Maintenance of the Body
simple diﬀusion. Tey are driven by the partial pressure gra-
dients of O
that exist on the opposite sides of the ex-
Check Your Understanding
You are given a sealed container of water and air. The
in the air are both 100 mm Hg. What are the
in the water? Which gas has more molecules
dissolved in the water? Why?
in the alveoli is about 56 mm Hg lower than in the
inspired air. Explain this difference.
Suppose a patient is receiving oxygen by mask. Are the
arterioles leading into the O
-enriched alveoli dilated or
constricted? What is the advantage of this response?
For answers, see Appendix H.
Transport of Respiratory
Gases by Blood
We have considered external and internal respiration consecu-
tively to emphasize their similarities, but keep in mind that it
is blood that transports O
between these two ex-
Describe how oxygen is transported in blood, and explain
how temperature, pH, BPG, and
affect oxygen loading
Molecular oxygen is carried in blood in two ways: bound to he-
moglobin within red blood cells and dissolved in plasma. Oxy-
gen is poorly soluble in water, so only about 1.5% of the oxygen
transported is carried in the dissolved form. Indeed, if this were
means of oxygen transport, a P
of 3 atm or a cardiac
output of 15 times normal would be required to provide the
oxygen levels needed by body tissues! Hemoglobin, of course,
solves this problem—98.5% of the oxygen is carried from lungs
to tissues in a loose chemical combination with hemoglobin.
Association of Oxygen and Hemoglobin
As we described in Chapter 17, hemoglobin (Hb) is composed
of four polypeptide chains, each bound to an iron-containing
heme group (see Figure 17.4). Because the iron atoms bind oxy-
gen, each hemoglobin molecule can combine with four mol-
ecules of O
, and oxygen loading is rapid and reversible.
Te hemoglobin-oxygen combination, called
bin), is written
that has released oxygen is called
, and is written
. A single reversible equa-
tion describes the loading and unloading of O
A±er the ﬁrst O
molecule binds to iron, the Hb molecule
changes shape. As a result, it more readily takes up two more
molecules, and uptake of the fourth is even more facilitated.
When one, two, or three oxygen molecules are bound, a hemo-
globin molecule is
. When all four of its heme
groups are bound to O
, the hemoglobin is
By the same token, unloading of one oxygen molecule en-
hances the unloading of the next, and so on. In this way, the
(binding strength) of hemoglobin for oxygen changes
with the extent of oxygen saturation, and both loading and un-
loading of oxygen are very eﬃcient.
Te rate at which Hb reversibly binds or releases O
lated by P
, temperature, blood pH, P
, and blood concen-
tration of an organic chemical called BPG. Tese factors interact
to ensure that adequate O
is delivered to tissue cells.
Inﬂuence of P
on Hemoglobin Saturation
hemoglobin dissociation curve
shows how local P
oxygen loading and unloading from hemoglobin.
Focus on the
Oxygen-Hemoglobin Dissociation Curve
you through this graph step by step, explaining how hemoglobin
ensures adequate oxygen delivery under a variety of conditions.
Under normal resting conditions (P
100 mm Hg), arte-
rial blood hemoglobin is 98% saturated, and 100 ml of systemic
arterial blood contains about 20 ml of O
arterial blood is written as 20 vol % (volume percent). As arte-
rial blood ﬂows through systemic capillaries, it releases about
5 ml of O
per 100 ml of blood, yielding an Hb saturation of 75%
and an O
content of 15 vol % in venous blood. Tis means that
substantial amounts of O
are normally still available in venous
), which can be used if needed.
Te nearly complete saturation of Hb in arterial blood ex-
plains why breathing deeply increases both the alveolar and
arterial blood P
but causes very little increase in the O
ration of hemoglobin. Remember, P
only the amount of O
dissolved in plasma, not the amount
bound to hemoglobin. However, P
values are a good index
of lung function, and when arterial P
is signiﬁcantly less than
some degree of respiratory impairment exists.
Inﬂuence of Other Factors on Hemoglobin Saturation
perature, blood pH, P
, and the amount of BPG in the blood
all inﬂuence hemoglobin saturation at a given P
. Red blood
cells (RBCs) produce BPG (2,3-bisphosphoglycerate) as they
break down glucose by the anaerobic process called glycolysis.
BPG binds reversibly with hemoglobin, and its levels rise when
oxygen levels are chronically low.
All of these factors inﬂuence Hb saturation by modifying
hemoglobin’s three-dimensional structure, thereby changing its
aﬃnity for O
. Generally speaking, an
, or BPG levels in blood lowers Hb’s aﬃnity for O
hancing oxygen unloading from the blood. Tis is shown by the
rightward shi± of the oxygen-hemoglobin dissociation curve in
on p. 832. (Te purple lines represent normal body
conditions, and the red lines show the shi± to the right.)
in any of these factors increases hemo-
globin’s aﬃnity for oxygen, decreasing oxygen unloading. Tis