The Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 35

in the bronchioles and foreign particles are removed by macrophages located in
the alveoli instead of mucociliary action;

• The amount of smooth muscle in the tube walls increases as the passageways

become smaller.

The passageway from the trachea, bifurcating into the right and left main bronchi,
which further divide into lobar, then segmental bronchi and continues this bifurcating
process down to the terminal bronchioles (which are the smallest airways without
alveoli) is called the conducting airways (Fig. 2.14 ). In this area, gas exchange does
not take place because no alveoli are present, and it contributes to the anatomic dead
space which takes up approximately 150 mL in volume.

Fig. 2.14 Schematic of the
airway generations in the
human adult lung. On
average, a total of
21-25 generations are found
between the trachea and the
alveoli. (Redrawn from
Weibel 1963)

The terminal bronchioles that divide into respiratory bronchioles are also called
transitional bronchioles as they have occasional alveoli present at the walls. The
respiratory bronchioles further divide into alveolar ducts which are completely lined
with alveoli. This region is the acinus (meaning berry in Latin) region because of the
cluster of cells that resemble a knobby berry, like a raspberry. It includes all parts dis-
tal to a single terminal bronchiole and on average is beyond the sixteenth generation
(Haefeli-Bleuer and Weibel 1988). The acinus is therefore comprised of respira-
tory airways and forms the functional tissue of the lung, or, the lung parenchyma.
It extends only a few millimetres for about eight generations with the first three