 Another organizing principle in GI anatomy is understanding which structures are inside the peritoneal cavity and which structures are in the retroperitoneal space. The peritoneum is a layer of tissue that lines the abdominal wall and some of the intra-abdominal organs. It forms a cavity within the abdomen inside which some, but not all, intra-abdominal organs are contained. So it's our job to know which structures are inside versus outside the peritoneal cavity. Just like upper versus lower GI tract, there's a clinical significance to whether structures are located inside or outside the peritoneum. We typically discuss structures as being either intra-peritoneal or retro-peritoneal, primarily because if a structure in one of these locations were to bleed or be injured, we know that free fluid or air would be present either within or behind the peritoneal cavity. I find the best approach is to memorize which structures are retro-peritoneal, so that way anything that's not on your list of retro-peritoneal structures must be intra-peritoneal. Now take the second to get oriented to our diagram here. If you're familiar with cross-sectional imaging, this may seem a bit unusual. Just note that the dorsal side of the patient is on top here and the ventral side is facing down. The peritoneum is visualized here as all of this blue tissue than the abdominal cavity. Now there's a very well-known mnemonic that will aid in your memorization of the retro-peritoneal structures. We didn't come up with it, but you'll see it in all sorts of review resources, and that mnemonic is sad pucker. The S standing for suprarino glands, which is another name for the adrenal glands. The A is for aorta and naturally also the IVC, which is located in close proximity. And remember any artery that branches off the aorta, for example the renal arteries or common iliac are also in the retro-peritoneal space. D is for duodenum. It's important to note that the first portion of the duodenum is intra-peritoneal, while the second through fourth portions are retro-peritoneal. P is for pancreas. U is for ureters, which carry urine from the kidneys to the bladder. C is for colon. Importantly, only the ascending and descending parts of the colon are outside the peritoneum. The transverse and sigmoid colon are intra-peritoneal. K is for kidneys. E is for esophagus. And R is for rectum. Now remember in this mnemonic, sad pucker is singular, not plural. This is important because if you make the mistake that I did on my GA exam in med school and assume there's an S at the end of sad pucker, as you might think that the spleen or stomach is retro-peritoneal. But remember there's only one S in sad pucker. You'll notice we left out a few of the pelvic organs. There's a few including the bladder and the uterus, which are actually sub-peritoneal structures, or below the peritoneal cavity. But anything else we didn't just discuss, for example the spleen, stomach, liver, gallbladder, etc., are all intra-peritoneal. You often get questions about a collection of blood or air in the peritoneum or in the retro-peritoneal space, which then asks you to pick out an organ which may have been responsible. Just keep in mind you're sad pucker mnemonic and you won't have any trouble answering these questions. This video has been a review of the organization of the GI tract and peritoneum. These are important foundational concepts that you'll need to understand the rest of the GI anatomy and get all the questions right. If you have any questions or anything was unclear, please make sure you let us know in the comments below. We look forward to seeing you next time.