Most oxygen
(98.5%)
binds to hemoglobin,
a protein in red blood cells; one hemoglobin mol-
ecule can bind four oxygen molecules. The result-
ing molecule, oxyhemoglobin, has a bright red color,
leading to the characteristic appearance of arte-
rial blood samples. Because hemoglobin has a very
high
a ffin ity fo r
(attraction to) oxygen, the blood can
sometimes still contain a very high level of O2, even
when there is a very low partial pressure of O2.
Figure 13.6 illustrates oxygen transport as
well as our next topic, carbon dioxide transport.
Most Carbon Dioxide Must
Be Converted to Bicarbonate
to Be Moved to the Lungs
Carbon dioxide travels through the blood
in the plasma (7%), bound to hemoglobin
(23%) to form carbaminohemoglobin, or
as bicarbonate ions (70%). When carbon
dioxide from the cells diffuses into red
blood cells in systemic capillaries, an en-
zyme called carbonic anhydrase com-
bines it with water to form carbonic acid
(H2CO3). Carbonic acid readily breaks
apart into bicarbonate ions (HCO3-) and
hydrogen ions (H+). Bicarbonate ions,
which are extremely soluble, quickly
move out of red blood cells and into
plasma. The hydrogen ions are rapidly
picked up by the hemoglobin molecule
to prevent shifts in the pH of the red
blood cells. Because venous blood has
more hemoglobin bound to CO2
or hy-
drogen and less that is still bound to
O2, venous blood is a maroon color.
When the blood enters the pulmo-
nary capillaries, the high PO
and low PCO
cause hemoglobin to pick up oxygen and
release carbon dioxide and hydrogen ions.
The hydrogen ions recombine with bicar-
bonate ions to form carbonic acid. Carbonic
anhydrase changes the carbonic acid into
water and carbon dioxide, which diffuses out
of the red blood cells and into the alveoli.
The
transformation
of
bicarbonate
back into carbon dioxide is necessary to
dispose of the carbon dioxide, as the highly
soluble bicarbonate would never leave the
blood. Carbon dioxide readily diffuses across the
membrane and into the air.
T r a n s p o r t o f o x y g e n a n d c a r b o n
d i o x i d e in t h e b l o o d
F ig u r e 1 3 . 6
In external respiration, carbon dioxide is released from hemoglobin in
exchange for oxygen, and bicarbonate is transformed back into carbon
dioxide. In internal respiration, the opposite chemical changes occur: He-
moglobin lets go of oxygen in exchange for carbon dioxide and hydrogen
ions, and bicarbonate is formed from carbon dioxide.
Transport of
CO 2
7% dissolved in plasma
23% as Hb-CO2 (carbaminohemoglobin)
70% as HCO3- (bicarbonate ions)
Transport of O 2
1.5% dissolved in plasma
98.5% as Hb-O2 (oxyhemoglobin)
To lungs
To right atrium
b. Internal respiration:
system ic gas
exchange
OL
(dissolved)
To left atrium
f .
To tissue cells
Systemic
capillaries
Systemic
tissue cells
384 CHAPTER 13
The Respiratory System
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