816
UNIT 4
Maintenance of the Body
22
Te
pulmonary veins
convey the freshly oxygenated blood
from the respiratory zone of the lungs to the heart. Teir tribu-
taries course back to the hilum both with the corresponding
bronchi and in the connective tissue septa separating the bron-
chopulmonary segments.
Te pulmonary circuit is a low-pressure, high-volume circu-
lation. Because
all
of the body’s blood passes through the lungs
about once each minute, the lung capillary endothelium is an
ideal location for enzymes that act on materials in the blood.
Examples include
angiotensin converting enzyme
, which acti-
vates an important blood pressure hormone, and enzymes that
inactivate certain prostaglandins.
Bronchial Circulation of the Lungs
In contrast to the pulmonary
circulation, the
bronchial arteries
provide oxygenated systemic
blood to lung tissue. Te bronchial arteries arise from the aorta,
enter the lungs at the hilum, and then run along the branching
bronchi. Tey provide a high-pressure, low-volume supply of oxy-
genated blood to all lung tissues except the alveoli. Te tiny bron-
chial veins drain some systemic venous blood from the lungs, but
there are multiple anastomoses between the two circulations, and
most venous blood returns to the heart via the pulmonary veins.
Innervation of the Lungs
Te lungs are innervated by para-
sympathetic and sympathetic motor fibers, and visceral sensory
fibers. Tese nerve fibers enter each lung through the
pulmo-
nary plexus
on the lung root and run along the bronchial tubes
and blood vessels in the lungs. Parasympathetic fibers cause the
air tubes to constrict, whereas sympathetic fibers dilate them.
The Pleurae
Te
pleurae
(ploo
9
re; “sides”) form a thin, double-layered se-
rosa. Te layer called the
parietal
pleura
covers the thoracic
wall and superior face of the diaphragm (Figure 22.10a, c). It
continues around the heart and between the lungs, forming the
lateral walls of the mediastinal enclosure and snugly enclosing
the lung root. From here, the pleura extends as the layer called
the
visceral pleura
to cover the external lung surface, dipping
into and lining its fissures.
Te pleurae produce
pleural fluid
, which fills the slitlike
pleu-
ral cavity
between them. Tis lubricating secretion allows the
lungs to glide easily over the thorax wall during our breathing
movements. Although the pleurae slide easily across each other,
the surface tension of the pleural fluid strongly resists their sepa-
ration. Consequently, the lungs cling tightly to the thorax wall
and expand and recoil passively as the volume of the thoracic
cavity alternately increases and decreases during breathing.
Te pleurae also help divide the thoracic cavity into three
chambers—the central mediastinum and the two lateral pleural
compartments, each containing a lung. Tis compartmentaliza-
tion helps prevent one mobile organ (for example, the lung or
heart) from interfering with another. It also limits the spread of
local infections.
Homeostatic Imbalance
22.6
Pleurisy
(ploo
9
rĭ-se), inflammation of the pleurae, o±en results
from pneumonia. Inflamed pleurae become rough, resulting
in friction and stabbing pain with each breath. As the disease
progresses, the pleurae may produce excessive amounts of fluid.
Tis increased fluid relieves the pain caused by pleural sur-
faces rubbing together, but may exert pressure on the lungs and
hinder breathing movements.
Other fluids that may accumulate in the pleural cavity in-
clude blood (leaked from damaged blood vessels) and blood
filtrate (the watery fluid that oozes from the lung capillaries
when le±-sided heart failure occurs). Te general term for fluid
accumulation in the pleural cavity is
pleural effusion
.
Check Your Understanding
4.
What features of the alveoli and their respiratory membranes
suit them to their function of exchanging gases by diffusion?
5.
A 3-year-old boy is brought to the emergency department
after aspirating (inhaling) a peanut. Bronchoscopy confirms
the suspicion that the peanut is lodged in a bronchus and
then it is successfully extracted. Which main bronchus was
the peanut most likely to be in? Why?
6.
The lungs are perfused by two different circulations. Name
these circulations and indicate their roles in the lungs.
For answers, see Appendix H.
Mechanics of Breathing
Breathing
, or
pulmonary ventilation
, consists of two phases:
inspiration
, the period when air flows into the lungs, and
expi-
ration
, the period when gases exit the lungs.
Pressure Relationships
in the Thoracic Cavity
Explain the functional importance of the partial vacuum
that exists in the intrapleural space.
Before we discuss the breathing process, it is important to un-
derstand that
respiratory pressures are always described relative to
atmospheric pressure
(
P
atm
)
, which is the pressure exerted by the
air (gases) surrounding the body. At sea level, atmospheric pres-
sure is 760 mm Hg (the pressure exerted by a column of mercury
760 mm high). Tis pressure can also be expressed in atmosphere
units: atmospheric pressure
5
760 mm Hg
5
1 atm.
A negative respiratory pressure in any respiratory area indicates
that the pressure in that region is lower than atmospheric pres-
sure. For instance, a respiratory pressure of
2
4 mm Hg indicates
a pressure that is lower than atmospheric pressure by 4 mm Hg
(760
2
4
5
756 mm Hg). In this case, 756 mm Hg is the absolute
pressure in that region. A positive respiratory pressure is higher
than atmospheric pressure, and zero respiratory pressure is equal
to atmospheric pressure. Now we are ready to examine the pres-
sure relationships that normally exist in the thoracic cavity.
Intrapulmonary Pressure (
P
pul
)
Te
intrapulmonary
(intra-alveolar)
pressure (
P
pul
)
is the
pressure in the alveoli. Intrapulmonary pressure rises and falls
with the phases of breathing, but it
always
equalizes with the
atmospheric pressure eventually
(Figure 22.12)
.
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