Physiology 5th Ed.

OXYGEN TRANSPORT IN BLOOD

Forms of O2 in Blood

O2 is carried in two forms in blood: dissolved and bound to hemoglobin. Dissolved O2 alone is inadequate to meet the metabolic demands of the tissues; thus, a second form of O2, combined with hemoglobin, is needed.

Dissolved O2

Dissolved O2 is free in solution and accounts for approximately 2% of the total O2 content of blood. Recall that dissolved O2 is the only form of O2 that produces a partial pressure, which, in turn, drives O2diffusion. (In contrast, O2 bound to hemoglobin does not contribute to its partial pressure in blood.) As described by Henry’s law, the concentration of dissolved O2 is proportional to the partial pressure of O2; the proportionality constant is simply the solubility of O2 in blood, 0.003 mL O2/100 mL blood/mm Hg. Thus, for a normal image of 100 mm Hg, the concentration of dissolved O2 is 0.3 mL O2/100 mL (100 mm Hg × 0.003 mL O2/100 mL blood/mm Hg).

At this concentration, dissolved O2 is grossly insufficient to meet the demands of the tissues. For example, in a person at rest, O2 consumption is about 250 mL O2/min. If O2 delivery to the tissues were based strictly on the dissolved component, then 15 mL O2/min would be delivered to the tissues (O2 delivery = cardiac output × dissolved O2 concentration, or 5 L/min × 0.3 mL O2/100 mL = 15 mL O2/min). Clearly, this amount is insufficient to meet the demand of 250 mL O2/min. An additional mechanism for transporting large quantities of O2 in blood is needed—that mechanism is O2 bound to hemoglobin.

O2 Bound to Hemoglobin

The remaining 98% of the total O2 content of blood is reversibly bound to hemoglobin inside the red blood cells. Hemoglobin is a globular protein consisting of four subunits. Each subunit contains a heme moiety, which is an iron-binding porphyrin, and a polypeptide chain, which is designated either α or β. Adult hemoglobin (hemoglobin A) is called α2β2; two of the subunits have α chains and two have β chains. Each subunit can bind one molecule of O2, for a total of four molecules of O2 per molecule of hemoglobin. When hemoglobin is oxygenated, it is called oxyhemoglobin; when it is deoxygenated, it is called deoxyhemoglobin. For the subunits to bind O2, iron in the heme moieties must be in the ferrous state (i.e., Fe2+).

There are several variants of the hemoglobin molecule:

image Methemoglobin. If the iron component of the heme moieties is in the ferric, or Fe3+, state (rather than the normal Fe2+ state), it is called methemoglobin. Methemoglobin does not bind O2. Methemoglobinemia has several causes including oxidation of Fe2+ to Fe3+ by nitrites and sulfonamides. There is also a congenital variant of the disease in which there is a deficiency of methemoglobin reductase, an enzyme in red blood cells that normally keeps iron in its reduced state.

image Fetal hemoglobin (hemoglobin F, HbF). In fetal hemoglobin, the two β chains are replaced by γ chains, giving it the designation of α2γ2. The physiologic consequence of this modification is that hemoglobin F has a higher affinity for O2 than hemoglobin A, facilitating O2 movement from the mother to the fetus. Hemoglobin F is the normal variant present in the fetus and is replaced by hemoglobin A within the first year of life.

image Hemoglobin S. Hemoglobin S is an abnormal variant of hemoglobin that causes sickle cell disease. In hemoglobin S, the α subunits are normal and the β subunits are abnormal, giving it the designation αAS2. In its deoxygenated form, hemoglobin S forms sickle-shaped rods in the red blood cells, distorting the shape of the red blood cells (i.e., sickling them). This deformation of the red blood cells can result in occlusion of small blood vessels. The O2 affinity of hemoglobin S is less than the O2 affinity of hemoglobin A.

O2-Binding Capacity and O2 Content

Because the majority of O2 transported in blood is reversibly bound to hemoglobin, the O2 content of blood is primarily determined by the hemoglobin concentration and by the O2-binding capacity of that hemoglobin.

