going to talk about the digestive system and answer the questions, what are the digestive system organs and what are their functions and what accessory digestive organs contribute to digestion and what's the vascular supply and innervation of the GI tract? Hello everyone, my name is Dr. Morton and I'm the Noted Anatomist. So let's begin by talking about digestive organs and specifically start in the oral cavity.
And so the oral cavity has teeth and the teeth masticate, which means it's the physical digestion or the physical breakdown of food. And what does that mean? Well, we have this mandible that allows us to chew.
So what happens is you get food and you put food in your mouth and you start chewing, that piece of pizza keeps going, breaking down and just becomes pizza, but just smaller pieces of pizza. That's physical digestion, taking food and making it smaller pieces of the same food, but no chemical bonds were broken. Now let's enter salivary glands in the oral cavity, and that's where saliva moistens food and it forms a bolus.
and the salivary amylase, let me start that again, salivary amylase in saliva starts the chemical digestion of carbohydrates. So there we have three glands, submandibular salivary gland, the parotid gland, and the subligual salivary gland. And these glands make saliva, but the submandibular, that makes the most by far, like 75 to 80% of the saliva. Parotid makes maybe 20, 15 to 20 percent and just 2 to 3 percent is that sublingual salivary gland.
And these glands are paired, three on each side. So if you zoom in, let's take a look. There's saliva coming from these three areas and what the saliva does is it takes these small pieces of food and it forms a ball and we call that ball a bolus and that's what you swallow down the pharynx into the esophagus. But then also salivary amylase within saliva chemically starts digesting carbohydrates and you break. chemical bonds.
So that's how polysaccharides become disaccharides or monosaccharides. That is chemical digestion, breaking chemical bonds in carbohydrates, proteins, or fats. You make about a liter and a half of saliva every single day.
That's like four and a half cans of soda. Just to give you an idea how much saliva you, you, me, we all make. Next, let's go to the esophagus.
The esophagus transports food from the oral cavity to the stomach. And during that transportation, no metabolic changes occur. It's simply being transported. So there's the esophagus. Now, the esophagus is interesting in that it has three different architects of muscle in its wall, depending on where we're at.
So proximally, there's skeletal muscle lines, the esophagus, from the pharynx down to the top of the esophagus. That's what allows you to voluntarily swallow food. Distally, the part of the esophagus that goes into the stomach, that's all smooth muscle.
That's involuntary, so you do not have any conscious control over it. And somewhere between the proximal and distal part, that skeletal and smooth muscle mix. So you go from voluntary to involuntary as you swallow that bolus of food down.
So there's that orange circle represents a bolus of food. And so swallowing voluntarily, we see that peristalsis is this movement that takes over after you swallow. So when you swallow, just... sit there and swallow your own spit you go boom you voluntarily did that that's skeletal muscle but then there's this process called peristalsis that's involuntary that takes over and that's how we move the food all the way through the rest of the gi tract all the way down until you get down to defecate and now you've got skeletal muscle that allows you to defecate voluntarily again as whatever's left over comes out and so that's a nice transition to a little tangent that we're going to talk about the layers of the GI tract. Here's a cross-section of the GI tract, and there's four layers that we're going to go over.
The first, mucosa, second, submucosa, third, muscularis externa, and finally, the serosa. Let's start with the mucosa there in pink. The mucosa is lined with epithelium. That's the part that's lining the lumen. And deep to all basement membranes of epithelium is loose connective tissue.
We call that the lamina propria. And the mucosa is there to protect or to help with absorption and secretion. There's epithelium. That's lining the lumen.
And there's the lamina propria. That's where the capillaries are found for absorption. Let's take one of those capillaries and blow it up. And there we've got the mucosa. And the mucosa consists of two parts.
Epithelium, in the case in this one, it's simple columnar epithelium that we see in the stomach, small intestine, and large intestine. The esophagus is stratified non-carotinized epithelium, but most of the gut tube is simple columnar. Deep to that is the laminopropion. That's loose connective tissue, and that loose connective tissue house the systemic capillaries that absorbed carbohydrates and amino acids, and the central lacteals, the lymphatic capillaries that absorb fats. So this motif is something we're going to see throughout the digestive system.
