Human Physiology, 14th edition (2016)

540 Chapter 16

required for subsequent breaths. However, if the baby is pre-

mature and its lungs have not matured sufficiently to produce

surfactant, that effort must be duplicated with every breath.

Surfactant and Respiratory

Distress Syndrome

Alveolar fluid contains a substance that reduces surface tension.

This substance is called surfactant —a contraction of surface

active agent. Surfactant is secreted into the alveoli by type II

alveolar cells ( fig.  16.11 ), and consists of phospholipids—

primarily phosphatidylcholine and phosphatidylglycerol —together

with hydrophobic surfactant proteins. Surfactant becomes inter-

spersed between water molecules at the water– air interface; this

reduces the hydrogen bonds between water molecules (shown in

chapter 2, fig. 2.7) at the surface and thereby reduces the sur-

face tension. As a result of this effect of pulmonary surfactant,

the surface tension of the alveoli is negligible. The surfactant

secreted into the alveoli by type II alveolar cells is removed by

alveolar macrophages.

The ability of surfactant to lower surface tension improves

as the alveoli get smaller during expiration. This may be because

the surfactant molecules become more concentrated as the alve-

oli get smaller. Surfactant thus prevents the alveoli from col-

lapsing during expiration, as would be predicted from the law

of Laplace. Even after a forceful expiration, the alveoli remain

open and a residual volume of air remains in the lungs. Since the

alveoli do not collapse, less surface tension has to be overcome

to inflate them at the next inspiration.

Surfactant begins to be produced in late fetal life. Even so,

a normal newborn with partially collapsed alveoli must over-

come great surface tension forces to take the first breath of life;

it must create a transpulmonary pressure 15 to 20 times that

Figure 16.11 The production of pulmonary

surfactant. Produced by type II alveolar cells, surfactant

appears to be composed of a derivative of lecithin combined

with protein.

Surfactant

Alveolus

Macrophage

Type II

alveolar cell

Capillary

endothelium

Basal
lamina

Type I

alveolar cell

CLINICAL APPLICATION

Respiratory distress syndrome ( RDS ), where the alveoli

are collapsed in a neonate due to lack of surfactant, occurs

in 60% of babies born at less than 28 weeks of gestation

(normal pregnancy lasts 37 to 42 weeks), 30% of babies

born at 28 to 34 weeks, and less than 5% of babies born

after 34 weeks. Most of these babies can be saved by the

use of mechanical ventilators and by exogenous surfactant

delivered into the baby’s lungs by an endotracheal tube.

When the baby’s lungs are sufficiently mature, they manu-

facture their own surfactant.

People with lung injury caused by septic shock may

develop acute respiratory distress syndrome ( ARDS ).

Inflammation causes increased capillary and alveolar perme-

ability, producing a protein-rich fluid in the lungs. This reduces

lung compliance, and surfactant production, and the decreased

surfactant further reduces compliance. This produces hypox-

emia (low blood oxygen) of the blood leaving the lungs. Severe

hypoxemia can be fatal, but survival has improved in recent

years due to improvements in medical interventions.

LEARNING OUTCOMES

After studying this section, you should be able to:

5. Explain how inspiration and expiration are
   accomplished.


6. Describe lung volumes and capacities, and explain
   how pulmonary function tests relate to pulmonary
   disorders.

| CHECKPOINT

3. Describe the changes in the intrapulmonary and
   intrapleural pressures that occur during inspiration,
   and use Boyle’s law to explain the reasons for these
   changes.
   4a. Explain how the compliance and elasticity of the
   lungs affect inspiration and expiration.
   4b. Describe pulmonary surfactant and discuss its
   significance.

16.3 MECHANICS OF BREATHING

Normal, quiet inspiration results from muscle contraction,

and normal expiration from muscle relaxation and elastic

recoil. The amount of air inspired and expired can be mea-

sured in a number of ways to test pulmonary function.