VENTILATION
AND ITS MEASUREMENT
Gases (air) are measured by virtue of
the pressures that they exert. When
gases are mixed they each contribute to a total
pressure. Each gas contributes a partial pressure. Total atmospheric air pressure at sea level,
at 15◦ C and zero humidity, is 760 mmHg (millimeters of mercury). At sea level partial pressure oxygen, written PO2 is 159 mmHg (20.93%), and partial pressure carbon dioxide, written PCO2, is 0.3 mmHg (less than 0.04%).
Air is
transported by means of airways. Airways branch out into smaller airways,
e.g., bronchioles, up to about 22
times, ultimately ending with the alveolus,
the fundamental respiratory unit. In the
initial airways there is no exchange of O2 and CO2, an
area known as the anatomical dead space. Before reaching the alveoli much of the branching is surrounded with smooth muscles, bronchial muscles, which contract and
dilate, making breathing easier or more difficult, a phenomenon known as airway resistance. There are about 300 million alveoli, surrounded
by about 280 billion pulmonary
capillaries.
Most of
the gas exchange, O2 and CO2, takes place in the alveolar-capillary unit. Normal inhalation, at sea level, increases alveolar PO2 (average PO2
in the alveoli) to about 104 mmHg.
Because the venous blood
arriving in the pulmonary capillary network contains only about 40 mmHg PO2,
rapid diffusion from the alveoli
takes place, resulting in an arterial PO2
(PaO2) of about 100 mmHg,
most of which (98.5%) is transported to the tissues by hemoglobin in the red blood cells.
Without pure oxygen (where PO2 = 760 mmHg) or hyperbaric
chamber pressure (where PO2 = 600 mmHg), the O2 dissolved
in blood plasma by itself is not adequate to support life.
Carbon
dioxide is transported to the lungs where it is (1) excreted into the alveoli of the lungs for discharge into the
atmosphere, and (2) reallocated to
the body for proper maintenance of acid-base physiology. Reallocation of CO2 means
reflexive coordination of breathing depth and rate, where arterial PCO2 (PaCO2),
which under normal circumstances, is maintained at about 40 mmHg for
normalizing blood plasma pH (about 7.4).
PCO2 in capillary venous blood, at rest, is about 46 to 48
mmHg, whereas inspired atmospheric air contains only about 0.3 mmHg PCO2. Because pulmonary capillary PCO2
equilibrates with alveolar PCO2
as a result of diffusion, alveolar PCO2 levels must also be
continuously maintained at about 40 mmHg.
Thus, if alveolar PCO2 increases, so too does arterial PCO2,
and if alveolar PCO2 drops as a result of overbreathing, so too does
arterial CO2. Bad breathing is when learned breathing behavior disturbs the
proper regulation of CO2 allocation.
Hypocapnia
(CO2 deficit), as a consequence of overbreathing behavior, is
measured with a capnograph (or capnometer), an instrument used to
measure average alveolar PCO2.
In a lung-healthy and cardiovascular-healthy person the alveolar PCO2
is equivalent to PaCO2. Generally,
PaCO2 levels below 35 mmHg constitute hypocapnia (CO2 deficit): 30-35 mmHg is mild to moderate, 25-30 mmHg is
serious, and 20-25 mmHg is severe hypocapnia.
These instruments are used worldwide in emergency medicine, in critical
care, and during surgery for gas monitoring and regulation purposes; these are
medical applications. The CapnoTrainer provides for educational applications. The CapnoTrainer is a capnometer specifically
designed and manufactured for evaluating, observing, and self-regulating overbreathing
behavior.
Click
here to learn more about
external respiration.
Copyrighted by
Behavioral Physiology Institute,