Deep to the mucosa is the submucosa. It means sub, like a submarine below. And this consists of dense connective tissue with lots of vessels and nerves, and there's some blood vessels.
These are the larger vessels, the macroscopic vessels that feed the capillaries that go into the lamina propria. The muscularis mucosa that I just outlined there is a thin layer of smooth muscle that is the fence between the mucosa and the submucosa. Next is the muscularis externa. The muscularis externa consists of an inner circular and outer longitudinal layer of smooth muscle.
Inner circular, outer longitudinal that go in opposite direction. Contraction of the inner circular and outer longitudinal layer of smooth muscle is what causes segmentation, the breaking down of the bolus into smaller. chunks, and peristalsis, that movement of food through the GI tract.
Notice between the inner circular and outer longitudinal layers is this nerve. We call it, it's a neuronal cell body, we call that our box plexus or the myenteric plexus. And then deep to the inner circular, there's another one we call that Meissner's plexus or the submucosal plexus because it's kind of in that, kind of it is between the inner circular layer and the submucosa.
These are describing... autonomic neurons that line the GI tract that help with its functioning. So we call these GI gastrointestinal intramural ganglia within the wall, like an intramural sports team is a sports team within your own school.
Extramural means you fight or not fight. You know what I mean? You play against a different school.
GI intramural ganglia are within the wall of this. But simply these are postganglionic parasympathetic neuronal cell bodies. That's what they are.
They're the second. neuron in the parasympathetic chain, but we have another name for the, it's just the enteric nervous system. All these words are talking about the same thing.
The neuronal cell bodies that line the wall of the gut tube, either in the submucosal plexus or my enteric plexus that help with its functioning. All of these are derived from neural crest cells. They're talking about the same thing. Yeah.
Thanks anatomists. So let's talk about peristalsis. It's like getting toothpaste out of a toothpaste container so here's the toothpaste container open the lid and we go like this squish and you notice that when you do that toothpaste comes out and then you push squish push squish push squish push squeezing and pushing that's where you get toothpaste out well peristalsis is like getting toothpaste out of a container here's the gi tract and the in and you see that you squeeze that's called segmentation you break the bolus into smaller pieces of bolus and then peristalsis is you move it like that.
So we see just like squeezing, which is the inner circular layer, and pushing, which is the outer longitudinal layer, that's how you get toothpaste out of a container, and that's how you get a bolus of food or the chyme to move through the GI tract. Finally is the serosa, which is a mesothelial lining. Another name is the visceral peritoneum. It's the serous lining that makes serous fluid.
All right, now here's an... H&E stain of the ileum we're going to zoom in on and we're going to focus on this area. So the mucosa consists of epithelium and there's the lamina propria and the dotted line is the basement membrane. The lamina propria houses the capillaries for absorption.
Then there's the muscularis mucosa that I just drew on a smooth muscle that separates the mucosa from the submucosa, which is primarily connective tissue, blood vessels, and nerves. Then we have the muscularis Externa, made a smooth muzzle of an inner circular and an outer longitudinal layer. And then finally, there's the serosa of mesothelium.
There are the layers of the GI tract. Let's talk about the stomach now. Like the stomach's topography is this between the esophagus and duodenum, as you go from mouth to anus.
So there's the stomach between the esophagus and the duodenum. But it's also superficial to our spleen, pancreas, and aorta, where there's the stomach. And... It's superficial, you know, it's hard to tell from that picture of the spleen is deeper to the stomach. And the pancreas lies deep to the stomach as does the abdominal aorta.
The function of the stomach. Well, peristalsis moves the movements, the peristalsis moves. Let me say that again. Peristaltic movements turn the gastric contents and it makes the bolus into chyme. And so this would help facilitate the physical breakdown of food and the mixing of secretions from gastric glands turn this bolus into chyme.
So goblet cells of the epithelium lining the stomach make mucus that protects from the acidity that's derived from parietal cells and parietal cells produce hydrochloric acid. with a pH of like one and a half to two. Also intrinsic factor, which helps with the reabsorption of vitamin B12 by the small intestine.
