606 SECTION VIIRespiratory Physiology
The lungs also activate one hormone; the physiologically
inactive decapeptide angiotensin I is converted to the pressor,
aldosterone-stimulating octapeptide angiotensin II in the pul-
monary circulation. The reaction occurs in other tissues as
well, but it is particularly prominent in the lungs. Large
amounts of the angiotensin-converting enzyme responsible
for this activation are located on the surface of the endothelial
cells of the pulmonary capillaries. The converting enzyme
also inactivates bradykinin. Circulation time through the pul-
monary capillaries is less than 1 s, yet 70% of the angiotensin I
reaching the lungs is converted to angiotensin II in a single
trip through the capillaries. Four other peptidases have been
identified on the surface of the pulmonary endothelial cells,
but their full physiologic role is unsettled.
Removal of serotonin and norepinephrine reduces the
amounts of these vasoactive substances reaching the systemic
circulation. However, many other vasoactive hormones pass
through the lungs without being metabolized. These include
epinephrine, dopamine, oxytocin, vasopressin, and angioten-
sin II. In addition, various amines and polypeptides are
secreted by neuroendocrine cells in the lungs.
CHAPTER SUMMARY
■ The pressure exerted by any one gas in a mixture of gases is de-
fined as its partial pressure. Partial pressures (P) of gases in air
at sea level are as follows: Po 2 = 149 mm Hg; Pco 2 = 0.3 mm Hg;
PN 2 (including other gases) = 564 mm Hg.■ Air enters the respiratory system in the upper airway, then pro-
ceeds to the conducting airway and on to the respiratory airway
that ends in the alveoli. In the upper airway, air is humidified
and warmed. The cross sectional area of the airway gradually
increases through the conducting zone, then rapidly increases
during the transition from conducting to respiratory zones
■ The epithelium that line the conducting airway include ciliated
cells that keep particulates from reaching the respiratory zone.
The epithelium that lines the alveoli consist of two cell types:
alveolar type I cells and alveolar type II cells. Type I cells are
flattened epithelial cells that provide approximately 95% of the
alveolar surface area and are the site of gas exchange. Type II
cells are cuboidal epithelial cells that secrete surfactants that line
the alveolar surface.
■ There are several important measures of lung volume, includ-
ing: tidal volume; inspiratory volume; expiratory reserve vol-
ume; forced vital capacity (FVC); the forced expiratory volume
in one second (FEV 1 ); respiratory minute volume and maximal
voluntary ventilation.
■ Lung compliance refers to the ability of lungs to stretch. How-
ever, many normal factors affect lung compliance and it is best
represented by a whole pressure-volume curve.
■ Surfactant is a lipid-protein mixture that is in the fluid lining the
alveolar epithelium. A primary function of surfactant is to in-
crease surface tension in the alveoli to keep them from deflating.
■ Both ventilation and perfusion are greater at the base of the lung
and lower at the apex of the lung. The ventilation/perfusion ratio
is lower at the base compared to the apex of the lung.
■ Not all air that enters the airway is available for gas exchange.
The regions where gas is not exchanged in the airway are termed
“dead space.” The conducting airway represents anatomical
dead space. Increased dead space can occur in response to
disease that affects air exchange in the respiratory zone.
■ The pressure gradient in the pulmonary circulation system is
much less than that in the systemic circulation. Because pulmo-
nary capillary pressure is much lower than oncotic pressure in
the plasma, fluid remains in the plasma as it traverses the lung.
■ The mucociliary escalator in the conducting airway helps to
keep particulates out of the respiratory zone.
■ There are a variety of biologically activated substances that are
metabolized in the lung. These include substances that are made
and function in the lung (eg, surfactant), substances that are
released or removed from the blood (eg, prostaglandins), and
substances that are activated as they pass through the lung (eg,
angiotensin II).MULTIPLE-CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.- On the summit of Mt. Everest, where the barometric pressure is
about 250 mm Hg, the partial pressure of O 2 is about
A) 0.1 mm Hg.
B) 0.5 mm Hg.
C) 5 mm Hg.
D) 50 mm Hg.
E) 100 mm Hg.
TABLE 35–5 Biologically active substances
metabolized by the lungs.
Synthesized and used in the lungs
Surfactant
Synthesized or stored and released into the blood
Prostaglandins
Histamine
Kallikrein
Partially removed from the blood
Prostaglandins
Bradykinin
Adenine nucleotides
Serotonin
Norepinephrine
Acetylcholine
Activated in the lungs
Angiotensin I → angiotensin II