The oxygen exchange in the lungs takes place across the membranes of small balloon-like structures called alveoli attached to the branches of the bronchial passages. These alveoli inflate and deflate with inhalation and exhalation. The behavior of the alveoli is largely dictated by LaPlace's law and surface tension. It takes some effort to breathe in because these tiny balloons must be inflated, but the elastic recoil of the tiny balloons assists us in the process of exhalation. If the elastic recoil of the alveoli is compromised, as in the case of emphysema, then it is difficult to exhale forcibly.