Pleurisy, also known as fluid accumulation in the pleura and pleurisy, is also defined as pleural effusion in medical language. Health Each individual has two membrane layers called the parietal pleura on the outside of the lung and the visceral pleura on the inside of the chest wall. Between these membranes is a slippery, protein-poor fluid that allows the lungs to inflate and deflate easily during breathing. This fluid is secreted from the outer membrane of the lung and is absorbed through the inner membrane that covers the outside of the lung. Approximately 20 ml. This fluid, which remains constant at the level of the pleura, is in constant motion between the two membranes called the pleural cavity. Some lung conditions and systemic diseases cause increased fluid secretion between the two membranes or blocked absorption, resulting in an increase in the amount of fluid between the membranes.