480
Chapter 13
a. This makes intrapleural pressure more subatmospheric,
increases transpulmonary pressure, and causes the lungs to
expand to a greater degree than they do between breaths.
b. This expansion initially makes alveolar pressure
subatmospheric, which creates the pressure difference
between the atmosphere and alveoli to drive air fl ow into
the lungs.
IV. During expiration, the inspiratory muscles cease contracting,
allowing the elastic recoil of the lungs to return them to
their original between-breath size.
a. This initially compresses the alveolar air, raising alveolar
pressure above atmospheric pressure and driving air out
of the lungs.
b. In forced expirations, the contraction of expiratory
intercostal muscles and abdominal muscles actively
decreases chest dimensions.
V. Lung compliance is determined by the elastic connective
tissues of the lungs and the surface tension of the fl uid lining
the alveoli. The latter is greatly reduced, and compliance
increased, by surfactant, produced by the type II cells of the
alveoli. Surfactant also stabilizes alveoli by increasing surface
tension in large alveoli.
VI. Airway resistance determines how much air fl ows into the
lungs at any given pressure difference between atmosphere
and alveoli. The major determinant of airway resistance is
the radii of the airways.
VII. The vital capacity is the sum of resting tidal volume,
inspiratory reserve volume, and expiratory reserve volume.
The volume expired during the fi rst second of a forced vital
capacity (FVC) measurement is the FEV
1
and normally
averages 80 percent of FVC.
VIII. Minute ventilation is the product of tidal volume and
respiratory rate. Alveolar ventilation = (tidal volume –
anatomic dead space)
×
respiratory rate.
Exchange of Gases in Alveoli and Tissues
I. Exchange of gases in lungs and tissues is by diffusion, as a
result of differences in partial pressures. Gases diffuse from
a region of higher partial pressure to one of lower partial
pressure. Oxygen consumption is approximately 250 ml per
minute whereas carbon dioxide production is approximately
200 ml per minute.
II. Normal alveolar gas pressure for oxygen is 105 mmHg and
for carbon dioxide is 40 mmHg.
a. At any given inspired
P
O
2
, the ratio of oxygen
consumption to alveolar ventilation determines alveolar
P
O
2
—the higher the ratio, the lower the alveolar
P
O
2
.
b. The higher the ratio of carbon dioxide production to
alveolar ventilation, the higher the alveolar
P
CO
2
.
III. The average value at rest for systemic venous
P
O
2
is 40
mmHg and for
P
CO
2
is 46 mmHg.
IV. As systemic venous blood fl ows through the pulmonary
capillaries, there is net diffusion of oxygen from alveoli to
blood and of carbon dioxide from blood to alveoli. By the
end of each pulmonary capillary, the blood gas pressures
have become equal to those in the alveoli.
V. Inadequate gas exchange between alveoli and pulmonary
capillaries may occur when the alveolar capillary surface area
is decreased, when the alveolar walls thicken, or when there
are ventilation-perfusion inequalities.
VI. Signifi cant ventilation-perfusion inequalities cause the systemic
arterial
P
O
2
to be reduced. An important mechanism for opposing
mismatching is that a low local
P
O
2
causes local vasoconstriction,
diverting blood away from poorly ventilated areas.
VII. In the tissues, net diffusion of oxygen occurs from blood to
cells, and net diffusion of carbon dioxide from cells to blood.
Transport of Oxygen in Blood
I. Each liter of systemic arterial blood normally contains 200 ml
of oxygen, more than 98 percent bound to hemoglobin and
the rest dissolved.
II. The major determinant of the degree to which hemoglobin is
saturated with oxygen is blood
P
O
2
.
a. Hemoglobin is almost 100 percent saturated at the normal
systemic arterial
P
O
2
of 100 mmHg. The fact that saturation
is already more than 90 percent at a
P
O
2
of 60 mmHg
permits relatively normal uptake of oxygen by the blood
even when alveolar
P
O
2
is moderately reduced.
b. Hemoglobin is 75 percent saturated at the normal systemic
venous
P
O
2
of 40 mmHg. Thus, only 25 percent of the oxygen
has dissociated from hemoglobin and entered the tissues.
III. The affi nity of hemoglobin for oxygen is decreased by an
increase in
P
CO
2
, hydrogen ion concentration, and temperature.
All these conditions exist in the tissues and facilitate the
dissociation of oxygen from hemoglobin.
IV. The affi nity of hemoglobin for oxygen is also decreased by
binding DPG, which is synthesized by the erythrocytes. DPG
increases in situations associated with inadequate oxygen
supply and helps maintain oxygen release in the tissues.
Transport of Carbon Dioxide in Blood
I. When carbon dioxide molecules diffuse from the tissues
into the blood, 10 percent remains dissolved in plasma and
erythrocytes, 30 percent combines in the erythrocytes with
deoxyhemoglobin to form carbamino compounds, and 60
percent combines in the erythrocytes with water to form
carbonic acid, which then dissociates to yield bicarbonate and
hydrogen ions. Most of the bicarbonate then moves out of the
erythrocytes into the plasma in exchange for chloride ions.
II. As venous blood fl ows through lung capillaries, blood
P
CO
2
decreases because of the diffusion of carbon dioxide out of the
blood into the alveoli, and the reactions are reversed.
Transport of Hydrogen Ions Between Tissues and
Lungs
I. Most of the hydrogen ions generated in the erythrocytes from
carbonic acid during blood passage through tissue capillaries
bind to deoxyhemoglobin because deoxyhemoglobin, formed
as oxygen unloads from oxyhemoglobin, has a high affi nity for
hydrogen ions.
II. As the blood fl ows through the lung capillaries, hydrogen ions
bound to deoxyhemoglobin are released and combine with
bicarbonate to yield carbon dioxide and water.
Control of Respiration
I. Breathing depends upon cyclical inspiratory muscle excitation
by the nerves to the diaphragm and intercostal muscles.
This neural activity is triggered by the medullary inspiratory
neurons.
II. The medullary respiratory center is composed of the dorsal
respiratory group, which contains inspiratory neurons, and
the ventral respiratory group, where the respiratory rhythm
generator is located.
III. The most important inputs to the medullary inspiratory
neurons for the involuntary control of ventilation are from the
peripheral chemoreceptors—the carotid and aortic bodies—
and the central chemoreceptors.
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