Respiratory Physiology
457
in tidal volume goes entirely toward increasing alveolar venti-
lation. These concepts have important physiological implica-
tions. Most situations that produce an increased ventilation,
such as exercise, refl exly call forth a relatively greater increase
in breathing depth than in breathing rate.
The anatomic dead space is not the only type of dead
space. Some fresh inspired air is not used for gas exchange with
the blood even though it reaches the alveoli because some alve-
oli may, for various reasons, have little or no blood supply. This
volume of air is known as
alveolar dead space.
It is quite small
in normal persons but may be very large in persons with several
kinds of lung disease. As we shall see, local mechanisms that
match air and blood fl
ows minimize the alveolar dead space.
The sum of the anatomic and alveolar dead spaces is known as
the
physiologic dead space.
This is also known as wasted ven-
tilation because it is air that is inspired but does not participate
in gas exchange with blood fl owing through the lungs.
Exchange of Gases in Alveoli
and Tissues
We have now completed the discussion of the lung mechan-
ics that produce alveolar ventilation, but this is only the fi rst
step in the respiratory process. Oxygen must move across the
alveolar membranes into the pulmonary capillaries, be trans-
ported by the blood to the tissues, leave the tissue capillaries
and enter the extracellular fl uid, and fi nally cross plasma mem-
branes to gain entry into cells. Carbon dioxide must follow a
similar path, but in reverse.
In the steady state, the volume of oxygen that leaves the
tissue capillaries and is consumed by the body cells per unit
time is equal to the volume of oxygen added to the blood in the
lungs during the same time period. Similarly, in the steady state,
the rate at which carbon dioxide is produced by the body cells
and enters the systemic blood is the same as the rate at which
carbon dioxide leaves the blood in the lungs and is expired.
The amount of oxygen the cells consume and the amount
of carbon dioxide they produce, however, are not necessarily
identical. The balance depends primarily upon which nutrients
are used for energy. The ratio of CO
2
produced to O
2
consumed
is known as the
respiratory quotient (RQ).
On a mixed diet,
the RQ is approximately 0.8; that is, 8 molecules of CO
2
are
produced for every 10 molecules of O
2
consumed. The RQ is 1
for carbohydrate, 0.7 for fat, and 0.8 for protein.
Figure 13–20
presents typical exchange values during
1 min for a person at rest, assuming a cellular oxygen con-
sumption of 250 ml/min, a carbon dioxide production of
200 ml/min, an alveolar ventilation of 4000 ml/min, and a
cardiac output of 5000 ml/min.
Because only 21 percent of the atmospheric air is oxygen,
the total oxygen entering the alveoli per min in our illustration
is 21 percent of 4000 ml, or 840 ml/min. Of this inspired oxy-
gen, 250 ml crosses the alveoli into the pulmonary capillaries,
and the rest is subsequently exhaled. Note that blood entering
the lungs already contains a large quantity of oxygen, to which
the new 250 ml is added. The blood then fl ows from the lungs
to the left side of the heart and is pumped by the left ventricle
through the aorta, arteries, and arterioles into the tissue capillar-
ies, where 250 ml of oxygen leaves the blood per minute for cells
to take up and utilize. Thus, the quantities of oxygen added to
the blood in the lungs and removed in the tissues are the same.
The story reads in reverse for carbon dioxide. A signifi -
cant amount of carbon dioxide already exists in systemic arte-
rial blood; to this is added an additional 200 ml per minute,
the amount the cells produce, as blood fl ows through tissue
capillaries. This 200 ml leaves the blood each minute as blood
fl ows through the lungs and is expired.
Blood pumped by the heart carries oxygen and carbon
dioxide between the lungs and tissues by bulk fl ow, but dif-
fusion is responsible for the net movement of these molecules
between the alveoli and blood, and between the blood and the
cells of the body. Understanding the mechanisms involved in
these diffusional exchanges depends upon some basic chemical
and physical properties of gases, which we will now discuss.
Partial Pressures of Gases
Gas molecules undergo continuous random motion. These
rapidly moving molecules collide and exert a pressure, the
magnitude of which is increased by anything that increases the
rate of movement. The pressure a gas exerts is proportional to
temperature (because heat increases the speed at which mol-
ecules move) and the concentration of the gas—that is, the
number of molecules per unit volume.
As
Dalton’s law
states, in a mixture of gases, the pres-
sure each gas exerts is independent of the pressure the oth-
ers exert. This is because gas molecules are normally so far
apart that they do not affect each other. Each gas in a mix-
ture behaves as though no other gases are present, so the total
Table 13–5
Effect of Breathing Patterns on Alveolar Ventilation
Subject
Tidal Volume
(ml/breath)
×
Frequency
(breaths/min)
=
Minute Ventilation
(ml/min)
Anatomic Dead-Space
Ventilation (ml/min)
Alveolar Ventilation
(ml/min)
A
150
40
6000
150
×
40 = 6000
0
B
500
12
6000
150
×
12 = 1800
4200
C
1000
6
6000
150
×
6 =
900
5100
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