Page 133 - Fundamentals of Air Pollution

III. The Human Respiratory System 103

In the normal interaction between the respiratory and circulatory sys-
tems, O 2 is moved into the body for use in biochemical oxidation and CO 2,
a waste product, is removed. Hemoglobin molecules in the blood play an
important role in both processes. Hemoglobin combines with O 2 and CO 2
as these gases are moved between the lung and the cells. The stability of
the hemoglobin-O 2 and hemoglobin-CO 2 complex is sufficiently strong to
transport the gases in the circulatory system but not strong enough to
prevent the release of CO 2 at the lung and O 2 where it is needed at the
cellular level. CO interferes with this normal interaction by forming a much
more stable complex with hemoglobin (COHb) (7). This process reduces
the number of hemoglobin molecules available to maintain the necessary
transport of O 2 and CO 2.
The baseline level of COHb is —0.5% for most individuals. Upon exposure
to elevated levels of atmospheric CO, the percentage of COHb will increase
in a very predictable manner. Analytical techniques are available to measure
COHb from <0.1 to >80% in the bloodstream, providing a very rapid
method for determining the total body burden. If elevated levels of CO are
reduced, the percentage of COHb will decrease over a period of time.
At low levels of COHb (0.5-2.0%) the body burden is measurable, but
research has not shown any substantive effects at these low levels. When
COHb increases to higher levels the body burden of CO is elevated, produc-
ing adverse effects on the cardiovascular system and reducing physical
endurance.

III. THE HUMAN RESPIRATORY SYSTEM

The primary function of the human respiratory system is to deliver O 2
to the bloodstream and remove CO 2 from the body. These two processes
occur concurrently as the breathing cycle is repeated. Air containing O 2
flows into the nose and/or mouth and down through the upper airway to
the alveolar region, where O 2 diffuses across the lung wall to the blood-
stream. The counterflow involves transfer of CO 2 from the blood to the
alveolar region and then up the airways and out the nose. Because of
the extensive interaction of the respiratory system with the surrounding
atmosphere, air pollutants or trace gases other than N 2 and O 2 can be
delivered to the respiratory system.
The anatomy of the respiratory system is shown in Fig. 7-1. This system
may be divided into three regions—the nasal, tracheobronchial, and pulmo-
nary. The nasal region is composed of the nose and mouth cavities and
the throat. The tracheobronchial region begins with the trachea and extends
through the bronchial tubes to the alveolar sacs. The pulmonary region is
composed of the terminal bronchi and alveolar sacs, where gas exchange
with the circulatory system occurs. Figure 7-1 illustrates the continued