What holds the visceral and parietal membranes together?
Surface Tension is technically what hods the membranes together. Not sure how technical you are going.
The pleural membrane is a double-layered serous membrane found in the thoracic cavity. The outer layer is called the parietal pleura and lines the thoracic wall, and the inner layer is called the visceral pleura and covers the lungs. The membranes produce lubricating serous fluid that causes them to adhere closely to one another. This holds the lungs to the thoracic wall allowing easier movement during breathing.