You should know about the internal structure of the lungs. The windpipe (trachea) branches into two bronchi (one to each lung). Cartilage rings in the walls of the windpipe help to keep it open however much you twist your neck. The bronchi split into smaller and smaller tubes called bronchioles. The bronchioles end in microscopic air sacs (alveoli). These air sacs are lined with mucus and are surrounded by a network of blood capillaries.