The mature respiratory tract is composed of the nasal
cavity, nasopharynx, larynx, the tubular conducting
airways including trachea, intra- and extrapulmonary
bronchi, bronchioles, and the gas exchange system
including terminal bronchioles and alveoli where gas
exchange occurs. The lungs and chest cavity are covered
in a thin, translucent pleural membrane.
There are two separate blood-conducting systems in
the lungs. The pulmonary artery system supplies venous
blood from the right ventricle to the capillary plexus
surrounding the pulmonary alveoli. The intimate association of this large vasculature network and the large
airspace of the alveoli can serve as a portal of entry for
pathogens into the body. The supporting structures
around the trachea, bronchi, and bronchioles are vascularized
with blood from the bronchial tree.