Lungs in a wireframe body, computer artwork. The windpipe (trachea, grey) runs downwards from the neck. It branches (upper centre) into two bronchi, one for each of the two lungs. Each bronchus branches to form bronchioles, which themselves branch to form smaller bronchioles. This repeated branching forms numerous airways in each lung. The airways terminate in tiny air sacs called alveoli (not visible). The alveoli provide a large surface area for gaseous exchange. In the alveoli, oxygen and carbon dioxide are exchanged between the blood and the lungs. The lungs expand and contract with each breath. The shapes that they form in the chest are shown (dark blue).