And the chief cells secrete this proenzyme called pepsinogen that when it mixes with hydrochloric acid becomes pipsin, starts the chemical digestion of proteins. So here are the different regions of the stomach, okay? And I've just drawn these on.
Well, there's the cardia because it's deep to the diaphragm, which above that is the heart. and the cardiac sphincter separates the esophagus from the stomach. The fundus is the dome-shaped area at the top of the stomach.
That's where gas usually goes. That's where the next belch is sitting. There's the body of the stomach, the big part in the middle, and then the funneling part at the end is called the pylorus, which gives rise to the pyloric sphincter, which is a thickening of the inner circular layer of the muscularis externa that helps keep contents from the stomach and the small intestine separate. Then we have this very large curvature we call the greater curvature and a smaller one called the lesser curvature on top. Now the lesser omentum is some mesothelium that attaches to the lesser curvature and the greater omentum is peritoneum that attaches to the greater curvature.
We take a cross section through the stomach and there's gastric rugae that if I draw it looks like this and it allows to increase the contents in the stomach up to like one and a half liters to allow you to consume more food, like at one of those old you-can-eat buffets. Now let's talk about small intestines. So small intestines are about seven meters long, but they don't get their name because they're short. They get their name because the lumen is small, the luminal diameter is small.
And this is where digestive processes are completed, where nutrients are chemically digested and then they're absorbed. And the small intestine has three... parts.
It has the duodenum, the jejunum, and the ileum. Let's start with the duodenum. It's the first part of the small intestine. It's distinguished by Brunner's glands, and it has four parts. So there's the duodenum.
It has these Brunner's glands, and these asterisks show these glands that help produce the bicarbonate that's secreted into the lumen. And then it has four parts. The first part of the duodenum, the superior part, has what's called the duodenal cap or bulb.
There's the pylorus of the stomach and the pyloric sphincter, and there's the first part, the duodenal bulb. And in this barium swallow, you can see the pylorus of the stomach, pyloric sphincter, and there's the duodenal cap or duodenal bulb. Duodenal or duodenal, tomato, tomato.
The first part is also smooth walled. So there you can see the first part is smooth, and the second part is not smooth walled because it has these circular folds. Smooth.
not smooth, okay? And then the first part is intraperitoneal, where the rest of the GI tract is retroperitoneal. So there's the hepatoduodenal ligament of the lesser omentum attaching to the first part of duodenum, that's intraperitoneal, where the rest of the duodenum, parts two, three, and four are retroperitoneal.
Now, part two of the duodenum is called the descending part. It starts as retroperitoneal. It has circular folds and it has what's called the major duodenal papilla.
So there's the second part, descending because the food descends vertically. Has a circular fold that you'll see characteristic for the entire small intestine for increasing surface area for absorption. And that's also where the common bile duct and the pancreatic duct form wonder twin powers to make the major duodenal papilla.
And that's how you get bile and pancreatic enzymes into the second part of the duodenum. At the end of the second part of the duodenum, we have the beginning of the horizontal or third part, that's the separation from the embryonic foregut, from midgut, more on that later. The third part, or transverse part of the duodenum, is where the superior mesenteric artery and vein cross over the duodenum. That's one of the clinical characteristic features traversed by the SMV and SMA. In this picture, you can see the parts of the duodenum, and you'll see then the superior mesenteric vein and artery crossing over.
the third part of the duodenum. And finally, the fourth part or ascending part of the duodenum is where the duodenal-jejunal junction occurs and where we go from retro back to intraperitoneal organs. So there's the fourth part and there's the ligamenta trites, which is skeletal muscle coming off the diaphragmatic crura.
And that's what points to where the duodenum and jejunum have the duodenal-jejunal junction. It's a surgical landmark that's often used. It's also where we see... There's the fourth part where the jejunum and ilium, that's the duodenal-jejunal junction.
Retroparoteneal to intraperitoneal. So there's our four parts of the duodenum. Now let's talk about the rest of the small intestines with the jejunum and the ilium.
