Maintenance of the Body
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-
Te pulmonary circuit is a low-pressure, high-volume circu-
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.
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
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
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.
re; “sides”) form a thin, double-layered se-
rosa. Te layer called the
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
to cover the external lung surface, dipping
into and lining its ﬁssures.
Te pleurae produce
, which ﬁlls the slitlike
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
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
Check Your Understanding
What features of the alveoli and their respiratory membranes
suit them to their function of exchanging gases by diffusion?
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?
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
, consists of two phases:
, the period when air ﬂows into the lungs, and
, the period when gases exit the lungs.
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-
respiratory pressures are always described relative to
, 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
760 mm Hg
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
4 mm Hg indicates
a pressure that is lower than atmospheric pressure by 4 mm Hg
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 (
pressure in the alveoli. Intrapulmonary pressure rises and falls
with the phases of breathing, but it
equalizes with the
atmospheric pressure eventually