Describe the pleural mesothelium.
The pleural mesothelium is an embryonic mesoderm-derived layer of cells which surrounds the lungs. There are two layers of pleural mesothelium. The most common illustration to explain the layers is that of a fist pushing into a balloon. As the fist presses inward, one layer of the balloon is directly against it. This is the visceral layer. The opposite side of the balloon, which mimics the shape of the inner layer is known as the parietal layer. Although the fist is surrounded by the balloon, it is not inside the balloon, or pleural cavity. The pleural cavity is classified as a serous cavity. The pleural cavities permit the lungs to contract and expand with minimal friction. The pleural cavity typically contains a small amount of air and fluid (approximately 10cc of fluid).