The jejunum is located in the upper left quadrant and the ilium is in the lower right quadrant. So here we have the small intestines, the greater momentum is reflected. Upper left quadrant, primarily that small intestine you see is the jejunum.
Lower right quadrant, that... primarily is the ilium. There we've got where they're basically located. And the jejunum and ilium are considered intraperitoneal organs, and they're tethered to the posterior abdominal wall by mesentery. This is a sagittal section of the abdominal cavity, and there we're focusing on all of those circles are showing cross-sections, or in this case, sagittal sections through the jejunum and ilium.
Notice that mesentery It anchors it to the back of the abdominal wall. And these two-layer mesentery is where we find the vessels and nerves coursing to the gut tube. Now, the jejunum is the second part of the small intestine, as we've shown here. And it has a lot of circular folds.
The fancy way of saying it is the plicae circularis. Lots of circular folds, as you can see there. So there's lots of absorption of the nutrients from the food we're eating. Now, the ileum is the third part. of the small intestine.
There are fewer circular folds in the jejunum. And so when we see the ileum and jejunum, lots of circular folds, fewer circular folds. They're there, but just not as many.
Here is an actual cadaveric small intestine opened up where there's a jejunum. Look at the number of those circular folds in contrast to the ileum where they're there. There's just fewer of them because a lot of the absorption has already occurred. All right. Now the ileum also histologically has something called Peyer's patches, which is...
dense lymphatic nodules in the lamina propria that you see all those purple dots are showing nuclei of lymphocytes, lots of them. And that's histologically how you distinguish the ileum from the jejunum and from the duodenum. So histologically, duodenum has Brunner's glands, ileum has pyrus patches, jejunum has neither.
And finally, the ileum has the ileocecal valve because there's the ileum, there's the cecum. So what do you call the valve between them? The ileocecal valve.
That That represents the termination, the end of the road for small intestines, which leads us now to the large intestines or the colon. What are some of the features of large intestines? Well, they're about one and a half meters long, so they get their name because the luminal diameter is large compared to the small intestine, even though they're far shorter.
The large intestines absorb water and salts and also vitamins that are produced by the intestinal bacteria flora. And the... The colon basically compacts and then eliminates feces.
The large intestine is then further characterized by tinea coli, hostra, and epiploic appendages. Let's go through each one of those. So the tinea coli are thickening of outer longitudinal layer of the muscularis externa, and there are three separate longitudinal bands, anterior, posterior, and lateral, and they run the entire length of the colon. And so we see...
There we can see the anterior and posterior and then the anterior again. We just don't see the lateral very good. And little tightening of the tinea coli form these hostra. And the hostra are small pouches in the colon that give it its segmented appearance, very characteristic of the colon or large intestine. And finally, the epiploc appendages, which are small outpouchings of fat-filled serosa covering the outside of the colon like that.
All right, so now let's talk about the cecum. It's located in the lower right quadrant of the abdomen. It's a blind-ended sac.
It's the start of the large intestines, and coming off the cecum is the vermiform appendix right there. It's attached to the cecum, and it has an unknown function for the most part, though. More literature showing that it might have very important intestinal flora inside and its variable length and position.
But the surface anatomy to locate the appendix is called McBurney's point. You find the ASIS, anterior superior iliac spine, find the umbilicus, and from the right side, I should say, the ASIS, you draw an imaginary line. And a third of the way up is McBurney's point.
If you push on that, that's where the origin of the appendix is off the cecum. The ascending colon arises from the cecum, courses vertically to the liver, it's on the right side of the abdomen, and it ends at the right colic or hepatic flexure. The transverse colon is an intraperitoneal organ.
It's the superior part of the abdomen, and it courses from the right colic flexure all the way to the left colic flexure. Did you hear that? That's my dog.
Hold on. And the transverse colon is anchored to the posterior abdominal wall by the transverse mesocolon. So something that happens in the transverse colon is there's a transition from the embryonic midgut to hindgut. So there's our transverse colon somewhere, we usually say two-thirds along the way, the midgut becomes the hindgut. Now why do we care about that?
Well that's going to become important when we start talking about vascular supply and innervation. Okay, all right, now the descending colon is on the left side of the abdomen. It courses from the left colic or splenic flexure down to the sigmoid colon.