The O2-binding capacity is the maximum amount of O2 that can be bound to hemoglobin per volume of blood, assuming that hemoglobin is 100% saturated (i.e., all four heme groups on each molecule of hemoglobin are bound to O2). The O2-binding capacity is measured by exposing blood to air with a high PO2 (so that hemoglobin will be 100% saturated) and by correcting for the small amount of O2 that is present in the dissolved form. (To correct for dissolved O2, remember that the solubility of O2 in blood is 0.003 mL O2/100 mL blood/mm Hg.) Other information needed to calculate the O2-binding capacity is that 1 g of hemoglobin A can bind 1.34 mL O2 and that the normal concentration of hemoglobin A in blood is 15 g/100 mL. The O2-binding capacity of blood is therefore 20.1 mL O2/100 mL blood (15 g/100 mL × 1.34 mL O2/g hemoglobin = 20.1 mL O2/100 mL blood).

The O2 content is the actual amount of O2 per volume of blood. The O2 content can be calculated from the O2-binding capacity of hemoglobin and the percent saturation of hemoglobin, plus any dissolved O2. (Recall that O2-binding capacity is determined at 100% saturation with all heme groups bound to O2 on all hemoglobin molecules.)

image

where

O2 content

= Amount of O2 in blood (mL O2/100 mL blood)

O2-binding capacity

= Amount of O2 bound to hemoglobin (mL O2/100 mL blood) measured at 100% saturation

Percent saturation

= % of heme groups bound to O2

Dissolved O2

= Unbound O2 in blood (mL O2/100 mL blood)

SAMPLE PROBLEM. A man who is anemic has a severely reduced hemoglobin concentration of 10 g/100 mL blood. Assuming that the patient has normal lungs and that the values of both image and image are normal at 100 mm Hg, what is the O2 content of his blood, and how does that value compare with the normal value? Assume that for a normal hemoglobin concentration of 15 g/100 mL, the O2-binding capacity is 20.1 mL O2/100 mL blood, and that hemoglobin is 98% saturated at a image of 100 mm Hg.

SOLUTION. (1) First, calculate the O2-binding capacity (the maximum amount of O2 that can be bound to hemoglobin) at a hemoglobin concentration of 10 g/100 mL blood. It is a given that at a normal hemoglobin concentration of 15 g/100 mL, O2-binding capacity is 20.1 mL O2/100 mL blood. Thus, at a hemoglobin concentration of 10 g/100 mL, O2-binding capacity is 10/15 of normal. Thus,

image

(2) Next, calculate the actual amount of O2 combined with hemoglobin by multiplying the O2-binding capacity by the % saturation. Thus,

image

(3) Finally, determine the total O2 content by calculating the dissolved O2 at image of 100 mm Hg and adding that amount to the O2 bound to hemoglobin. The solubility of O2 in blood is 0.003 mL O2/100 mL/mm Hg. Thus,

image

image

An O2 content of 13.4 mL O2/100 mL blood is severely depressed. Compare this value with the O2 content of 20.0 mL O2/100 mL blood calculated at the normal hemoglobin concentration of 15 g/100 mL and 98% saturation. (Bound O2 is 20.1 mL O2/100 mL × 98% = 19.7 mL O2/100 mL, and dissolved O2 is 0.3 mL O2/100 mL. Thus, normal total O2 content is the sum, or 20.0 mL O2/100 mL blood.)

O2 Delivery to Tissues

The amount of O2 delivered to tissues is determined by blood flow and the O2 content of blood. In terms of the whole organism, blood flow is considered to be cardiac output. O2 content of blood, as already described, is the sum of dissolved O2 (2%) and O2-hemoglobin (98%). Thus, O2 delivery is described as follows:

image

O2-Hemoglobin Dissociation Curve

As a review, recall that O2 combines reversibly and rapidly with hemoglobin, binding to heme groups on each of the four subunits of the hemoglobin molecule. Each hemoglobin molecule, therefore, has the capacity to bind four molecules of O2. In this configuration, saturation is 100%. If fewer than four molecules of O2 are bound to heme groups, then saturation is less than 100%. For example, if, on average, each hemoglobin molecule has three molecules of O2 bound, then saturation is 75%; if, on average, each hemoglobin has two molecules of O2 bound, then saturation is 50%; and if only one molecule of O2 is bound, saturation is 25%.

