Essentials of Anatomy and Physiology

350 The Respiratory System

Figure 15–5. (A) Alveolar structure showing type I and type II cells, and alveolar

macrophages. The respiratory membrane: the structures and substances through which

gases must pass as they diffuse from air to blood (oxygen) or from blood to air (CO 2 ).

(B) Sections of human lungs embedded in plastic. On the left is a normal adult lung; on

the right is a smoker’s lung. (Photograph by Dan Kaufman.)

QUESTION:Which cells shown here are part of the respiratory membrane? Which cells are

not, and what are their functions?

Elastin fibers

Macrophage

Type I cell

Type II

surfactant

cell

Surfactant

and tissue fluid

Alveolar epithelium

Capillary endothelium

Basement membrane

of capillary endothelium

Capillary

Red blood cells

Interstitial

space

Exhalation

Inhalation

Respiration

Oxygen (O )

Carbon dioxide

(CO )

2

2

Primary

bronchi

B

A

Alveolus

Respiratory

membrane

presses the alveoli. As intrapulmonic pressure rises
above atmospheric pressure, air is forced out of the
lungs until the two pressures are again equal.
Notice that inhalation is an active process that
requires muscle contraction, but normal exhalation is

a passive process, depending to a great extent on the

normal elasticity of healthy lungs. In other words,

under normal circumstances we must expend energy

to inhale but not to exhale (see Box 15–4: Emphy-

sema).