What allows the lungs to inflate and deflate without friction?

The lungs inflate and deflate without friction due to the presence of a slippery fluid called pleural fluid, which is found in the pleural cavity between the lung surface and the chest wall. This fluid reduces surface tension and allows the lungs to glide smoothly against the thoracic cavity during breathing. Additionally, the elastic properties of lung tissue and the negative pressure in the pleural space facilitate efficient lung expansion and contraction.