Welcome. In the early embryo, there is a cavity, the intraembryonic coelomic cavity. This cavity develops into cavities around the heart, the lungs, and the intestines. Respectively the pericardial, pleural, and peritoneal cavities. These cavities, in turn, are lined by membranes, respectively the pericardium, the pleura, and the peritoneum. Collectively they are known as serous membranes. Serous membranes are slippery membranes that line the inside of the body cavities, and cover the organs that are embedded in these cavities. Hence the heart, the lungs, and the intestines are all lined by these serous membranes. The serous membranes allow the ensheathed organs to move in relation to their surroundings while they also partially fix the position of the organs. The serous membrane of the gut is the peritoneum. In this video we'll explain the general relation between the serous membranes and the organs that they surround, especially the relation between the peritoneum and the gut. The relation of the serous membranes to the ensheathed organs Is traditionally explained via a balloon model. Even though it's probably not 100% correct from a biological development point of view, it provides a conceptually easy to understand model which is why we'll demonstrate this balloon model here. Imagine we originally have a completely empty abdominal cavity. The spleen is still in place in this model, because we can't remove it here, but you should imagine it's also gone. Now I'm going to introduce the peritoneum. This balloon stands for the peritoneum. Inside the balloon stands for the peritoneal cavity. Imagine that the balloon, that's the peritoneum, is completely blown up, and I'll place it in the abdomen, and thus it fills the complete abdomen from inside, and thus the peritoneum lines the inside of the body wall. Now I'll introduce the gut. This white tube stands for the gut, the gut is in fact behind the peritoneum in the abdomen. Now I'm going to take both the gut and the peritoneum out to demonstrate to you what happens. And I'm going to turn it a little bit. The gut gets, over time, embryologically, gets pushed into the peritoneum, and in that way there is a layer of peritoneum that comes to lay adjacent to the gut. This layer, if you might be able to see here, that's the so called visceral layer, named after the word viscus, that means intestine. Now remember that this peritoneum was also in the abdomen. So we also had a layer on the outside, adjacent to the body wall. That's called the parietal layer, called from the Latin word paries which means wall. Now, in fact, these two layers were lying adjacent to each other because there is no air in the peritoneum, in fact, like there is in this balloon. But there's only a slightly bit of slippery fluid and the balloon is completely filled with convoluted and long intestines. These intestines have pushed into the balloon like this and because, it's completely filled with these intestines, the layer that lies on the intestine, the visceral layer, lies directly adjacent to the layer that's on the outside, the parietal layer, and with only this little bit of slippery fluid in between, these layers can move in relation to each other. And that means that this organ, in this case the gut, can move in relation to its surroundings. And that's the great thing of this configuration, that it allows organs to move in relation to its surroundings. And that's so far about this model. So far in this model, we can schematically draw the balloon model with this cross section. Once again we see the gut, the visceral and the parietal peritoneum. And the gut was pushed into balloon in this direction. Imagine the gut now being pushed even deeper into the balloon such that the sides of the balloon collided again behind the gut. Thus forming a double layer, such a double layer is called a mesentery. By definition, a mesentery is double layer of peritoneum that arouse by invagination of an organ, and that connects the organ with the body wall, and that gives pathway for neurovascular structures from the body wall to the organ. The great thing about this construction is that the gut can move freely, but at the same time there are limits to it's freedom because it's fixed to the back wall, so the chances of getting entangled are slim. The second great thing is that the blood vessels, the lymphatic vessels, and the nerves can reach the gut, as they have a pathway through the mesentery. Summarizing, we discussed that the heart, the lungs, and the intestines are surrounded by the pericardial, pleural, and peritoneal cavities, and these cavities may be viewed as a balloon, into which the heart, the lungs, and the intestine have been pushed from the outside, and the balloon material representing respectively the pericardium, the pleura, and the peritoneum, those get adhered on one side to the organs and on the other side to the body wall. And the sides can slide over each other, this enabling movement of the embedded organs.