The mammalian lung is made air tight by the pleural membrane. The pleural membrane is a thin, double-layered membrane that covers the lungs and lines the inside of the chest cavity. It creates a sealed, air-tight space around the lungs called the pleural cavity, which helps to maintain the pressure gradient necessary for breathing. The pleural membrane also produces a lubricating fluid that reduces friction and allows the lungs to expand and contract smoothly during breathing. Therefore, it is the pleural membrane that makes the mammalian lung air tight.