762
UNIT FIVE
An optical tool called “frequency combs” can detect trace amounts
of more than a thousand diF
erent compounds in an exhaled breath.
The sets of chemicals detected and their concentrations can provide
clues to health. A special laser sends short pulses of light into a device
holding a breath. An algorithm uses the pattern of light wavelengths
absorbed to identify speci±
c chemical compounds that may serve as
biomarkers (see chapter 2 vignette on page 51). ²or example, a signa-
ture of nitric oxide, carbon monoxide, nitrites, nitrates, pentane, eth-
ane, and hydrogen peroxide indicates asthma, but a diF
erent set of
compounds in exhaled breath indicates cystic ±
brosis.
PRACTICE
37
Describe the structure of the respiratory membrane.
38
What is partial pressure?
39
What causes oxygen and carbon dioxide to move across the
respiratory membrane?
19.7
GAS TRANSPORT
The blood transports oxygen and carbon dioxide between
the lungs and the body cells. As these gases enter the blood,
they dissolve in the liquid portion, the plasma, or combine
chemically with other atoms or molecules.
Oxygen Transport
Almost all the oxygen (over 98%) is carried in the blood bound
to the protein
hemoglobin
in red blood cells. The oxygen bound
to iron in hemoglobin provides the color of these blood cells.
The remainder of the oxygen is dissolved in the blood plasma.
Hemoglobin consists of two types of components called
heme
and
globin
(see chapter 18, p. 720). Globin is a protein
of 574 amino acids in four polypeptide chains. Each chain
is associated with a heme group, and each heme group sur-
rounds an atom of iron. Each iron atom can loosely bind an
The respiratory membrane is normally so thin that cer-
tain soluble chemicals other than CO
2
may diffuse into alveo-
lar air and be exhaled. This is why breath analysis can reveal
alcohol in the blood or acetone can be smelled on the breath
of a person who has untreated diabetes mellitus. Breath
analysis may also detect substances associated with kidney
failure, certain digestive disturbances, and liver disease.
AS
AS
BM
IS
EP
RBC
Blood flow
(to body
tissues)
Alveolus
P
CO
2
= 45 mm Hg
P
O
2
= 40 mm Hg
P
CO
2
= 40 mm Hg
P
O
2
= 104 mm Hg
Diffusion of CO
2
Diffusion of O
2
P
CO
2
= 40 mm Hg
P
O
2
= 104 mm Hg
Capillary
Alveolar
wall
Blood flow
(from body
tissues)
FIGURE 19.34
²alsely colored electron micrograph of a capillary
located between alveoli (7,000×). (
AS,
alveolar space;
RBC,
red blood
cell;
BM,
basement membrane;
IS,
interstitial connective tissue;
EP,
epithelial nucleus.)
FIGURE 19.35
Gases are exchanged between
alveolar air and capillary blood because of diF
erences
in partial pressures.
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