532
U N I T 6
Respiratory Function
U N D E R S T A N D I N G
Oxygen Transport
All body tissues rely on oxygen (O
2
) that is transported in the blood to meet
their metabolic needs. Oxygen is carried in two forms: dissolved and bound to
hemoglobin. About 98% of O
2
is carried by hemoglobin and the remaining 2%
is carried in the dissolved state. Dissolved oxygen is the only form that diffuses
across cell membranes and produces a partial pressure (PO
2
), which, in turn, drives
diffusion.The transport of O
2
involves (1) transfer from the alveoli to the pulmonary
capillaries in the lung, (2) hemoglobin binding and transport, and (3) the dissociation
from hemoglobin in the tissue capillaries.
Alveoli-to-Capillary Transfer.
In the lung, O
2
moves from the
alveoli to the pulmonary capillar-
ies as a dissolved gas. Its movement
occurs along a concentration gradi-
ent, moving from the alveoli, where
the PO
2
is about 100 mm Hg, to the
venous end of the pulmonary capil-
laries with their lesser O
2
concentra-
tion and lower PO
2
. The dissolved
O
2
moves rapidly between the alve-
oli and the pulmonary capillaries,
such that the PO
2
at the arterial end
of the capillary is almost if not the
same as that in the alveoli.
1
Lung
Alveolus
Pulmonary
capillary
Red blood cell
O
2
O
2
PO
2
HbO
2
Hemoglobin Binding and
Transport.
Oxygen, which is rel-
atively insoluble in plasma, relies
on hemoglobin for transport in
the blood. Once oxygen has dif-
fused into the pulmonary capillary,
it moves rapidly into the red blood
cells and reversibly binds to hemo-
globin to form HbO
2
. The hemo-
globin molecule contains four heme
units, each capable of attaching
an oxygen molecule. Hemoglobin
is 100% saturated when all four
units are occupied and is usually
about 97% saturated in the sys-
temic arterial blood. The capacity of
the blood to carry O
2
is dependent
both on hemoglobin levels and the
ability of the lungs to oxygenate the
hemoglobin.
2
α
1
α
2
β
1
β
2
Heme
O
2
