Physicists suggest how evolution has shaped our lungs through successive optimizations of physical parameters such as conservation of energy and speed of delivery. Our respiratory system consists of a bronchial tree designed to transport air through the lungs combined with an alveolar system designed to capture the oxygen. Both are subjected to different type of optimizations. Only tree-like structures, the paper shows, are able to efficiently feed organs above a small size, below which organs are solely fed by diffusion.