++
The equilibrium point of the reversible reaction of hemoglobin and oxygen is, of course, dependent on how much oxygen the hemoglobin in blood is exposed to.

This corresponds directly to the partial pressure of oxygen in the plasma under the conditions in the body. Thus, the

of the plasma
determines the amount of oxygen that binds to the hemoglobin in the erythrocytes.
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The Oxyhemoglobin Dissociation Curve
++
One way to express the proportion of hemoglobin that is bound to oxygen is as percent saturation. This is equal to the content of oxygen in the blood (minus that part physically dissolved) divided by the oxygen-carrying capacity of the hemoglobin in the blood times 100%:
+
++
Note that the oxygen-carrying capacity of an individual depends on the amount of hemoglobin in that person’s blood. The blood oxygen content also depends on the amount of hemoglobin present (as well as on the

). Both content and capacity are expressed as milliliters of oxygen per 100 mL of blood. On the other hand, the percent hemoglobin saturation expresses only a percentage and not an amount or volume of oxygen. Therefore, “percent saturation” is not interchangeable with “oxygen content.” For example, 2 patients might have the same percent of hemoglobin saturation, but if one has a lower blood hemoglobin concentration because of anemia, he or she will have a lower blood oxygen content.
++
The relationship between the

of the plasma and the percent of hemoglobin saturation is demonstrated graphically as the
oxyhemoglobin dissociation curve. An oxyhemoglobin dissociation curve for normal blood is shown in
Figure 7–1.
++
++
The oxyhemoglobin dissociation curve is really a plot of how the availability of one of the reactants, oxygen (expressed as the

of the plasma), affects the reversible chemical reaction of oxygen and hemoglobin. The product, oxyhemoglobin, is expressed as percent saturation—really a percentage of the maximum for any given amount of hemoglobin.
++
As can be seen in Figure 7–1, the relationship between

and HbO
2 is not linear; it is an S-shaped curve, steep at the lower

and nearly flat when the

is above 70 mm Hg.

It is this S shape that is responsible for several very important physiologic properties of the reaction of oxygen and hemoglobin. The reason that the curve is S-shaped and not linear is that it is actually a plot of 4 reactions rather than 1. That is, each of the 4 subunits of hemoglobin can combine with 1 molecule of oxygen. Indeed, it may be more correct to write the following equation:
+
++
The reactions of the 4 subunits of hemoglobin with oxygen do not appear to occur simultaneously. Instead they are believed to occur sequentially in 4 steps, with an interaction between the subunits occurring in such a way that during the successive combinations of the subunits with oxygen, each combination facilitates the next (“positive cooperativity”). Similarly, dissociation of oxygen from hemoglobin subunits facilitates further dissociations. The dissociation curve for a single monomer of hemoglobin is far different from that for the tetramer (see Figure 7–4C).
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As already stated, for hemoglobin to participate in the transport of oxygen from the lungs to the tissues, it must combine with oxygen in the pulmonary capillaries and then release oxygen to the metabolizing tissues in the systemic capillaries. The oxyhemoglobin dissociation curve in Figure 7–1 shows how this is accomplished.
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Loading Oxygen in the Lung
++
Mixed venous blood entering the pulmonary capillaries normally has a

of about 40 mm Hg, as discussed in
Chapter 5. At a

of 40 mm Hg, hemoglobin is about 75% saturated with oxygen, as seen in
Figure 7–1. Assuming a blood hemoglobin concentration of 15 g Hb/100 mL of blood, this corresponds to 15.08 mL O
2/100 mL of blood
bound to hemoglobin plus an additional 0.12 mL O
2/100 mL of blood
physically dissolved, or a
total oxygen content of approximately 15.2 mL O
2/100 mL of blood.
++
Oxygen-carrying capacity is
+
++
Oxygen bound to hemoglobin at a

of 40 mm Hg (37°C, pH 7.4) is
+
++
Oxygen physically dissolved at a

of 40 mm Hg is
+
++
Total blood oxygen content at a

of 40 mm Hg (37°C, pH 7.4) is
+
++
As the blood passes through the pulmonary capillaries, it equilibrates with the alveolar

of about 100 mm Hg. At a

of 100 mm Hg, hemoglobin is about 97.4% saturated with oxygen, as seen in
Figure 7–1. This corresponds to 19.58 mL O
2/100 mL of
blood bound to hemoglobin plus 0.3 mL O
2/100 mL of blood
physically dissolved, or a
total oxygen content of 19.88 mL O
2/100 mL of blood.
++
Oxygen bound to hemoglobin at a

of 100 mm Hg (37°C, pH 7.4) is
+
++
Oxygen physically dissolved at a

of 100 mm Hg is
+
++
Total blood oxygen content at a

of 100 mm Hg (37°C, pH 7.4) is
+
++
Thus, in passing through the lungs, each 100 mL of blood has loaded (19.88 – 15.20) mL O2, or 4.68 mL O2. Assuming a cardiac output of 5 L/min, this means that approximately 234 mL O2 is loaded into the blood per minute:
+
++
Note that the oxyhemoglobin dissociation curve is relatively flat when

is greater than approximately 70 mm Hg. This is very important physiologically because it means that there is only a small decrease in the
oxygen content of blood equilibrated with a

of 70 mm Hg instead of 100 mm Hg. In fact, the curve shows that at a

of 70 mm Hg, hemoglobin is still approximately 94.1% saturated with oxygen. This constitutes an important safety factor because a patient with a relatively low alveolar or arterial

of 70 mm Hg (owing to hypoventilation or intrapulmonary shunting, for example) is still able to load oxygen into the blood with little difficulty. A quick calculation shows that at 70 mm Hg the total blood oxygen content is approximately 19.12 mL O
2/100 mL of blood compared with the 19.88 mL O
2/100 mL of blood at a

of 100 mm Hg. These calculations show that

is often a more sensitive diagnostic indicator of the status of a patient’s respiratory system than is the arterial oxygen content. Of course, the oxygen content is more important physiologically to the patient.
++
It should also be noted that since hemoglobin is approximately 97.4% saturated at a

of 100 mm Hg, raising the alveolar

above 100 mm Hg can combine little additional oxygen with hemoglobin (only about 0.52 mL O
2/100 mL of blood at a hemoglobin concentration of 15 g/100 mL of blood). Hemoglobin is fully saturated with oxygen at a

of about 250 mm Hg.
+++
Unloading Oxygen at the Tissues
++
As blood passes from the arteries into the systemic capillaries, it is exposed to lower

, and oxygen is released by the hemoglobin. The

in the capillaries varies from tissue to tissue, being very low in some (eg, myocardium) and relatively higher in others (eg, kidney). As can be seen in
Figure 7–1, the oxyhemoglobin dissociation curve is very steep in the range of 40 to 10 mm Hg. This means that a small decrease in

can result in a substantial further dissociation of oxygen and hemoglobin, unloading more oxygen for use by the tissues. At a

of 40 mm Hg, hemoglobin is about 75% saturated with oxygen, with a total blood oxygen content of 15.2 mL O
2/100 mL of blood (at 15 g Hb/100 mL of blood). At a

of 20 mm Hg, hemoglobin is only 32% saturated with oxygen. The total blood oxygen content is only 6.49 mL O
2/100 mL of blood, a decrease of 8.71 mL O
2/100 mL of blood for only a 20-mm Hg decrease in

.
++
The unloading of oxygen at the tissues is also facilitated by other physiologic factors that can alter the shape and position of the oxyhemoglobin dissociation curve. These include the pH,

, temperature of the blood, and concentration of 2,3-BPG (2,3 bisphosphoglycerate) in the erythrocytes.