And the sigmoid colon then is on the left side of the pelvis. It's an intraperitoneal organ anchored to the pelvic wall through the sigmoid mesocolon. And then finally, the rectum is the terminal straight portion of the GI tract.
Rectum or rectus means straight, like rectus abdominis or erector spinae. The rectum is a retroperitoneal organ. So here in my one little illustration, there's the rectum and there's parietal peritoneum.
And notice that the rectum is behind it. Like that, okay? Now, the anus is the exit of the GI tract, and there's the anus. And so the anus has skeletal muscle for the external anal sphincter. That's what allows you to defecate when you want to.
The internal anal sphincter is smooth muscle that's involuntary, and as more feces backs up behind the anus into the rectum, the more pressure is on the internal anal sphincter, the more it becomes relaxed, and the more you feel like, oh. I got to make it to the toilet and that's where the external anal sphincter comes into play to help you until you're ready to sit down. Let's talk about the accessory digestive organs, okay, and start with the liver. Now the liver is located in the upper right quadrant of the abdomen and it produces bile. Now bile is what we say emulsifies fat, takes big fat droplets and breaks them.
Think of cracking a rock into tiny pieces. It's still a rock, they're just smaller pieces and that enables to reduce so that digestive enzymes can better break the chemical bonds in these smaller droplets of fat. But the liver has a ton of other functions like it helps with cholesterol metabolism, it takes part in the urea cycle, produces proteins like albumin, it produces clotting factor, helps to detoxify blood, and so forth.
The liver has the following lobes, right, left lobe, quadrate, and caudate. Here's a visceral inferior view of the liver. There's the falciform ligament and the gallbladder, and there's the portal triad, more on that later, and that's the groove for the inferior vena cava. To the right of the gallbladder is the right lobe. To the left of the falciform ligament is the left lobe.
Between the gallbladder and falciform ligament is the quadrate lobe, and between the inferior vena cava and the left lobe is the caudate lobe. There are the four anatomic lobes of the liver. Now, the portal triad consists of the proper hepatic artery, the portal triad, and the the portal vein and common bile duct. So there, and we'll view in a little bit closer, there is the proper hepatic artery that comes off the common hepatic artery, a branch off the celiac trunk.
And then there's the portal vein, which is basically the portal vein, the union of the splenic and superior mesenteric vein. This is what's draining all blood from the GI tract. And then the common bile duct there, that's what's delivering bile from the liver and gallbladder into the second part of the duodenum.
So the right and left hepatic duct feed into the common hepatic duct and they join the cystic duct to make the common bile duct and that's what we see there. All right, so there's our liver, there's the gallbladder, there's the stomach, and there's the duodenum. Between that, we're going to draw on the lesser omentum coming off the lesser curvature of the stomach. which consists of two parts because anatomists name everything. The hepatoduodenal ligament Hepato for liver to the duodenum is the free edge of the lesser omentum and then the hepatogastric ligament is the rest.
And we talk about this because inside the hepatoduodenal ligament of the lesser omentum is the portal triad. Okay. But this, I wanted to show an H&E stain of a microscopic section high power of the liver because All those cells you see around the side are hepatocytes, but look here in the middle is a portal triad. Right there, there's a hepatic duct, hepatic artery, and portal vein.
So what we see microscopically is the same relationship that you see grossly. I think that's really cool. All right, now the mesenteries of the liver. So we zoom in.
There is the visceral peritoneum that lines the liver, parietal peritoneum that lines the wall, and they reflect upon each other. But notice, There is this space right there where there is no mesentery. We call it the bare areas and bare naked, bare area.
There's no mesentery. And that is now where when you have high pressure from ascites, where fluid can push through that area from the abdominal serous fluid into plural, to become plural fluid or fluid in the plural space, I should say. And there's the bare area of the liver and often these illustrations that you'll see. There's also this falciform ligament right here, this ventral mesentery coming from the liver to the anterior abdominal wall.