Percent saturation of hemoglobin is a function of the Po2 of blood, as described by the O2-hemoglobin dissociation curve (Fig. 5-20). The most striking feature of this curve is its sigmoidal shape. In other words, the percent saturation of heme sites does not increase linearly as PO2 increases. Rather, percent saturation increases steeply as PO2 increases from zero to approximately 40 mm Hg, and it then levels off between 50 mm Hg and 100 mm Hg. For convenience, Table 5-2 gives the values of percent saturation that correspond to various values of PO2.

image

Figure 5–20 O2-hemoglobin dissociation curve. P50 is the partial pressure of O2 at which hemoglobin is 50% saturated.

Table 5–2 Values of Po2 and Corresponding Values of Percent Saturation of Hemoglobin

Po2 (mm Hg)

Saturation (%)

10

25

20

35

25

50

30

60

40

75

50

85

60

90

80

96

100

98

The PO2 that corresponds to 50% saturation of hemoglobin is called P50.

Sigmoidal Shape

The shape of the steepest portion of the curve is the result of a change in affinity of the heme groups for O2 as each successive O2 molecule binds: Binding of the first molecule of O2 to a heme group increases the affinity for the second O2 molecule, binding of the second O2 molecule increases the affinity for the third O2 molecule, and so forth. Affinity for the fourth, and last, molecule of O2 is highest and occurs at values of PO2 between approximately 60 and 100 mm Hg (where saturation is nearly 100%, corresponding to four molecules of O2 per one molecule of hemoglobin). This phenomenon is described aspositive cooperativity.

P50

A significant point on the O2-hemoglobin dissociation curve is the P50. By definition, P50 is the PO2 at which hemoglobin is 50% saturated (i.e., where two of the four heme groups are bound to O2). A change in the value of P50 is used as an indicator for a change in affinity of hemoglobin for O2. An increase in P50 reflects a decrease in affinity, and a decrease in P50 reflects an increase in affinity.

Loading and Unloading of O2

The sigmoidal shape of the O2-hemoglobin dissociation curve helps to explain why O2 is loaded into pulmonary capillary blood from alveolar gas and unloaded from systemic capillaries into the tissues (Fig. 5-21). At the highest values of PO2 (i.e., in systemic arterial blood), the affinity of hemoglobin for O2 is highest; at lower values of PO2 (i.e., in mixed venous blood), affinity for O2 is lower.

image

Figure 5–21 Hemoglobin saturation as a function of PO2 in systemic arterial blood and mixed venous blood.

Alveolar air, pulmonary capillary blood, and systemic arterial blood all have a PO2 of 100 mm Hg. On the graph, a PO2 of 100 mm Hg corresponds to almost 100% saturation, with all heme groups bound to O2, and affinity for O2at its highest value due to positive cooperativity. On the other hand, mixed venous blood has a PO2 of 40 mm Hg (because O2 has diffused from systemic capillaries into the tissues). On the graph, a PO2 of 40 mm Hg corresponds to approximately 75% saturation and a lower affinity of hemoglobin for O2. Thus, the sigmoidal shape of the curve reflects changes in the affinity of hemoglobin for O2, and these changes in affinity facilitate loading of O2 in the lungs (where PO2 and affinity are highest) and unloading of O2 in the tissues (where PO2 and affinity are lower).

image In the lungs,image is 100 mm Hg. Hemoglobin is nearly 100% saturated (all heme groups are bound to O2). Due to positive cooperativity, affinity is highest and O2 is most tightly bound (the flat portion of the curve). The high affinity makes sense because it is important to have as much O2 as possible loaded into arterial blood in the lungs. Also, because O2 is so tightly bound to hemoglobin in this range, relatively less O2 is in the dissolved form to produce a partial pressure; by keeping the PO2 of pulmonary capillary blood lower than the PO2 of alveolar gas, O2 diffusion into the capillary will continue. The flat portion of the curve extends from 100 mm Hg to 60 mm Hg, which means that humans can tolerate substantial decreases in alveolar PO2 to 60 mm Hg (e.g., caused by decreases in atmospheric pressure) without significantly compromising the amount of O2 carried by hemoglobin.

image In the tissues,image is approximately 40 mm Hg, much lower than it is in the lungs. At a PO2 of 40 mm Hg, hemoglobin is only 75% saturated and the affinity for O2 is decreased. O2 is not as tightly bound in this part of the curve, which facilitates unloading of O2 in the tissues.

