Surface Anatomy of the Small Intestine

00:01 So now let's move on to the small intestine. The small intestine occupies a large part of the abdomen and it is the natural continuation from the stomach. We really have 3 parts to the small intestine. We can see here that it's surrounded by the large intestine we'll come to later. But there's really the duodenum, which is a direct continuation of this tube from the stomach. Leads on to the jejunum and they're quite distinctive, the transition between the duodenum and the jejunum. Whereas the transition from the jejunum to the ileum is much more discrete and it's harder to actually visualize. The ileum then passes into the large intestine. Here, we can see the distal end of the pyloric canal and that's really important regulatory structure that can eliminate and control the amount of ingested food that's passing from the stomach into this region. The termination of the small intestines is the ileocecal junction where the ileum joins the cecum which is the beginning of the large intestine. The small intestine can also vary in length from person to person, but on average it's around 6-7 meters long and this incredible length really does optimize this organ for absorbing all of the nutrients that have been ingested by rough food, so the incredibly long and coiled up structure. The main body of the small intestines, like I said, is that mass length for 6-7 meters.

01:24 Let's have a look at how the small intestine can project itself on to the surface of anatomy and where it lies. So here we've added the thoracic cavity. You can see here we've got the stomach that's sitting up a little bit under the ribs, but within the abdomen underneath the diaphragm. And you can see how the stomach gives rise to the first part of the duodenum at this transpyloric plane. It then descends next to the lumbar vertebrae between L2 and L3, you have that descending portion of the duodenum. Remember there are 4 parts of the duodenum. We just saw the first part coming away from the pyloric region of the stomach. It's the superior bit. Leads on to this descending portion that runs down alongside the 2nd and 3rd lumbar vertebrae. It then runs horizontally across the lumbar vertebrae. Its 3rd part are really around L3. So from the descending part L2 to L3, it then curves towards the left where it then is the horizontal portion that runs across the lumbar vertebrae at L3. There's then a very short portion which is sensed back up to L2 and that is the ascending 4th part of the duodenum. So there's 4 parts of the duodenum there and they form this C-shaped structure.

02:42 This C-shaped structure of the duodenum then leads on to the jejunum and then on to the ileum. And as you can see, the small intestine very much sits in all of the 4 quadrants. You can see that it's sitting up in the left quadrant here and the right quadrant. And what is really depicting there is the upper left to lower right root of the small intestine. So the actual small intestine is noticed floating within the abdomen, but it's rooted to the posterior abdominal wall by the mesentery, a double layer with peritoneum to come to later on. And that anchors the small intestine, it holds it in position. And that is very much coming from the upper left quadrant down to the lower right quadrant. So going really from the duodenojejunal junction all the way down to the ileocecal junction on the upper left to the lower right quadrant. So now let's look at the small intestines and its relationship to the 9 regions that we described previously. Essentially, the small intestine occupies a large portion of the abdomen and it can, to a certain extent, be found in all of those 9 regions. If anything, it may not be present in the upper right and left hypochondriac regions, but pretty much it's not uncommon for it to creep up into those regions. During the lifetime, the abdomen moves and churns a lot, and a lot of the anatomy you see in the textbooks is not always what you will find if you were to have a look into a cadaver or into a living patient. It is very malleable in its movement across the abdominal cavity.