Flow in the Lungs - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 261

The “slope” values that appear before the Δ P in the second and third relationships are

known as the compliance coefficients of the alveolar wall. Under disease conditions, these
compliance values can change significantly, and this is what effectively changes gas move-
ment from the atmosphere to blood.

9.3 PRESSURE-VOLUME RELATIONSHIP FOR AIR FLOW
IN THE LUNGS

We have briefly discussed in a previous section that air movement into the lungs is
caused by changes in the lung volume. Following Boyle's Law, the gas pressure must
change inversely with the change in volume. Here we will briefly discuss some of the
mechanics of breathing that bring about the changes in volume. Recall that we have stated
that during inspiration, the rib cage moves outward and upward and the diaphragm
moves downward. The movement of the rib cage is controlled by the intercostal muscles
and the diaphragm is a skeletal muscle. During inspiration, the diaphragm and the exter-
nal intercostal muscles contract causing an increase in the thoracic cavity volume. The con-
traction of the diaphragm accounts for approximately 75% of the air movement during
normal breathing. During strenuous activity, the increase in speed of rib movement and
the increase in rib displacement are controlled by the pectoralis minor, the scalene mus-
cles, and the sternocleidomastoid, which completely accounts for the increased volume of
the thoracic cavity. During expiration, the contraction of the internal intercostal muscles
brings the rib cage back to its normal position and the abdominal muscles contract to
assist the internal intercostal muscles and to force the diaphragm upward.
The respiratory system can adapt rapidly to the oxygen demands of the body. The rate
of breathing as well as the amount of air moved with each breath can vary significantly. It
is possible that the amount of air moved by the respiratory system can exceed 50 times the
normal breathing capacity during strenuous exercise. The respiratory rate is defined as the
number of breaths that are taken within 1 minute. Under normal resting conditions, this is
close to 12 to 15 breaths per minute. The tidal volume is the amount of air that is inhaled
during one breath, and this is approximately 500 mL for an average adult. Therefore, at
rest, the amount of air moved into the lungs per minute is 6000 mL to 7500 mL. Of the
500 mL of air that enters the respiratory system during each breath, only approximately
350 mL of this enters the alveolar space. The remaining 150 mL fills up the dead space
(nose, trachea, and bronchi, among others) within the conducting system and is never
used during gas exchange. Therefore, the alveolar space is ventilated with 4200 mL to
5250 mL of air each minute. Interestingly, the respiratory rate and the tidal volume can be
controlled independently of each other. If the respiratory rate increases to 25 breaths per
minute, then the tidal volume must drop to 300 mL to maintain the same lung ventilation
per minute (7500 mL). However, because the dead space of the conducting system does
not change, the alveolar ventilation rate drops to 3750 mL (as compared with 5250 mL)
because 150 mL stays within the dead space during each breath.
Under extreme conditions, the respiratory system can move 4.8 L of air during each
breath. This is termed the vital capacity of the lungs. The lungs never fully deflate because
the alveolar wall would adhere to itself and then it would be difficult to inflate the alveoli