Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 35Pulmonary Function 599

The frictional resistance to air movement is relatively small
during quiet breathing, but it does cause the intrapleural pres-
sure changes to lead the lung volume changes during inspira-
tion and expiration (Figure 35–6), producing a hysteresis loop
rather than a straight line when pressure is plotted against vol-
ume (Figure 35–15). In this diagram, area AXBYA represents
the work done to overcome airway resistance and lung viscos-
ity. If the air flow becomes turbulent during rapid respiration,
the energy required to move the air is greater than when the
flow is laminar.
Estimates of the total work of quiet breathing range from
0.3 up to 0.8 kg-m/min. The value rises markedly during exer-
cise, but the energy cost of breathing in normal individuals
represents less than 3% of the total energy expenditure during
exercise. The work of breathing is greatly increased in diseases
such as emphysema, asthma, and congestive heart failure with
dyspnea and orthopnea. The respiratory muscles have length–
tension relations like those of other skeletal and cardiac mus-
cles, and when they are severely stretched, they contract with
less strength. They can also become fatigued and fail (pump
failure), leading to inadequate ventilation.

DIFFERENCES IN VENTILATION & BLOOD

FLOW IN DIFFERENT PARTS OF THE LUNG

In the upright position, ventilation per unit lung volume is
greater at the base of the lung than at the apex. The reason for
this is that at the start of inspiration, intrapleural pressure is
less negative at the base than at the apex (Figure 35–16), and
since the intrapulmonary intrapleural pressure difference is
less than at the apex, the lung is less expanded. Conversely, at
the apex, the lung is more expanded; that is, the percentage of
maximum lung volume is greater. Because of the stiffness of
the lung, the increase in lung volume per unit increase in pres-
sure is smaller when the lung is initially more expanded, and

ventilation is consequently greater at the base. Blood flow is

also greater at the base than the apex. The relative change in

blood flow from the apex to the base is greater than the relative

change in ventilation, so the ventilation/perfusion ratio is low

at the base and high at the apex.

The ventilation and perfusion differences from the apex to

the base of the lung have usually been attributed to gravity;

they tend to disappear in the supine position, and the weight

of the lung would be expected to make the intrapleural pres-

sure lower at the base in the upright position. However, the

inequalities of ventilation and blood flow in humans were

found to persist to a remarkable degree in the weightlessness

of space. Therefore, other factors also play a role in producing

the inequalities.

DEAD SPACE & UNEVEN VENTILATION

Because gaseous exchange in the respiratory system occurs

only in the terminal portions of the airways, the gas that occu-

pies the rest of the respiratory system is not available for gas

exchange with pulmonary capillary blood. Normally, the vol-

ume (in mL) of this anatomic dead space is approximately

equal to the body weight in pounds. As an example, in a man

who weighs 150 lb (68 kg), only the first 350 mL of the 500 mL

inspired with each breath at rest mixes with the air in the alve-

oli. Conversely, with each expiration, the first 150 mL expired

is gas that occupied the dead space, and only the last 350 mL is

gas from the alveoli. Consequently, the alveolar ventilation;

FIGURE 35–15 Pressure volume relationships in breathing.
Diagrammatic representation of pressure and volume changes during
quiet inspiration (line AXB) and expiration (line BZA). Line AYB is the
compliance line.

− 2

0

− 4 − 6

500

Z

Y

X

A

Intrapleural pressure

(mm Hg)

Tidal volume (mL)

CB

FIGURE 35–16 Intrapleural pressures in the upright

position and their effect on ventilation. Note that because intrapul-

monary pressure is atmospheric, the more negative intrapleural pres-

sure at the apex holds the lung in a more expanded position at the

start of inspiration. Further increases in volume per unit increase in in-

trapleural pressure are smaller than at the base because the expanded

lung is stiffer. (Reproduced with permission from West JB: Ventilation/Blood Flow

and Gas Exchange, 3rd ed. Blackwell, 1977.)

+10 0

0

50%

100%

–10 –20 –30

Lung volume

Intrapleural pressure (cm H 2 O)

–2.5 cm H 2 O

–10 cm H 2 O

Intrapleural

pressure