The respiratory system allows humans to breathe and obtain oxygen while removing carbon dioxide. It includes the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and lungs. The nasal cavity and mouth allow air to enter, which then passes through the pharynx and larynx before entering the trachea and branching bronchi that lead to the lungs. Within the lungs are clusters of tiny alveoli sacs where the exchange of oxygen and carbon dioxide occurs through thin walls, allowing oxygen to enter the bloodstream and carbon dioxide to be removed.