Chapter 22
The Respiratory System
823
22
In healthy people, AVR is usually about 12 breaths per minute
times the difference of 500
2
150 ml per breath, or 4200 ml/min.
Because anatomical dead space is constant in a particular
individual, increasing the volume of each inspiration (breathing
depth) enhances AVR and gas exchange more than raising the
respiratory rate. AVR drops dramatically during rapid shallow
breathing because most of the inspired air never reaches the ex-
change sites. Furthermore, as tidal volume approaches the dead
space value, effective ventilation approaches zero, regardless of
how fast a person is breathing.
Table 22.2
summarizes the ef-
fects of breathing rate and breathing depth on alveolar ventila-
tion for three hypothetical patients.
Nonrespiratory Air Movements
Many processes other than breathing move air into or out of the
lungs, and these processes may modify the normal respiratory
rhythm. Most of these
nonrespiratory air movements
result from
reflex activity, but some are produced voluntarily.
Table 22.3
de-
scribes the most common of these movements.
Check Your Understanding
10.
Resistance in the airways is typically low. Why? (Give at least
two reasons.)
11.
Premature infants often lack adequate surfactant. How does
this affect their ability to breathe?
12.
Explain why slow, deep breaths ventilate the alveoli more
effectively than do rapid, shallow breaths.
For answers, see Appendix H.
AVR
5
frequency
3
(TV
2
dead space)
(ml/min)
(breaths/min)
(ml/breath)
involved in alveolar ventilation. ±e remaining 150 ml of the
tidal breath is in the anatomical dead space.
If some alveoli cease to act in gas exchange (due to alveolar
collapse or obstruction by mucus, for example), the
alveolar
dead space
is added to the anatomical dead space. ±e sum of
the nonuseful volumes is the
total dead space
.
Pulmonary Function Tests
±e various lung volumes and capacities are o²en abnormal
in people with pulmonary disorders. ±e original clinical mea-
suring tool, a
spirometer
(spi-rom
9
ĕ-ter), was a cumbersome
instrument utilizing a hollow bell inverted over water. Now pa-
tients simply blow into a small electronic measuring device.
Spirometry is most useful for evaluating losses in respira-
tory function and for following the course of certain respira-
tory diseases. It cannot provide a specific diagnosis, but it can
distinguish between
obstructive pulmonary disease
involving
increased airway resistance (such as chronic bronchitis) and
re-
strictive diseases
involving reduced total lung capacity. (±ese
changes might be due to diseases such as tuberculosis, or to
fibrosis due to exposure to certain environmental agents such
as asbestos). In obstructive diseases, TLC, FRC, and RV may
increase because the lungs hyperinflate, whereas in restrictive
diseases, VC, TLC, FRC, and RV decline because lung expan-
sion is limited.
We can obtain more information by assessing the
rate
at
which gas moves into and out of the lungs.
Forced vital capacity (FVC)
measures the amount of gas ex-
pelled when a subject takes a deep breath and then forcefully
exhales maximally and as rapidly as possible.
Forced expiratory volume (FEV)
determines the amount of
air expelled during specific time intervals of the FVC test.
For example, the volume exhaled during the first second is
FEV
1
. ±ose with healthy lungs can exhale about 80% of the
FVC within 1 second. ±ose with obstructive pulmonary dis-
ease exhale considerably less than 80% of the FVC within 1 sec-
ond, while those with restrictive disease can exhale 80% or more
of FVC in 1 second even though their FVC is reduced.
Alveolar Ventilation
±e
minute ventilation
is the total amount of gas that flows into
or out of the respiratory tract in 1 minute. During normal quiet
breathing, the minute ventilation in healthy people is about 6 L/
min (500 ml per breath multiplied by 12 breaths per minute).
During vigorous exercise, the minute ventilation may reach
200 L/min.
Minute ventilation values provide a rough yardstick for as-
sessing respiratory efficiency, but the
alveolar ventilation rate
(AVR)
is a better index of effective ventilation. ±e AVR takes
into account the volume of air wasted in the dead space and
measures the flow of fresh gases in and out of the alveoli dur-
ing a particular time interval. We can compute AVR using this
equation:
Table 22.2
Effects of Breathing Rate and Depth on Alveolar Ventilation of Three Hypothetical Patients
BREATHING PATTERN OF
HYPOTHETICAL PATIENT
DEAD SPACE
VOLUME
(DSV)
TIDAL
VOLUME
(TV)
RESPIRATORY
RATE*
MINUTE
VENTILATION
(MVR)
ALVEOLAR
VENTILATION
(AVR)
% EFFECTIVE
VENTILATION
(AVR/MVR)
I—Normal rate and depth
150 ml
500 ml
20/min
10,000 ml/min
7000 ml/min
70%
II—Slow, deep breathing
150 ml
1000 ml
10/min
10,000 ml/min
8500 ml/min
85%
III—Rapid, shallow breathing
150 ml
250 ml
40/min
10,000 ml/min
4000 ml/min
40%
*Respiratory rate values are artificially adjusted to provide equivalent minute ventilation as a baseline for comparing alveolar ventilation.
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