Human Physiology, 14th edition (2016)

Respiratory Physiology 539

decreases in size, its surface tension (the numerator in the equa-

tion) is decreased at the same time that its radius (the denomi-

nator) is reduced. The reason for the decreased surface tension,

which prevents the alveoli from collapsing, is described next.

Surface Tension

The forces that act to resist distension include elastic resistance
and the surface tension that is exerted by fluid in the alveoli.
The lungs both secrete and absorb fluid in two antagonistic pro-
cesses that normally leave only a very thin film of fluid on the
alveolar surface. Fluid absorption is driven (through osmosis)
by the active transport of Na^1 , while fluid secretion is driven
by the active transport of Cl^2 out of the alveolar epithelial
cells. Research has demonstrated that people with cystic fibro-
sis have a genetic defect in one of the Cl^2 channels (called the
cystic fibrosis transmembrane regulator, or CFTR; chapter 6,
section 6.2). This results in an imbalance of fluid absorption
and secretion, so that the airway fluid becomes excessively vis-
cous (with a lower water content) and difficult to clear.

Figure 16.9 A pneumothorax of the right lung. The

right side of the thorax appears more uniformly dark because it is

filled with air. The spaces between the ribs are also greater on the

right side due to release from the elastic tension of the lungs.

CLINICAL APPLICATION

A pneumothorax ( fig. 16.9 ) occurs when air enters the pleu-

ral space, raising the intrapleural pressure so that the pres-

sure difference keeping the lung against the chest wall is

abolished. The lung then collapses due to its elastic recoil.

A spontaneous pneumothorax may occur without disease

or trauma, but it can also be caused by air leaking from the

lung due to puncture from a broken rib or to lung disorders

such as COPD (section 16.3), cystic fibrosis, or rupture of a

lung blister. An open chest wound can also allow air to enter

the pleural space, and if air continues to enter and not leave,

it produces a serious tension pneumothorax. Because each

lung is in a separate pleural compartment, a pneumothorax

usually occurs in only one lung.

Clinical Investigation CLUES

Peter was stabbed in the chest.

• What condition did this produce, and how?


• How did the physician treat this condition?

The thin film of fluid normally present in the alveolus has
a surface tension, produced because water molecules at the sur-
face are attracted more to other water molecules than to air. As
a result, the surface water molecules are pulled tightly together
by attractive forces from underneath. This surface tension acts
to collapse the alveolus, and in the process increases the pres-
sure of the air within the alveolus. As described by the law of
Laplace, the pressure thus created is directly proportional to
the surface tension and inversely proportional to the radius of
the alveolus ( fig. 16.10 ). According to this law, the pressure in a
smaller alveolus would be greater than in a larger alveolus if the
surface tension were the same in both. The greater pressure of
the smaller alveolus would then cause it to empty its air into the
larger one. This does not normally occur because, as an alveolus

Figure 16.10 The law of Laplace. According to the

law of Laplace, the pressure created by surface tension should

be greater in the smaller alveolus than in the larger alveolus. This

implies that (without surfactant) smaller alveoli would collapse

and empty their air into larger alveoli.

P =

Law of Laplace

2 × T

r

P = P =^2 × 1 T

P = T P = 2T

2 × T

2

Bronchioles

Alveoli

r = 1

r = 2