And that falciform ligament has at the bottom the ligamentum teres that's housing the obliterated umbilical vein. And there we have it, often what you'll see in the liver. Okay, now let's talk about the gallbladder, which is on the visceral surface of the liver, and it stores and concentrates bile. So here we have the gallbladder, and there's the cystic duct.
and the common bile duct, and there's the main pancreatic duct, and they form together the major duodenal papilla. Now, that's in the second part of the duodenum. Now, I want to see what happens, like in the liver, bile's produced. And so what happens is bile moves down these ducts to get down to go into the liver.
Let me say that again, to get into the duodenum. But that opening is not always patent. That major duodenal papilla has a sphincter of odi that keeps it closed at times.
And unless cholecystokinin is secreted, that will remain, that opening will remain closed. So bile backs up and goes into the cystic duct where the gallbladder stores and then concentrates that bile. Let's talk about the pancreas. And so the pancreas is located deep to the stomach.
There's the stomach. There's the duodenum. And there I've just highlighted the pancreas. And there if we cut the stomach, you see that the pancreas is retroperitoneal.
So we see these two lines of the parietal peritoneum. And between them, there's the pancreas. But the pancreas is behind the parietal peritoneum, except for the tail. That tail, it touches the spleen.
That little bit, it's like your toes sticking out the end of your bedsheets. That's what they consider intraperitoneal. But the bulk of the pancreas is retroperitoneal.
And it's about 15 centimeters long. And I just include that because I remember when I first learned digestive system, I had in my mind. The pancreas is like the length of my index finger, and it's pretty hefty. Now, the pancreas has both exocrine and endocrine glands.
Exocrine glands, or any exocrine glands, secretes their product to an epithelial surface. So in this case, the pancreas secretes digestive enzymes into the epithelial line duodenum. And all those yellow circles represent acinar glands, and they produce digestive enzymes that go in the main pancreatic duct of Wurzung.
which then goes into the duodenum, that second part. So Johann Worsang was, he's a German anatomist that was dissecting in an anatomy lab in Padua, England. And in 1642, he discovered this duct in the pancreas. And he's like, ta-da, I'm going to name this.
And so he did name it. And instead of publishing in journals, he decided he's going to put them on copper plates instead. So all these copper plates have this investigation. The next year, this other anatomy...
A student, Giacomo Cambier, discovers the same duck and he starts writing about it and someone says, no, no, no, actually, Johan Wersung already did it. And through very hard feelings, Giacomo got so angry that one night when Johan was going to home, Dr. Wersung was shot in the back by Giacomo and killed over this structure. So, Jokamal went to jail, Dr. Wurtsang died, the structure was continued in his name, and five years later, Dr. Wurtsang's graduate student said, actually, I'm the one who discovered it, but I think there was enough death and violence, and so he just let it go. Dr. Wurtsang is what it's named after.
Now, those digestive enzymes in the duodenum from the pancreas is what I want to talk about next. These digestive enzymes that I've now schematically shown as scissors, what they do is you've got pancreatic proteases. And what these enzymes do is they take proteins and break the chemical bonds between individual amino acids. And that's how we chemically digest proteins into their individual amino acids. That's one of the ways the pancreas does this.
It also does it through pancreatic amylase, which then breaks the chemical bonds between these different monosaccharides and disaccharides in these long-chained polysaccharides. And that's how we get glucose. And then finally, the pancreatic lipase that takes and breaks the bonds between these fatty acids and glycerol in the duodenum. And so that is one of the ways that the pancreas helps with digestion, is making those digestive enzymes. Now, the pancreas also contains endocrine glands.
And now, in contrast, endocrine glands secrete their products into the bloodstream, which then circulates them. The pancreas secretes insulin and glucagon. We have these things that I've drawn in orange circles. They're called pancreatic islets of Langerhans.
And that is what's producing the hormones insulin and glucagon. And so insulin will take macromolecules like sugar, glucose, and put them into the cells. Glucagon will take macromolecules like glycogen and break them down and dump glucose into the bloodstream.
So the pancreas has... exocrine digestive enzymes, and endocrine for insulin and glucagon. All right. Let's talk about the spleen. Now, the spleen actually is located in the upper left quadrant like that.
