Biomedical Engineering Reference
In-Depth Information
The TLC can be measured using the gas dilution technique. In this method, patients
inspire to TLC from a gas mixture containing a known amount of an inert tracer gas, such
as helium, and hold their breath for 10 s. During this time, the inert gas becomes evenly
distributed throughout the lungs and airways. Due to conservation of mass, the product
of initial tracer gas concentration (which is known) times the amount inhaled (which is
measured) equals the product of final tracer gas concentration (which is measured during
expiration) times the TLC. Body plethysmography, which provides the most accurate
method for measuring lung volumes, uses an airtight chamber in which the patient sits
and breathes through a mouthpiece. This method uses Boyle's Law, which states that the
product of pressure and volume for gas in a chamber is constant under isothermal condi-
tions. Changes in lung volume and pressure at the mouth when the patient pants against
a closed shutter can be used to calculate the functional residual capacity. Since the expira-
tory reserve volume can be measured, the residual volume can be calculated by subtracting
it from the functional residual capacity.
EXAMPLE PROBLEM 3.10
A patient is allowed to breathe a mixture from a 2-liter reservoir that contains 10 percent of an
inert gas—that is, a gas that will not cross from the lungs into the circulatory system. At the end of
a period that is sufficient for the contents of the reservoir and the lungs to equilibrate, the concen-
tration of the inert gas is measured and is found to be 2.7 percent. What is the patient's total lung
capacity?
Solution
The total amount of inert gas is the same at the beginning and end of the measurement, but
its concentration has changed from 10 percent (C
1
) to 2.7 percent (C
2
). At the beginning, it is
confined to a 2-liter reservoir (V
1
). At the end, it is in both the reservoir and the patient's lungs
(V
2
¼
V
1
þ
TLC).
C
1
V
1
¼
C
2
V
2
ð
0
:
1
Þð
2
l
Þ¼ð
0
:
027
Þð
2
l
þ
TLC
Þ
0
:
2
l
0
:
054
l
¼
0
:
027
TLC
5
:
4
l
¼
TLC
External respiration occurs in the lungs when gases are exchanged between the blood
and the alveoli (Figure 3.26). Each adult lung contains about 3.5
10
8
alveoli, which results
in a large surface area (60-70 m
2
) for gas exchange to occur. Each alveolus is only one cell
layer thick, so the air-blood barrier is only two cells thick (an alveolar cell and a capillary
endothelial cell), which is about 2
m. The partial pressure of oxygen in the alveoli is higher
than the partial pressure of oxygen in the blood, so oxygen moves from the alveoli into
the blood. The partial pressure of carbon dioxide in the alveoli is lower than the partial
pressure of carbon dioxide in the blood, so carbon dioxide moves from the blood into
the alveoli. During internal respiration, carbon dioxide and oxygen move between the
blood and the extracellular fluid surrounding the body's cells. The direction and rate of
m
