In humans the respiratory tract is the part of the anatomy that has to do with the process of respiration.
The respiratory tract is divided into 3 segments:
The respiratory tract is a common site for infections. Upper respiratory tract infections are probably the most common infections in the world.
Most of the respiratory tract exists merely as a piping system for air to travel in the lungs; alveoli are the only part of the lung that exchanges oxygen and carbon dioxide with the blood.
Moving down the respiratory tract starting at the trachea, the tubes get smaller and divide more and more. There are estimated to be about 20 to 23 divisions, ending up at an alveolus.
Even though the cross-sectional area of each bronchus or bronchiole is smaller, because there are so many, the total surface area is larger. This means there is less resistance at the terminal bronchioles. (Most resistance is around the 3-4 division from the trachea due to turbulence.)