It is not a digestive organ, but it has the same blood supply because it forms in the dorsal mesogastrium. So I mention it here because it has the same blood supply. It's about the size of a fist, and it stores blood.
Phagocytosis form blood particles and it produces PMNs. And its other big function is it removes senescent and defective red blood cells. All right. Let's now talk about the vascular supply and innervation of GI organs and begin by talking about this embryonic foregut, midgut, hindgut.
Here's a schematic of the development of the gut tube in yellow. And so there is what's called the superior mesenteric artery. And this gut tube that's forming from mouth to anus has this part in the middle of your abdomen where the superior mesenteric artery has this part of the gut tube that wraps, that it herniates out. And so embryologists said, why don't we call this the middle gut, the mid gut?
So the part before it's the foregut and the part after it's called the hindgut. So foregut organs include stomach to the first couple parts of the duodenum and all the accessory digestive glands. The mid gut is the second part. the third and fourth part of the duodenum, all the way to the transverse colon, and the hindgut's transverse colon to the rectum. Knowing this is helpful.
Now, why do we care about learning foregut, midgut, hindgut? Because if you know those, it's very easy, or I should say straightforward, to learn the arterial supply. So the celiac trunk supplies all foregut organs.
Here we have the celiac trunk coming off the abdomen, abdominal aorta. So there's the stomach and there's the duodenum and the pancreas and the liver and gallbladder and the spleen. All those organs, they're supplied by the celiac trunk through those branches.
Okay. Now the superior mesenteric artery supplies all mid-gut organs. There's the SMA.
So there's jejunum and there's the ileum and there's the cecum and appendix and there's the ascending and transverse colon. All of those organs are supplied by the SMA through those branches. And finally, oh, pardon me. there is an anastomotic connection between the foregut and midgut. So here we've got the celiac trunk and there is its branches called the superior pancreatic duodenal arteries and there's the superior mesenteric artery with the inferior pancreatic duodenal arteries.
So between these pancreatic duodenal arteries we have an anastomotic connection between the foregut and midgut arterial supply. Now the IMA supplies all hindgut organs. So there's the IMA and so there's the transverse colon, descending and sigmoid colon in the rectum.
All of those are supplied by branches off the IMA. Now there's an asthmatic connection between the midgut and hindgut. Okay, and so there in red I show this thing it's called the marginal artery of Drummond. And as we can see there it kind of parallels the picture frame of the colon.
And so the superior mesenteric artery has this middle colic artery and the inferior mesenteric artery has this left colic. And then watch, we've got this very cool anastomotic connection through the marginal artery of Drummond. And finally, lymphatics.
Now, lymph from the GI tract drains to lymph nodes arising from the organs arterial supply. So there we have celiac lymph nodes. So where is that lymph coming from? All the foregut organs.
There's the superior mesenteric nodes. Where's the lymph coming from? All the midgut organs.
There are inferior mesenteric nodes. Where's that lymph coming from? All the hindgut organs.
Now, the venous drainage of the GI tract. To do that, to talk about that, we need to discuss the hepatic portal system. The hepatic portal system mirrors the arterial branches for the most part.
So for the foregut, you have the celiac trunk supplies it and the gastric, gastro-mental, splenic veins drain foregut. The midgut supplied by the SMA, midgut's drained by the SMV. Hindgut's supplied by the IMA, it's drained by the IMV.
So we see that the aorta supplies the foregut, midgut, hindgut. And then the portal vein drains the gut tube. So the foregut, through the gastric, gastro-mental, and splenic veins, and they all drain into the hepatic portal vein. So there's the foregut with the stomach and spleen, though there's other organs.
I'm just going to focus on those two. And you have the splenic vein, and there's the portal vein. And you see all those veins that have drained the stomach and the spleen all end up going to the portal vein, which drains into the liver. The superior mesenteric vein receives blood from the midgut.
So the superior mesenteric vein goes to the portal vein and into the liver. So there's the midgut with the jejunum and ileum and the cecum ascending and transverse colon. And there's the superior mesenteric vein collecting blood from all the midgut organs into the portal vein, into the liver.
