Exploring Anatomy: the Human Abdomen

Week 2 core lecture transcript: The gastrointestinal tract part 2

So now, let's move from the duodenum and pass into the jejunum. The jejunum starts at the duodenal‐jejunal flexure. And the jejunum then travels up, from the upper left quadrant down to the lower right quadrant. So it passes diagonally down the abdomen-- or, at least, its mesentery does. That suspends it.

The jejunum is continuous with the ileum. There's no clear demarcation that separates the jejunum from the ileum. There's various features that enable you to differentiate the jejunum from the ileum, but there's no clear bend or kink like the duodenal-jejunal flexure. The jejunum, for example, is redder. It has a greater vasculature than the ileum. The blood vessels for the jejunum assume a specific arrangement, compared to the ileum. The calibre of the jejunum is that much greater. Its diameter is wider. Its walls are thicker. So, although there's no clear demarcation between jejunum and ileum, there's a gradation of change, so that, by the time you get to the distal ileum, it looks very different to the proximal jejunum. The small intestines are important in absorbing the nutrients from the ingested food, and, because of this, their internal surface is thrown into a whole series of elevations, called plicae circulares. And what these do, these undulations increase the surface area. They increase the surface area, so there's more opportunity for the ingested food to come into contact with the absorptive surface. The small intestines help to absorb the food. The small intestines are, then, continuous with the large intestine. The large intestine, or the colon and the caecum, pass from this lower right quadrant to the upper right quadrant. It then passes across, to the upper left quadrant, before it descends to the lower left quadrant. It then passes towards the midline, where it's continuous with the rectum. So it takes this inverted-U path. The ileum passes into the large intestines, as we can see, as the ileocaecal junction. So the beginning of the large intestines is this little sac-like pouch, which is your caecum. And passing from the ileum to the caecum, we have the remainder of the ingested food. Once all the nutrients have been taken out, we have the dregs of it passing into the caecum. Also, coming away from the caecum, is a small, blind-ended little pouch. This is known as your appendix. The appendix doesn't really do a great deal in humans. It can store some lymph and this is important in the lymphatic system. But it doesn't have a great deal of digestive function in the human.

The caecum is then continuous, superiorly, with the ascending colon. The caecum and the ascending colon are retroperitoneal organs. So they're adhered to the posterior abdominal wall, with a layer of peritoneum lying over them. As the ascending colon ascends toward the liver, as it passes from the lower right to the upper right, it then takes a sharp turn to the left, and the ascending colon then becomes the transverse colon. That turn, from ascending colon to the transverse colon, by the liver, is known as the hepatic flexure. The transverse colon then travels, transversely, across the abdomen, towards the spleen-- spleen, up in its upper left quadrant-- it heads toward the spleen, and then it descends. So we can see it running across the abdominal cavity, heading toward the spleen before, then, descending, as the descending colon. The descending colon, again-- retroperitoneal. The descending colon then becomes the sigmoid colon. And the sigmoid colon passes into the pelvis, where it's continuous with the rectum. And the rectum is, then, continuous with the anal canal. And then you have your anal opening. The caecum, the ascending colon, and the descending colon, as I've said, are retroperitoneal. The transverse colon and the sigmoid colon have a mesentery. So they're suspended from the posterior abdominal wall. We have the transverse mesocolon and the sigmoid mesocolon. These are mesenteries suspending either the transverse colon or the sigmoid colon.

At the beginning, I mentioned the separation between the foregut and the midgut. And now, we need to detail the separation between the midgut and the hindgut. There is no specific landmark. Previously, I mentioned the major duodenal papilla separated the foregut from the midgut, but approximately 2/3 of the way along the transverse colon is midgut, and then the final third of the transverse colon is hindgut. The hindgut, then, continues as the descending colon, the sigmoid colon and the rectum.

So, just somewhere 2/3 of the way along the transverse colon, is where the midgut stops and the hindgut starts. So finally, you've opened up the abdomen, and you could see the greater omentum. You've removed the greater momentum, and you could see the small intestines. How can you differentiate the small intestines from the large intestine? Well, you can do this three ways. On the external surface of the large intestine, there are three characteristics. There's the taeniae coli, there's appendices epiploicae, and there's haustra. So what are they? The taeniae coli are thin, narrow bands of longitudinal muscle. And, as we can see, they originate at the appendix. And we have a thin band that doesn't cover the whole colon-- just a thin band that runs along the colon. We have three bands of these-- three bands of longitudinal muscle. The second feature are appendices epiploicae. These are just fatty tags. So you have peritoneum lining over the colon, and then they're just fatty, peritoneal-lined tags. But you only find them on the large intestine. You don't find them in the small intestine. The final structure, to differentiate a piece of large intestine from a piece of small intestine, are the haustra. You can see that the whole of the large intestine is sacculated. It has these sacculations. Why is that? That's because the taeniae coli-- these pieces of longitudinal muscle-- are shorter than the length of the large intestine. So what they've done is, they've contracted it into these little sacs. Continued contraction of these help to shuffle the faeces around the large intestine.

So, taeniae coli, appendices epiploicae, and the sacculations-- how to differentiate a piece of large intestine from a piece of small intestine. And there's the gastrointestinal tract. From the oesophagus and the stomach to the duodenum, jejunum, ileum, and finally to the large intestines. [end of transcript]