The Respiratory System
In healthy people, AVR is usually about 12 breaths per minute
times the diﬀerence of 500
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.
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
reﬂex activity, but some are produced voluntarily.
scribes the most common of these movements.
Check Your Understanding
Resistance in the airways is typically low. Why? (Give at least
Premature infants often lack adequate surfactant. How does
this affect their ability to breathe?
Explain why slow, deep breaths ventilate the alveoli more
effectively than do rapid, shallow breaths.
For answers, see Appendix H.
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
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
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
obstructive pulmonary disease
increased airway resistance (such as chronic bronchitis) and
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
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
. ±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.
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
Minute ventilation values provide a rough yardstick for as-
sessing respiratory eﬃciency, but the
alveolar ventilation rate
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
Effects of Breathing Rate and Depth on Alveolar Ventilation of Three Hypothetical Patients
BREATHING PATTERN OF
I—Normal rate and depth
II—Slow, deep breathing
III—Rapid, shallow breathing
*Respiratory rate values are artiﬁcially adjusted to provide equivalent minute ventilation as a baseline for comparing alveolar ventilation.