Inferior view of a transverse section of the thorax, highlighting the lungs. The trachea, cardiac impression, and transverse sections of the thoracic aorta and esophagus are visible, as is the bronchial tree. When air is inhaled into the lungs, it flows through large tubes called bronchi, then branches into smaller tubes known as bronchioles, and ends up in the alveoli, small pouches in the respiratory system where oxygen is transferred from the air to the bloodstream.