Human Physiology, 14th edition (2016)

560 Chapter 16

light differently (has a different absorption spectrum ). This

gives oxyhemoglobin a tomato juice red color, whereas car-

boxyhemoglobin has a color similar to cranberry juice. The

oxyhemoglobin saturation is commonly measured using a

pulse oximeter (described on page 550), but it can be measured

more precisely on a sample of arterial blood using a blood-gas

machine.

with oxygen and cannot participate in oxygen transport. Blood
normally contains only a small amount of methemoglobin, but
certain drugs can increase this amount.
In carboxyhemoglobin, another abnormal form of hemo-
globin, the reduced heme is combined with carbon monoxide
instead of oxygen. Because the bond with carbon monoxide is
about 210 times stronger than the bond with oxygen, carbon
monoxide tends to displace oxygen in hemoglobin and remains
attached to hemoglobin as the blood passes through systemic
capillaries.
The percent oxyhemoglobin saturation (the percentage
of oxyhemoglobin to total hemoglobin) is measured to assess
how well the lungs have oxygenated the blood. The normal
value for arterial blood is about 97%, with varying amounts of
deoxyhemoglobin, carboxyhemoglobin, and methemoglobin
composing the remainder. The oxyhemoglobin saturation and
the proportion of these other forms can be measured because
each hemoglobin type has a unique color; each absorbs visible

Figure 16.32 The structure of hemoglobin. ( a ) An
illustration of the three-dimensional structure of hemoglobin in
which the two alpha and two beta polypeptide chains are shown.
The four heme groups are represented as flat structures with
atoms of iron ( spheres ) in the centers. ( b ) The structural formula
for heme.

Heme

CH CH CH

C C

3 2

3

2

HC C

C

CC

C C

CC

C

N

N

N

C

C

CC

C

N

CH

CH CH

CH

CH

CH

CH

CH

CH CH

CH

3

2

2

23

2

Fe

HC

COOH

COOH

Hemoglobin

Beta chain

Beta chain

Alpha chain

Alpha chain

Heme group

(b)

(a)

Clinical Investigation CLUES

Peter collapsed in his closed garage when he left his car

motor running.

• What is the likely reason for Peter’s collapse, and
  how might this have killed him if he hadn’t received
  rapid treatment?

FITNESS APPLICATION

Carbon monoxide poisoning may be suspected when the

blood carboxyhemoglobin is greater than 10% in smokers

and 3% in nonsmokers (federal clean air standards specify

that active nonsmokers should have no more than 1.5%

carboxyhemoglobin). Carbon monoxide poisoning is usually

caused by the burning of fossil fuels by faulty appliances or

vehicles within an enclosed space. Symptoms may include

headache, weakness, dizziness, nausea, and confusion;

more severe symptoms include neurological disturbances

such as memory loss, psychiatric conditions, and cardiac

injury. People may be treated for carbon monoxide poison-

ing by breathing 100% oxygen at 1 atmosphere pressure, or

by breathing 100% oxygen at greater than 1.4 atmospheres

pressure within a hyperbaric chamber.

Hemoglobin Concentration

The oxygen-carrying capacity of whole blood is determined by

its concentration of hemoglobin. If the hemoglobin concentra-

tion is below normal—in a condition called anemia —the oxy-

gen content of the blood will be abnormally low. Conversely,

when the hemoglobin concentration rises above the normal

range—as occurs in polycythemia (high red blood cell count)—

the oxygen-carrying capacity of blood is increased accordingly.

This can occur as an adaptation to life at a high altitude.

The production of hemoglobin and red blood cells in bone

marrow is controlled by the hormone erythropoietin (chapter 13,

section 13.2), produced by the kidneys in response to tissue

hypoxia. The secretion of erythropoietin—and thus the produc-

tion of red blood cells—is stimulated when the amount of oxy-

gen delivered to the kidneys is lower than normal. Red blood

cell production is also promoted by androgens, which explains

why the hemoglobin concentration is from 1 to 2 g per 100 ml

higher in men than in women.