The function of the respiratory system is to exchange O2 and CO2 with the blood. To understand this system from a bioengineering viewpoint, we will first discuss the gross anatomy of the lungs and their associated structures, and then discuss the mechanics of breathing.
We divide the respiratory system into two subsystems: the conducting airways and the associated structures.
The conducting airways and pulmonary vasculature
The conducting airways form a fantastically complex branching tree designed to transport air efficiently into the alveoli, the smallest air-filled structures in the lung where blood/gas exchange takes place. Air enters through the mouth or nose then passes through (in order): the pharynx (the throat), the larynx (the voice box), and the trachea (the large tube passing down the neck). The trachea splits to form two bronchi (singular: bronchus), each of which feed air to one of the lungs (Fig. 7.1, color plate).
Each bronchus splits to form bronchioles, which, in turn, split to form smaller bronchioles, and so on (Fig. 7.2). After about 16 levels of branching, we reach the terminal bronchioles, which are the smallest structures that have a purely air-conducting function, that is, in which essentially no blood/gas exchange takes place (Fig. 7.3). In adult lungs, the structures distal to the terminal bronchioles consist of several generations of respiratory bronchioles, alveolar ducts and alveolar sacs, which collectively are known as the acinus; this is where the gas exchange occurs (Fig. 7.4).