Maintenance of the Body
Check Your Understanding
What is the driving force for pulmonary ventilation?
What causes the intrapulmonary pressure to decrease during
What causes the partial vacuum (negative pressure) inside
the pleural cavity? What happens to a lung if air enters the
pleural cavity? What is the clinical name for this condition?
For answers, see Appendix H.
Physical Factors Inﬂuencing
List several physical factors that inﬂuence pulmonary
As we have seen, the lungs are stretched during inspiration and
recoil passively during expiration. Te inspiratory muscles con-
sume energy to enlarge the thorax. Energy is also used to over-
come various factors that hinder air passage and pulmonary
ventilation. We examine these factors next.
source of resistance to gas ﬂow is friction,
or drag, encountered in the respiratory passageways. Te fol-
lowing equation gives the relationship between gas ﬂow (
), and resistance (
Notice that the factors determining gas ﬂow in the respira-
tory passages and blood ﬂow in the cardiovascular system are
equivalent. Te amount of gas ﬂowing into and out of the alveoli
is directly proportional to Δ
in pressure, or pres-
sure gradient, between the external atmosphere and the alveoli.
Normally, very small diﬀerences in pressure produce large
changes in the volume of gas ﬂow. Te average pressure gradi-
ent during normal quiet breathing is 2 mm Hg or less, and yet
it is suﬃcient to move 500 ml of air in and out of the lungs with
But, as the equation also indicates, gas ﬂow changes
with resistance. In other words, gas ﬂow decreases as resistance
increases. As in the cardiovascular system, resistance in the
respiratory tree is determined mostly by the diameters of the
conducting tubes. However, as a rule, airway resistance is insig-
niﬁcant for two reasons:
Airway diameters in the ﬁrst part of the conducting zone are
huge, relative to the low viscosity of air.
As the airways get progressively smaller, there are progres-
sively more branches. As a result, although individual bron-
chioles are tiny, there are an enormous number of them in
parallel, so the total cross-sectional area is huge.
Consequently, the greatest resistance to gas ﬂow occurs in
the medium-sized bronchi
. At the terminal
bronchioles, gas ﬂow stops and diﬀusion takes over as the main
force driving gas movement, so resistance is no longer an issue.
Smooth muscle of the bronchiolar walls is exquisitely sensitive
to neural controls and certain chemicals. For example, inhaled
irritants activate a reﬂex of the parasympathetic division of the
nervous system that causes vigorous constriction of the bron-
chioles and dramatically reduces air passage. During an acute
, histamine and other inﬂammatory chemicals
can cause such strong bronchoconstriction that pulmonary
ventilation almost completely stops, regardless of the pressure
gradient. Conversely, epinephrine released during sympathetic
nervous system activation or administered as a drug dilates
bronchioles and reduces airway resistance. Local accumulations
of mucus, infectious material, or solid tumors in the passage-
ways are important sources of airway resistance in those with
Whenever airway resistance rises, breathing movements
become more strenuous, but such compensation has its limits.
When the bronchioles are severely constricted or obstructed,
even the most magniﬁcent respiratory eﬀorts cannot restore
ventilation to life-sustaining levels.
Alveolar Surface Tension
At any gas-liquid boundary, the molecules of the liquid are more
strongly attracted to each other than to the gas molecules. Tis
unequal attraction produces a state of tension at the liquid sur-
, that (1) draws the liquid molecules
(stage of branching)
Resistance in respiratory passageways.
resistance peaks in the medium-sized bronchi and then declines
sharply as the total cross-sectional area of the airways increases