Biomedical Engineering Reference
In-Depth Information
fifteenth generation, gas diffusion is relatively more important. With the low gas velocities
that occur in diffusion, dimensions of the space over which diffusion occurs (alveolar space)
must be small for adequate oxygen delivery into the walls; smaller alveoli are more efficient
in transfer of gas than are larger ones. Thus, animals with high levels of oxygen consump-
tion are found to have smaller-diameter alveoli compared with animals with low levels of
oxygen consumption.
Alveoli are the structures through which gases diffuse to and from the body. To ensure
that gas exchange occurs efficiently, alveolar walls are extremely thin. For example, the total
tissue thickness between the inside of the alveolus to pulmonary capillary blood plasma is
only about 0.4
10 6 m. Consequently, the principal barrier to diffusion occurs at the
plasma and red blood cell level, not at the alveolar membrane.
Molecular diffusion within the alveolar volume is responsible for mixing of the enclosed
gas. Due to small alveolar dimensions, complete mixing probably occurs in less than
10 ms, fast enough that alveolar mixing time does not limit gaseous diffusion to or from
the blood.
Of particular importance to proper alveolar operation is a thin surface coating of surfac-
tant. Without this material, large alveoli would tend to enlarge and small alveoli would
collapse. It is the present view that surfactant acts like a detergent, changing the stress-
strain relationship of the alveolar wall and thereby stabilizing the lung.
Certain physical properties, such as compliance, elasticity, and surface tension, are char-
acteristic of lungs. Compliance refers to the ease with which lungs can expand under pres-
sure. A normal lung is about 100 times more distensible than a toy balloon. Elasticity refers
to the ease with which the lungs and other thoracic structures return to their initial sizes
after being distended. This aids in pushing air out of the lungs during expiration. Surface
tension is exerted by the thin film of fluid in the alveoli and acts to resist distention. It cre-
ates a force that is directed inward and creates pressure in the alveolus, which is directly
proportional to the surface tension and inversely proportional to the radius of the alveolus
(Law of Laplace). Thus, the pressure inside an alveolus with a small radius would be higher
than the pressure inside an adjacent alveolus with a larger radius and would result in air
flowing from the smaller alveolus into the larger one. This could cause the smaller alveolus
to collapse. This does not happen in normal lungs because the fluid inside the alveoli con-
tains a phospholipid that acts as a surfactant. The surfactant lowers the surface tension in
the alveoli and allows them to get smaller during expiration without collapsing. Premature
babies often suffer from respiratory distress syndrome because their lungs lack sufficient
surfactant to prevent their alveoli from collapsing. These babies can be kept alive with
mechanical ventilators or surfactant sprays until their lungs mature enough to produce
surfactant.
Breathing, or ventilation, is the mechanical process by which air is moved into (inspira-
tion) and out of (expiration) the lungs. A normal adult takes about 15 to 20 breaths per
minute. During inspiration, the inspiratory muscles contract and enlarge the thoracic cavity,
the portion of the body where the lungs are located. This causes the alveoli to enlarge and
the alveolar gas to expand. As the alveolar gas expands, the partial pressure within the
respiratory system drops below atmospheric pressure by about 3 mmHg so air easily flows
in (Boyle's Law). During expiration, the inspiratory muscles relax and return the thoracic
cavity to its original volume. Since the volume of the gas inside the respiratory system
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