And finally, the inferior mesenteric vein receives blood from the hindgut and the IMV goes to the splenic, which goes to the hepatic portal vein, which goes to the liver. So there's our hindgut with the descending colon, sigmoid colon, and rectum. And there's the inferior mesenteric vein going into the splenic vein, going into the portal vein, into the liver.
Now let's conclude with the innervation of the gut tube. Autonomic innervation can either be sympathetic or parasympathetic. So sympathetic innervation will reduce peristalsis or decrease it and decrease secretions, but increase sphincter tone. You're basically slowing down digestion.
Where in contrast... Parasympathetic innervation will increase peristalsis and increase secretions, but reduce finger tone, basically increasing digestion. So remember, parasympathetics are rest and digest. So let's talk about the way that the autonomics will help. So here we have in this illustration the foregut.
I'm just showing the stomach, but it's basically all foregut organs. The sympathetic innervation comes from the greater splanchnic nerve which comes from the lateral horn gray matter of T5 to T9, courses through the sympathetic trunk without synapsing and then continues as the greater splanchnic nerve to synapse in the celiac ganglion and then post ganglionic sympathetic neurons wrap around like Christmas lights on a Christmas tree branch, arteries from the celiac trunk that then supply sympathetics to the foregut organs. Now, parasympathetic innervation will come from the vagus nerve that's arising from the medulla, course through the celiac ganglia without synapsing, hug along the same arteries, and then synapse with postganglionic parasympathetics in the wall of the foregut. That's the intramural ganglia, the Arbox and Meissner's plexuses.
There. Okay. Now, the midgut organs. These are going to be supplied by the lesser splanchnic, so coming from the lateral horn of T10 and T11, course through the sympathetic trunk without synapsing.
The lesser splanchnic nerve will then synapse in the superior mesenteric ganglion. Postganglionic sympathetics follow the arteries to synapse in the wall of all the midgut organs, slowing down digestion. Whereas the vagus nerve for parasympathetics courses through the superior mesenteric ganglia wraps around the arteries of the SMA to supply midgut organs. Finally, we have the hindgut organs, which are going to be like transverse descending and sigmoid colon and rectum.
Those preganglionic sympathetics arise in the lateral horn of L1 and L2, course through the sympathetic trunk without synapsing, and these lumbar splanchnics will synapse in the inferior mesenteric ganglia. and then follow the arteries out to the hindgut, or through the superior hypogastric plexus and the inferior hypogastric plexus, course out to hindgut organs and innervate them. Parasympathetics arise from the S234 spinal cord levels in the lateral horn, and then they course to the hindgut via these pelvic splanchnic nerves, okay, from S234, and then coursing in the hypogastric and inferior hypogastric plexuses.
supplies the hindgut organs. So there we have it. So the GI tract in a nutshell. We have the oral cavity, which is where teeth are that masticate and physically digest the food, where the salivary gland secretes saliva with salivary amylase that moistens the food and forms a bolus. That bolus then moves down the esophagus through peristalsis into the stomach, which has goblet cells that make mucus, parietal cells that make hydrochloric acid, an intrinsic factor, and chief cells that make pipsinogen.
which then that food stuff, the chyme, goes into the duodenum. This is the location of biliary action and the enzymatic chemical digestion from the pancreas. So the liver filters the blood but also produces bile, where it's stored in the gallbladder and concentrated, and that's where the bile emulsifies fat.
The pancreas produces and secretes digestive enzymes into the duodenum. So the liver, the gallbladder, and the pancreas all dump their products into the duodenum. which is the location of biliary action and enzymatic chemical digestion. The rest of the small intestine, duodenum, jejunum, and ileum, is where the absorption of nutrients take place. So we have amino acids and carbohydrates that go into the systemic capillaries and fats that go into the central lacteals.
The large intestine, cecum, the colon, and rectum, this is where the absorption of water and salts take place. And so salts, water, and fat, soluble vitamins, and then the rectum. anus and we're out. So if we then take a look at the gut tube and know the foregut, midgut, hindgut in the individual organs that make up those regions, it's easier to understand the arterial supply, the venous drainage, and the innervation.
And that, my friends, is the digestive system in a nutshell.