144 Chapter 8heme groupcoiled and twisted globin protein © Cengage LearninghoW does blood transport oXygen?- Hemoglobin in red blood cells carries oxygen.
- Oxygen binds to iron in heme groups in each hemoglobin
molecule. - The relative amounts of oxygen and carbon dioxide present in
blood and the temperature and acidity of tissues affect how
much oxygen hemoglobin binds—and therefore the amount of
oxygen available to tissues.
taKe-Home messageHow blood transports oxygen
linked polypeptide chains, and each chain is associated
with a heme group. It is the iron molecule at the center of
each heme group that binds oxygen. Therefore, each hemo-
globin molecule can carry four oxygen atoms.
Oxygen in the lungs diffuses into the blood plasma and
then into individual red blood cells. There it binds with
the iron in hemoglobin. This oxyhemoglobin is deep red.
Hemoglobin that is depleted of oxygen looks purplish,
especially when it is observed through skin and the walls
of blood vessels.n Transporting oxygen is a key function of blood, and the key
to oxygen transport is the protein called hemoglobin.
n Link to Protein function 2.12Hemoglobin is the oxygen carrier
If you were to analyze a liter of blood drawn from an artery,
you would find only a quarter teaspoon of oxygen dis-
solved in the plasma—just 3 milliliters. Yet, like all large,
active, warm-bodied animals, we humans require a lot
of oxygen to maintain the metabolic activity of our cells.
The protein hemoglobin (Hb) meets this need. In addition
to the small amount of dissolved
oxygen, a liter of arterial blood
usually carries around sixty-five
times more O 2 bound to the heme
groups of hemoglobin molecules.
This oxygen-bearing hemoglobin
is called oxyhemoglobin.What determines how much oxygen
hemoglobin can carry?
As conditions change in different tissues and organs, so
does the tendency of hemoglobin to bind with and hold on
to oxygen. Several factors influence this process. The most
important factor is how much oxygen is present relative to
the amount of carbon dioxide. Other factors are the tem-
perature and acidity of tissues. Hemo globin is most likely
to bind oxygen in places where blood plasma contains a
relatively large amount of oxygen, where the temperature
is relatively cool, and where the pH is roughly neutral. This
is exactly the environment in our lungs, where the blood
must take on oxygen. By contrast, metabolic activity in cells
uses oxygen. It also increases both the temperature and the
acidity (lowers the pH) of tissues. Under those conditions,
the oxyhemoglobin of red blood cells arriving in tissue cap-
illaries tends to release oxygen, which then can enter cells.
We can summarize these events this way:Hb+more O 2
cooler
less acidic
O 2 HbO 2LUNGS
less O 2
warmer
more acidicHbO 2TISSUESHb+O 2The protein portion of hemoglobin also carries some of
the carbon dioxide wastes that cells produce, along with
hydrogen ions (H+) that affect the pH of body fluids. You’ll
read more about hemoglobin in Chapter 10, where we
examine how the respiratory system carries out exchanges
of oxygen and carbon dioxide.
You can see the structure of a hemoglobin molecule in
Figure 8.3. Notice that it has two parts: the protein globin
and heme groups that contain iron. Globin is built of fourFigure 8.3 Animated! The iron in hemoglobin binds oxygen.
This diagram represents hemoglobin, which is a globular protein
that has four iron-containing heme groups. Oxygen binds to the
iron in heme groups, which is one reason why humans require
iron as a mineral nutrient.David Scharf/Peter Arnold, Inc.8.2
hemoglobin The iron-
containing protein in red
blood cells that binds to
oxygen.
oxyhemoglobin
Hemoglobin that is carrying
oxygen.
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