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U N I T 6
Respiratory Function
Exchange of Gases Within
the Lungs
The primary functions of the lungs are oxygenation of
the blood and removal of carbon dioxide. Pulmonary
gas exchange is conventionally divided into three pro-
cesses: (1) ventilation or the flow of gases into and out
of the alveoli of the lungs, (2) perfusion or flow of blood
in the adjacent pulmonary capillaries, and (3) diffusion
or transfer of gases between the alveoli and the pulmo-
nary capillaries.
Alveolar Ventilation
The ultimate importance of the alveolar ventilation is
to continually renew the air in the gas exchange areas
of the lungs where the air is in close proximity to the
blood. These areas include the alveoli, alveolar sacs,
alveolar ducts, and respiratory bronchioles. It is affected
by body position and lung volume as well as by disease
conditions that affect the heart and respiratory system.
Distribution of Alveolar Ventilation
The distribution of ventilation between the base (bot-
tom) and apex (top) of the lung varies with body posi-
tion and reflects the effects of gravity on intrapleural
pressure and lung compliance. Compliance reflects the
change in volume that occurs with a change in intra-
pleural pressure. It is lower in fully expanded alveoli,
which have difficulty accommodating more air, and
greater in alveoli that are less inflated and can more eas-
ily expand to accommodate more air. In the seated or
standing position, gravity exerts a downward pull on
the lung, causing intrapleural pressure at the apex of the
lung to become more negative. As a result, the alveoli at
the apex of the lung are more fully expanded and less
compliant than those at the base of the lung. The same
holds true for lung expansion in the dependent portions
of the lung in the supine or lateral position. In the supine
position, ventilation in the lowermost (posterior) parts
of the lung exceeds that in the uppermost (anterior)
parts. In the lateral position (i.e., lying on the side), the
alveoli in the dependent lung is better ventilated.
The distribution of ventilation also is affected by lung
volumes. During full inspiration (high lung volumes) in
the seated or standing position, the airways are pulled
open and air moves into the more compliant portions
of the lower lung. At low lung volumes, the opposite
occurs. At functional residual capacity, the intrapleural
pressure at the base of the lung exceeds airway pressure,
compressing the airways so that ventilation is greatly
reduced. In contrast, the airways in the apex of the lung
remain open, and the alveoli in this area of the lung are
well ventilated.
Even at low lung volumes, some air remains in the
alveoli of the lower portion of the lungs, preventing
their collapse. According to the law of Laplace (dis-
cussed previously), the pressure needed to overcome
the tension in the wall of a sphere or an elastic tube is
inversely related to its radius; therefore, the small air-
ways close first, trapping some air in the alveoli. This
trapping of air may be increased in older persons and
persons with chronic lung disease owing to a loss in the
elastic recoil properties of the lungs. In these persons,
airway closure occurs at the end of normal instead of
low lung volumes, trapping larger amounts of air that
cannot participate in gas exchange.
Dead Air Space
Dead space refers to the air that must be moved with
each breath but does not participate in gas exchange.
The movement of air through dead space contributes to
the work of breathing but not to gas exchange. Some of
the air that enters the respiratory tract during breath-
ing fails to reach the alveoli. This volume (about 150
to 200 mL), which remains in the conducting airways
of the nose, pharynx, trachea, bronchi, and bronchioles
and does not participate in gas exchange, is referred to
as
anatomic dead space
. A second type of dead space,
physiological dead space,
consists of the total amount of
air that does not participate in gas exchange. It includes
the anatomic dead space plus the dead space in alveoli
that are perfused, but not ventilated. Physiologic dead
space tends to be the same as anatomic dead space in
persons with normal respiratory function, but can be
considerably larger in the presence of lung disease.
Perfusion
The term
perfusion
is used to describe the flow of
blood through the gas exchange portion of the lung.
Deoxygenated blood enters the lung through the pul-
monary artery, which has its origin in the right side of
the heart and enters the lung at the hilus, along with the
primary bronchus. The pulmonary arteries branch in a
manner similar to that of the airways. The small pulmo-
nary arteries accompany the bronchi as they move down
the lobules and branch to supply the capillary network
that surrounds the alveoli (see Fig. 21-7). The oxygen-
ated capillary blood is collected in the small pulmonary
veins of the lobules, and then it moves to the larger veins
to be collected in the four large pulmonary veins that
empty into the left atrium.
The pulmonary blood vessels are thinner, more com-
pliant, and offer less resistance to flow than those in the
systemic circulation, and the pressures in the pulmo-
nary system are much lower (e.g., 22/8 mm Hg versus
120/70 mm Hg). The low pressure and low resistance of
the pulmonary circulation accommodate the delivery of
varying amounts of blood from the systemic circulation
without producing signs and symptoms of congestion.
The volume in the pulmonary circulation is approxi-
mately 500 mL, with approximately 100 mL of this
volume located in the pulmonary capillary bed. When
the input of blood from the right heart and output of
blood to the left heart are equal, pulmonary blood flow
remains constant. Small differences between input and
output can result in large changes in pulmonary vol-
ume if the differences continue for many heartbeats.
