The airways comprises the conducting zone and the
Trachea, bronchi, bronchioles and terminal bronchioles.
These airways contain no alveoli and take no part and
thus constitute the anatomic dead space.
Comprises the respiratory bronchioles which have alveoli
budding from their walls, the alveolar ducts which are
completely lined with alveoli. This zone is also called the
acinus which is the functional unit of the lung.
The distance from the terminal bronchiole to the most distal
alveolus is only a few mm but this zone makes up the most
of the lung. Its volume is about 2.5-3 litres during rest.
During inspiration, the thoracic cavity is increased due
to contraction of the diaphragm and action of the
intercostal muscles to raise the ribs upwards and
Airflow in the conducting zone is by bulk flow.
However diffusion takes over in the respiratory zone
due to increased cross-sectional are brought about by
increased branching. This makes the forward velocity
Elasticity of the lung enables it to return to its pre-
B. STABILITY OF THE ALVEOLI
Relatively large forces develop on the alveoli that lead to its
collapse due to surface tension of the liquid lining the
Instability is thus an expected consequence with the high
number of alveoli (about 500 x 10^6).
Stability is however brought about by surfactant produced
by the type II alveolar epithelial cells. Mechanism is by
reducing surface tension.
The lung presents a large surface of exposure due to its
large surface area and its direct communication with
the external environment.
PARTICLE SIZE FILTRATION SITE
Small Mucociliary apparatus
Very small Type I alveolar cells.
Surface area of the blood-gas barrier (BGB) is 50-100 sq
metres. This is obtained by wrapping of the capillaries
around the enormous number of alveoli. There are 500
x 10^6 alveoli each with almost 1/3 mm in diameter.
BGB is extremely thin <2 micron.
Initially; the arteries, veins and bronchi run close
together but towards the periphery of the lung, the
veins move away to pass between the lobules whereas
the arteries and bronchi travel together down to the
centres of the lobules.
The capillaries form a dense network on the walls of
the alveoli. The diameter is around 7-10 microns, just
large enough to accommodate a red blood cell.
The lengths of the segments are so short that the
dense network forms an almost continuous sheet of
blood in the alveolar wall, a very efficient arrangement
for gaseous exchange.
The thin BGB predisposes the capillaries to damage as in
increased capillary pressure or increased lung volume
during excessive inflation. This leads to leakage of plasma
and red blood cells (rbcs) into the alveolar spaces.
Each rbc spends 0.75 seconds in the capillary network and
during this time it traverses 2-3 alveoli. So efficient is the
anatomy of gas exchange between the alveolar gas and
capillary blood that this brief time is sufficient for virtually
complete equilibration of O2 and CO2.
The pulmonary artery receives the whole output of the
heart but the resistance is small. It thus has a mean
pressure of almost 20 cm H2O (15mmHg) required for
a flow of 6 litres/min.
Bronchial circulation is an additional blood system.
The bronchial arteries are derived from the aorta and
the intercostal arteries. They mainly supply the
conducting zones as the respiratory zone is supplied
by the pulmonary artery.