Final answer:
Serous fluid is normally found between the visceral and parietal layers of the pleura, serving as a lubricant during respiration.
Step-by-step explanation:
Normally, between the visceral and parietal layers of the pleura, you will find serous fluid. This fluid acts as a lubricant, reducing friction between the two layers as the lungs contract and relax during breathing. The pleurae are serous membranes; the visceral pleura covers the surface of the lungs and extends into the lung fissures, while the parietal pleura lines the thoracic wall, the mediastinum, and the diaphragm. This configuration allows the lungs to move smoothly within the thoracic cavity during respiration. When these layers become inflamed, a condition known as pleurisy can occur, which is painful due to increased pressure and decreased lung volume.