520
U N I T 6
Respiratory Function
Innervation
The lung is innervated by both the sympathetic and
parasympathetic divisions of the autonomic nervous
system. The parasympathetic fibers, which are derived
from vagal nerves, and the sympathetic fibers, which
originate in the upper thoracic and cervical ganglia,
form the pulmonary plexuses that enter the lung in the
region of the hilus. Fibers from the plexus follow the
major bronchi and blood vessels into the lung to inner-
vate bronchial smooth muscle cells, blood vessels, and
epithelial cells (including the goblet and submucosal
glands). There is no voluntary motor innervation of the
lung, nor are there any pain fibers. Pain fibers are found
only in the pleura.
The parasympathetic (cholinergic) fibers are excit-
atory neurons that respond to acetylcholine. Stimulation
of the parasympathetic nervous system is responsible
for airway constriction, blood vessel dilation, and
increased glandular secretion. The sympathetic nervous
system, which responds to the catecholamines norepi-
nephrine and epinephrine, produces bronchodilation,
blood vessel constriction, and inhibition of glandular
secretion.
Exchange of Gases Between the
Atmosphere and the Lungs
The exchange of gases between the atmosphere and the
lungs occurs along a pressure gradient, moving from an
area of higher pressure to one of lower pressure.
Basic Properties of Gases
The air we breathe is made up of a mixture of gases,
mainly nitrogen and oxygen. These gases exert a com-
bined pressure called the
atmospheric pressure.
The
pressure at sea level is defined as 1 atmosphere, which
is equal to 760 millimeters of mercury (mm Hg) or 14.7
pounds per square inch (PSI). Respiratory pressures—
the pressures within the alveoli and other respiratory
structures—are always expressed relative to atmo-
spheric pressure, which is assigned a value of 0 mm
Hg. This means that a respiratory pressure of +15 mm
Hg is 15 mm Hg above atmospheric pressure, and a
respiratory pressure of −15 mm Hg is 15 mm Hg less
than atmospheric pressure. Respiratory pressures often
are expressed in centimeters of water (cm H
2
O) because
of the small pressures involved (1 mm Hg = 1.35 cm
H
2
O pressure).
The pressure exerted by a single gas in a mixture is
called the
partial pressure.
The capital letter “P” fol-
lowed by the chemical symbol of the gas (e.g., PO
2
) is
used to denote its partial pressure. The law of partial
pressures states that the total pressure of a mixture
of gases, as in the atmosphere, is equal to the sum of
the partial pressures of the different gases in the mix-
ture. If the concentration of oxygen at 760 mm Hg
(1 atmosphere) is 21%, its partial pressure is 160 mmHg
(760 × 0.21).
Water vapor is different from other types of gases;
its partial pressure is affected by temperature but not
atmospheric pressure. The relative humidity refers to
the percentage of moisture in the air compared with the
amount that the air can hold without causing conden-
sation (100% saturation). Warm air holds more mois-
ture than cold air. This is the reason that precipitation
in the form of rain or snow commonly occurs when the
relative humidity is high and there is a sudden drop in
atmospheric temperature. The air in the alveoli, which
remains 100% saturated at normal body temperature,
SUMMARY CONCEPTS
■■
The primary function of the respiratory system
is gas exchange, with oxygen from the air being
transferred to the blood and carbon dioxide
from the blood being eliminated into the
atmosphere.
■■
Functionally, the respiratory system can be
divided into two parts: the conducting airways
(nasopharynx, oropharynx, larynx, trachea, and
bronchi and bronchioles), through which air
moves as it passes between the atmosphere
and the lungs, and the respiratory airways of the
lungs (terminal bronchioles and alveoli), where
gas exchange takes place.
■■
There are two types of alveolar cells: type I and
type II.Type I alveolar cells provide the surface
area for the gas exchange function of the lung.
Type II alveolar cells secrete surface-active
surfactants that serve to decrease alveolar surface
tension (surfactants B and C) and mediate the
immune destruction of pathogens that have
entered the lung (surfactants A and D).
■■
The lungs are provided with a dual blood supply:
the pulmonary circulation, which provides for
the gas exchange function of the lungs; and the
bronchial circulation, which supplies blood to the
conducting airways and supporting structures of
the lung.
■■
Innervation of the lungs occurs by way of the
sympathetic and parasympathetic divisions of
the autonomic nervous system. Parasympathetic
innervation produces airway constriction
and an increase in respiratory secretions;
whereas sympathetic innervation produces
bronchodilation and a decrease in respiratory
tract secretions.
