820
UNIT 4
Maintenance of the Body
22
Check Your Understanding
7.
What is the driving force for pulmonary ventilation?
8.
What causes the intrapulmonary pressure to decrease during
inspiration?
9.
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
Pulmonary Ventilation
List several physical factors that influence pulmonary
ventilation.
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.
Airway Resistance
Te major
nonelastic
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 (
F
),
pressure (
P
), and resistance (
R
):
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
P
, the
difference
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
each breath.
But, as the equation also indicates, gas flow changes
inversely
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
(Figure 22.15)
. 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.
Homeostatic Imbalance
22.8
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
asthma attack
, 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
respiratory disease.
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-
face, called
surface tension
, that (1) draws the liquid molecules
F
5
Δ
P
R
Airway generation
(stage of branching)
Medium-sized
bronchi
Terminal
bronchioles
Conducting
zone
Respiratory
zone
1
5
10
15
20
23
Resistance
Figure 22.15
Resistance in respiratory passageways.
Airway
resistance peaks in the medium-sized bronchi and then declines
sharply as the total cross-sectional area of the airways increases
rapidly.
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