Digestive System Basics - Duodenum to Anus

Author: Dr Peter de Souza
Last modified: 14 December 2021


After the partially digested food known as chyme exits the stomach, it enters into the small intestine. The small intestine extends from the pylorus of the stomach all the way to the cecum, which is this little pouch at the beginning of the large intestine.   It’s this little bit here.

I'll just isolate the small intestine by fading away the large intestine. You can see how it's this long, convoluted tube. The small intestine consists of three parts. The first part is known as the duodenum. It's this short little c-shaped part here. And then you've got the second part called the jejunum and the distal part is called the ileum.

It's not really important to clearly mark out the jejunum and the ileum, but you just need to know that the jejunum makes up the second part of the small intestine and the ileum makes up the distal part, so the third part of the small intestine. The jejunum roughly lies in the left upper quadrant and the ileum roughly lies in the right lower quadrant and it makes up the distal three-fifths, whereas the jejunum makes up the proximal two-fifths.

But the duodenum is this first part. It's this c-shaped part which emerges at the end of the stomach, at the pylorus of the stomach. It curves around the head of the pancreas.

The pancreas is one of these three accessory organs of digestion that I mentioned in the first part of this tutorial. This organ sits behind the stomach as you can see here.   If I just fade away the other organs, you can see the pancreas. You can see how it sits in this c-shaped part of the duodenum.   The head of the pancreas nests itself into this curve of the duodenum.

You've got ducts from the pancreas and ducts from the gallbladder which open up into the duodenum. This allows bile from the gallbladder and pancreatic secretions from the pancreas to enter into the duodenum and this helps in digestion.

The gallbladder is this little structure here, which sits underneath the liver (if you remember that triangular-shaped liver which sat on the right side and covered a bit of the stomach).

The duodenum ends at about this point where it then becomes the jejunum. The jejunum forms the proximal two-fifths of the small intestine and then the distal, the last three-fifths are called the ileum.

As you can see, the small intestine is highly convoluted. There's all these folds and it's really long and folded. This really serves to increase the surface area. By having a larger surface area, there's much more possibility for absorption.

The small intestine is a major site for chemical digestion and absorption. And as I showed you just now, the duodenum receives bile and pancreatic secretions to help with this chemical digestion.

As I told you before, the small intestine starts at the pylorus of the stomach and it ends at the cecum. The cecum is this little pouch and it marks the beginning of the large intestine.

The large intestine lies around the edges of the abdominal cavity. We actually can see it in relation to the skin.

The function of the large intestine is to absorb water.   First, you've got this cecum, which I showed you. It's this little pouch here. This is where the ileum joins the large intestine. At this junction between the end of the ileum and the cecum, you've got the ileocecal valve.

And just below, you can see this little worm-like structure. This is called the vermiform appendix. 'Vermiform' in Latin just means 'worm-shaped'. It kind of looks like a little earthworm.   This is the appendix.

And then the colon is separated into four parts. You've got this ascending colon, which is this first part on the right, which ascends. And then you've got transverse colon, which runs horizontally, so it's called 'transverse' colon. And then you've got the descending colon, which descends down into the last part called the sigmoid colon, which is this curved part of the colon.

The sigmoid colon gets its name because it's kind of s-shaped. The greek symbol, sigma is the letter s. It's kind of s-shaped, so that's the sigmoid colon.

Where the ascending colon meets the transverse colon, there's this kind of bend. This bend is called flexures. The liver sits over this right side. This bend is given the name the hepatic flexure (hepatic referring to the liver).

And on the left side, you've got this other bend where the transverse colon meets the descending colon. This is called the splenic flexure because this is where the spleen lies on the left side.

You can see that here with the liver and the spleen, so they're the hepatic flexure and the splenic flexure.

After the colon, after the sigmoid colon, you've got the rectum. And then you've got the anal canal and the anus right at the end.

The rectum serves as a temporary holding site for feces. And then the feces is expelled out through the rectum into the anal canal. And then it exits the body through the anus.   Intra-abdominal pressure is increased to force out the feces from the rectum and into the anal canal and finally out of the anus.   The anus is the opposite end of the mouth. It's the exit of the digestive tract.