 Hey everybody, Dr. O'Neill here. This video we're going to talk about your body cavities. So you'll see here that we'll talk about the dorsal and ventral body cavities, but we're definitely going to focus on your ventral body cavities. There's actually some discussion or argument about whether or not the dorsal body cavities technically exist. So I'm just going to mention them, then we'll spend all of our time on the ventral body cavities. So ventral means towards the front, towards the belly, really dorsal means then towards the back. So your dorsal body cavities, if they exist, are the cranial cavity where their brain is and the vertebral cavity where your spinal cord is. So let's just leave that there. So let's talk about the ventral body cavities. So your ventral body cavities, these are going to be double membrane cavities lined with a cirrus fluid, a very watery fluid, and the three we have are going to be the pericardial cavity, which lines the heart, the pleural cavity, which lines each lung. You see these as purple here, and then we have the peritoneal cavity, which lines most of your abdominal public organs. So later on we'll learn that some of our organs are retroperitoneal, which means they're behind the peritoneal cavity. The pancreas would be a good example, but we'll cover that when we get to those organs. So first let's, there's a few more terms here, and then I'll actually talk about this double membrane system, so I'll make sure we cover all the terms here. So first of all, the diaphragm. So the diaphragm is your primary muscle of respiration, but the reason it's important here is the diaphragm separates your thoracic cavity, where your heart and lungs are, or your pericardial and pleural membranes are, cavities are, and it separates them from the abdominal pelvic cavity. So sometimes you can just call it the abdominal pelvic cavity, but as you can see here, it can be separated into the abdominal cavity and the pelvic cavity. So as we learn individual organs, we'll talk about where they are. So the diaphragm is important. Another term here that will come up again, especially when we're talking about the heart, will be the medius thymium. So yes, your pleural cavity or where your two lungs are, the pericardial cavity is where your heart is, but the medius thymium is the entire space between your two lungs. So the heart's in there, but so are the great vessels like the aorta that come off the heart, your thymus gland would be there, and other structures as well. So remember the diaphragm separates the thoracic cavity from the abdominal pelvic cavity. Remember the medius thymium is the space between your two lungs. So let's, with these ventral body cavities, each of them is going to have two layers. So here's an example. This is the pericardial cavity. There's going to be a visceral layer and a parietal layer. So viscera means organs. This is going to be the layer that's on the organ itself. Like you see here, the visceral pericardium. Parietal means body walls. This is going to be the portion of the cavity that lines the cavity or the body wall that the organ is in. Reason this is so important, you always see this picture of someone's fist in a balloon. The reason that this is so important is that space between these two cavities is full of a very watery cirrus fluid. And the reason that's important is because it reduces friction. When your organs move, you want them gently gliding with the cirrus fluid. You don't want there to be friction every time that these structures move. So a good example would be like pleurisy. If someone gets pleurisy, there's some irritation and friction every time you breathe, and that can be very painful. So we certainly don't want those. We don't want that. So this here would be the visceral and parietal pericardium. You'd see the same thing with the visceral and parietal peritoneum, which would be in your abdominal pelvic organs, least most of them. Then you'd see the same thing with your visceral and parietal pleura around your lungs. All right, so those are your ventral body cavities. Have a wonderful day. Be blessed.