Biomedical Engineering Reference
In-Depth Information
FIGURE 9-5
Scanning electron
microscope view
of lung alveoli,
magnified ×750.
(a) Full capacity.
(b) After normal
expiration. (Murray
and Nadel 2008)
copyright W. B.
Saunders (Elsevier)
reproduced with
permission
The barrier to diffusion has three components. On the inside surface of the alveoli is
a thin layer of secretion about 0.15
μ
m thick, containing a phospholipid surfactant that
lowers the surface tension of the lungs and allows them to be stretched by relatively low
pressure. Then there is the epithelial cell layer of the alveoli and finally the capillary en-
dothelium. Cells of a different type, the alveolar macrophages, are found within the cavities
of the alveoli; their function is to ingest and remove solid particles, such as those of smoke.
9.3
THE MECHANICS OF RESPIRATION
Air flows from a region of higher to lower pressure by bulk flow. When total gas pressure
in the alveoli,
P
A
(Pa), is the same as the atmospheric pressure
P
B
(Pa), no air flow occurs
because no pressure gradient exists. To initiate air flow into gas exchange sites of the lung,
P
A
must decrease below
P
B
,or
P
B
must increase above
P
A
, as occurs during mechanical
ventilation.
During respiratory muscle contraction, the volume of the thoracic cavity and lungs
increases, which decreases
P
A
. Air then moves into the alveoli by bulk flow until the pres-
sure equalizes at the end of inspiration. The lung is expanded elastically during inspiration,
so when the respiratory muscles relax it recoils to compress the alveolar gas volume. This
elevates
P
A
above
P
B
, so air is expelled until equilibrium is again achieved at the end of
expiration.
When the respiratory muscles contract, several opposing forces must be overcome to
bring about enlargement of the lungs. These are related to the physical characteristics of
the lungs, friction resistance of moving air, and inertia of both the lungs and the air. Nerve
impulses initiating contraction of the respiratory muscles originate in the medulla of the
brain stem. The force generated by the respiratory muscles can be increased by increasing
the frequency of discharge in individual motor units, activating additional motor units,
or calling on the accessory muscles of respiration. These are muscles not normally used
during normal breathing.
During inspiration, contraction of the diaphragm and external intercostal muscles
enlarges the chest cavity, which leads to an expansion and stretching of the lung. As the
lung inflates, potential energy is stored in the stretched elastic structures of the lung so
that with relaxation of the respiratory muscles expiration occurs from elastic recoil of the
lung-chest cage apparatus and no muscle contraction is required.
During contraction, the central portion of the diaphragm becomes flattened by about
1.5 cm during a normal breath and up to 10 cm for a deep breath. This increases the volume
