Schematic diagram of the pathways for normal pleural liquid turnover. In all mammals, pleural liquid is mainly a filtrate from capillaries in the parietal pleura lining the chest wall ( left ). Arrows indicate direction of flow. In large animals with thick visceral pleura, some pleural liquid is filtered from the bronchial capillaries of the visceral pleura lining the lung ( bottom right ). In small animals with thin visceral pleura, a small amount of pleural liquid might be reabsorbed by pulmonary capillaries ( top right ). Drainage of pleural liquid from the pleural space occurs via lymphatic stomata in the parietal pleura. The pleural space, visceral pleura, and alveoli are thicker in large animals than in small animals.