Biomedical Engineering Reference
In-Depth Information
Trachea
Left main bronchus
Bronchiole
Tributary of pulmonary vein
Lymphatic vessels
Pulmonary artery
Bronchiole
Bronchial artery
Conducting
Te rminal
Respiratory
Bronchiole
Te rtiary
Duct
Sac
Surface
of lung
Alveolus
Visceral pleura
Alveolar
Bronchial
arteries
Intrasegmental
Pulmonary
arteries
Bronchial
capillaries
Pulmonary
vein
Alveolar
capillary
plexus
Pulmonary vein
(intersegmental)
Te rminal
bronchiole
Respiratory
bronchiole
Alveolar
duct
Alveolar sac
(plexus
removed)
Septum
Alveolar
sac
Visceral pleura
bronchial
artery
Bronchopulmonary segment
(schematic)
FIGURE 11.2
Structural organization of lungs. (Reproduced from
Pediatric Critical Care
. 14 ed. D'Angelis
CA et al., Structure of the respiratory system. Lower respiratory tract. Copyright 2011, with permission from
Elsevier Saunders.)
The walls of the conducting airways are coated by an adhesive, viscoelastic mucus layer
(thickness: 5-55 μm), secreted by mucus cells. The major components of mucus are glycopro-
teins and water. This mucus fulfills important functions, including the protection of respiratory
epithelium from dehydration, the saturation of inhaled air with water in the mucus, the exertion
of antibacterial effects via mucus proteins and peptides such as defensins and lysozyme, and
the protection of the airways from inhaled xenobiotics or chemicals [20-22]. The “mucociliary
escalator” serves as an important protective mechanism for removing small, inhaled particles
from the lungs. The composition, thickness, viscosity, and clearance of mucus are often altered
in patients suffering from airway diseases. The majority of insoluble particles deposited in the
upper airways are eliminated by mucociliary clearance [23-25]. The most prominent defense
mechanism of the respiratory region is macrophage clearance. The particles deposited in the
deeper lung will be taken up by alveolar macrophages, which slowly migrate out of the lung,
either following the broncho-tracheal escalator or by migrating through the lymphatic system.
Particle clearance by macrophages appears most efficient for particles having a geometric size
between 1 and 3 μm [26].
The blood supply to the lung is provided by both pulmonary and systemic circulations. Both
circulations are anatomizing at the level of the junction between the conducting and respiratory
