HUMAN BIOLOGY

the RespiRatoRy system 185

hoW are blood gases exchanged

and transported?

• Oxygen and carbon dioxide diffuse into and out of capillaries
  following their partial pressure gradients.


• Hemoglobin in red blood cells greatly increases the oxygen-
  carrying capacity of the blood.


• Hemoglobin and blood plasma also carry carbon dioxide.


• In plasma, most carbon dioxide is transported in the form
  of bicarbonate.


• Buffers help prevent the blood from becoming too acid due to
  H+ being released when bicarbonate forms.

takE-homE mEssaGE

carbaminohemoglobin

Hemoglobin with carbon

dioxide bound to it.

oxyhemoglobin

Hemoglobin with oxygen

bound to it.

respiratory membrane

Two-layer membrane

between the walls of lung

capillaries and alveoli; blood

gases diffuse across it.

hemoglobin and blood plasma
both carry carbon dioxide

As you know, aerobic res piration in cells pro-
duces carbon dioxide as a waste. For this
reason, there is more carbon dioxide in metabolically active
tissues than in the blood in the nearby capil laries. So, fol-
lowing its pressure gradient, carbon dioxide diffuses into
these capillaries. It will be carried toward the lungs in three
ways. About 7 percent stays dissolved in plasma. About
another 23 percent binds with hemoglobin in red blood
cells, forming the compound carbaminohemoglobin
(HbCO 2 ). Most of the carbon dioxide, about 70 percent,
combines with water to form bicarbonate (HCO 32 ). The
reaction has two steps. First carbonic acid (H 2 CO 3 ) forms;
then it dissociates (that is, it separates) into bicarbonate ions
and hydrogen ions:

This reaction occurs in blood plasma and red blood
cells. However, it is faster in red blood cells, which contain
carbonic anhydrase. This enzyme increases the reaction
rate by at least 250 times. Newly formed bicarbonate in red
blood cells diffuses into the plasma, which will carry it to
the lungs. The reactions rapidly “sop up” carbon dioxide in
the blood and help maintain the gradient that keeps CO 2
diffusing from tissue fluid into the bloodstream.
The reactions that make bicarbonate are reversed in
alveoli, where the partial pressure of carbon dioxide is
lower than it is in surrounding capillaries. The CO 2 that
forms as the reactions go in reverse diffuses into the alveoli
and is exhaled.
If you look again at the chemical reactions outlined in
the pink shaded area above, you can see that the steps
that form bicarbonate also produce some H^1 , which makes

CO 2 + H 2 O H 2 CO 3

carbonic acid

HCO 3 –

bicarbonate

+ H+

Figure 10.14 Partial pressure gradients for oxygen and carbon

dioxide change as blood travels through the cardiovascular system.

Recall that each gas moves from higher to lower partial pressure areas.

(© Cengage Learning)

Pulmonary

veins

Pulmonary

arteries

4640

4640

>46<40

4640

27120

Dry inhaled

air

Moist exhaled

air

0.23160

Start of capillaries

in body tissues

Start of veins

in body tissues

O 2

O 2

CO 2

CO 2

alveolar

sacs

capillaries

entering

lungs

cells of

body

tissues

capillaries

entering

tissues

40100

40100 40100

40100

O 2 CO 2

O 2 CO 2

KEY

Partial pressure of CO 2 (PCO 2 )

Partial pressure of O 2 (PO 2 )

blood more acid. What happens to

these hydrogen ions? Hemoglobin

binds some of them and thus acts as

a buffer (Section 2.7). Certain pro-

teins in blood plasma also bind H^1.

These buffering mechanisms are

extremely important in homeo stasis

because they help prevent an abnor-

mal decline in blood pH.

© Cengage Learning

The four heme groups

that can bind oxygen

F i g u r e 10.13 oxyhemoglobin forms when oxygen
molecules bind to the heme groups in hemoglobin.
(© Cengage Learning)

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).