Path 3000 lecture 15 february 12, 2020 respiratory disease i. Infections: upper respiratory tract infections, lower infections (pneumonia) The respiratory system starts at the nose and the paranasal sinuses at the back of the nose. The oropharynx is at the back of the nose and goes down to the larynx (voice box), making the upper respiratory tract. The middle respiratory tract is the trachea, mainstem bronchi (left and right bronchi). Everything below that is the lower respiratory tract. It is important to divide the respiratory system because different epithelium lines these structures and different disease affects different parts of the respiratory tract. The upper respiratory tract consists of the nose, paranasal sinuses, pharynx and larynx. The nose and paranasal sinuses are lined by respiratory epithelium. The pharynx and larynx (at the back of the throat) are lined by squamous epithelium. The alveoli is not lined by respiratory epithelium but are consist of the pneumocytes.