What is the name of the double-layered sac that encases the lungs?

The double-layered sac that encases the lungs is known as the pleura. This structure consists of two main layers: the visceral pleura, which directly covers the lungs, and the parietal pleura, which lines the chest wall and the mediastinum. The space between these two layers is called the pleural cavity, which contains pleural fluid that reduces friction during respiratory movements and helps maintain surface tension.

Understanding the pleura is important in the context of respiratory physiology and pathology. It plays a critical role in facilitating lung expansion and contraction. Any issues affecting the pleura, such as inflammation or fluid accumulation (pleural effusion), can significantly impact breathing efficiency and overall lung function.

The other terms listed have distinct meanings. The alveolus refers to the tiny air sacs in the lungs where gas exchange occurs. The hilum is the area on the lung where blood vessels and bronchi enter and exit. The diaphragm is a muscle that separates the thoracic cavity from the abdominal cavity and plays a crucial role in breathing mechanics. Each of these structures is important in respiratory anatomy, but the pleura specifically is the correct term for the double-layered sac encasing the lungs.