HUMAN BIOLOGY

respiration Processes that
together deliver oxygen
from the air to body cells
and move waste carbon
dioxide to the outside.

respiratory surface The
thin, moist surface across
which oxygen and carbon
dioxide diffuse during res-
piration; the thin walls of
alveoli in the lungs provide
this surface.

respiration 5 Gas Exchange

only diffuse rapidly over short distances. The respiratory

surface must be moist because gases can’t diffuse across

it unless they are dissolved in fluid. The thin walls of the

millions of alveoli in the lungs meet these requirements.

Oxygen and carbon dioxide also move between body

cells and tissue fluid. Blood circulated by the cardiovascu-

lar system carries these gases to and from the tissue fluid

that bathes cells (Figure 10.6B).

Two factors affect how many gas molecules can move

into and out of lung alveoli in a given period of time. The

first is surface area, and the second is the partial pressure

gradient across it. Diffusion occurs faster when the surface

area is large and the gradient is steep. The millions of

alveoli in your lungs provide a huge surface area for gas

exchange. As we see next, the interaction between hemo-

globin and oxygen helps maintain a steep gradient that in

turn helps bring oxygen into the lungs.

n All living cells in the body rely on respiration to supply them

with oxygen and dispose of carbon dioxide wastes.

n Links to Mitochondria 3.8, Cellular respiration 3.15

Chapter 3 described aerobic cellular respiration—the

process inside cell mitochondria that uses oxygen and pro-

duces carbon dioxide wastes that enter the bloodstream.

Respiration, in contrast, refers to overall gas exchange: the

processes that deliver oxygen in inhaled air to body cells and

remove waste carbon dioxide from the body (Figure 10.4).

in gas exchange, oxygen and carbon dioxide

diffuse down a pressure gradient

Gas exchange in the body relies on the tendency of oxy-

gen and carbon dioxide to diffuse down their respective

concentration gradients—or, as we say for gases, their

pressure gradients. When molecules

of either gas are more concentrated

outside the body, they tend to move

inside and vice versa.

At sea level air is about 78 per-

cent nitrogen, 21 percent oxygen,

0.04 percent carbon dioxide, and

0.96 percent other gases. Atmos-

pheric pressure there is about

760 mm Hg, as measured by a mer-

cury barometer (Figure 10.5). Each

gas accounts for only part of the

total pressure exerted by the whole mix of gases. Oxygen’s

partial pressure is 21 percent of 760, about 160 mm Hg.

Carbon dioxide’s partial pressure is about 0.3 mm Hg.

Gases are exchanged across a thin, moist

respiratory surface

Meeting the metabolic needs of a large, active animal such

as a human requires extremely efficient gas exchange. Vari-

ous factors influence the process. To start with, gases enter

and leave the body by crossing a respiratory surface of

thin, moist epithelium (Figure 10.6A). The surface must

be thin—at most, one or two cells thick—because gases

O 2

CO 2

Cellular respiration

in mitochondria

Whole body

respiration

O 2

CO 2

Figure 10.4 Respiration is the exchange of inhaled oxygen

for waste carbon dioxide, which is exhaled. (© Cengage Learning)

78% N 2

Partial pressure of

N 2 = 600 mm Hg

21% O 2

Partial pressure of

O 2 = 160 mm Hg

1% CO 2 , other gases

760 mm Hg

Total atmospheric pressure = 760 mm Hg

Figure 10.5 Each gas in air exerts part of the total air

pressure. This is the meaning of “partial pressure.” Hg is the

chemical symbol for the element mercury. (© Cengage Learning)

cells of the

respiratory

surface

other

body

cells

External environment Internal environment

(tissue fluid)

O 2

CO 2

a Cells of the respiratory

surface exchange gases

with both the external and

internal environment.

B Other body cells

exchange gases with the

internal environment.

Figure 10.6 Gases are exchanged in the lungs and in tissues.

(© Cengage Learning)

10.2

10.3

180 ChapteR 10

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).