  The partial pressure gradient for O2 diffusion into the tissues is maintained in two ways: First, the tissues consume O2, keeping their PO2 low. Second, the lower affinity for O2 ensures that O2 will be unloaded more readily from hemoglobin; unbound O2 is free in blood, creates a partial pressure, and the PO2 of blood is kept relatively high. Because the PO2 of the tissue is kept relatively low, the partial pressure gradient that drives O2 diffusion from blood to tissues is maintained.

Changes in the O2-Hemoglobin Dissociation Curve

The O2-hemoglobin dissociation curve can shift to the right or shift to the left, as illustrated in Figure 5-22. Such shifts reflect changes in the affinity of hemoglobin for O2 and produce changes in P50. Shifts can occur with no change in O2-binding capacity, in which case the curve moves right or left, but the shape of the curve remains unchanged. Or, a right or left shift can occur in which the O2-binding capacity of hemoglobin also changes and, in this case, the shape of the curve changes.

image

Figure 5–22 Shifts of the O2-hemoglobin dissociation curve. A, Shifts to the right are associated with increased P50 and decreased affinity. B, Shifts to the left are associated with decreased P50 and increased affinity.

Shifts to the Right

Shifts of the O2-hemoglobin dissociation curve to the right occur when there is decreased affinity of hemoglobin for O2 (see Fig. 5-22A). A decrease in affinity is reflected in an increase in P50, which means that 50% saturation is achieved at a higher-than-normal value of PO2. When the affinity is decreased, unloading of O2 in the tissues is facilitated. Physiologically, the factors that cause a decrease in affinity and a right shift of the O2-hemoglobin dissociation curve are understandable: In each case, it is advantageous to facilitate unloading of O2 in the tissues.

image Increases in PCO2 and decreases in pH. When metabolic activity of the tissues increases, the production of CO2 increases; the increase in tissue PCO2 causes an increase in H+ concentration and a decrease in pH. Together, these effects decrease the affinity of hemoglobin for O2, shift the O2-hemoglobin dissociation curve to the right, and increase the P50, all of which facilitates unloading of O2 from hemoglobin in the tissues. This mechanism helps to ensure that O2 delivery can meet O2 demand (e.g., in exercising skeletal muscle). The effect of PCO2 and pH on the O2-hemoglobin dissociation curve is called the Bohr effect.

image Increases in temperature. The increases in temperature also cause a right shift of the O2-hemoglobin dissociation curve and an increase in P50, facilitating unloading of O2 in the tissues. Considering the example of exercising skeletal muscle, this effect also is logical. As heat is produced by the working muscle, the O2-hemoglobin dissociation curve shifts to the right, providing more O2 to the tissue.

image Increases in 2,3-diphosphoglycerate (2,3-DPG) concentration. 2,3-DPG is a byproduct of glycolysis in red blood cells. 2,3-DPG binds to the β chains of deoxyhemoglobin and reduces their affinity for O2. This decrease in affinity causes the O2-hemoglobin dissociation curve to shift to the right and facilitates unloading of O2 in the tissues. 2,3-DPG production increases under hypoxic conditions. For example, living at high altitude causes hypoxemia, which stimulates the production of 2,3-DPG in red blood cells. In turn, increased levels of 2,3-DPG facilitate the delivery of O2 to the tissues as an adaptive mechanism.

Shifts to the Left

Shifts of the O2-hemoglobin dissociation curve to the left occur when there is increased affinity of hemoglobin for O2 (see Fig. 5-22B). An increase in affinity is reflected in a decrease in P50, which means that 50% saturation occurs at a lower-than-normal value of PO2. When the affinity is increased, unloading of O2 in the tissues is more difficult (i.e., binding of O2 is tighter).

image Decreases in PCO2 and increases in pH. The effect of decreases in PCO2 and increases in pH is the Bohr effect again. When there is a decrease in tissue metabolism, there is decreased production of CO2, decreased H+concentration, and increased pH, resulting in a left shift of the O2-hemoglobin dissociation curve. Thus, when the demand for O2 decreases, O2 is more tightly bound to hemoglobin and less O2 is unloaded to the tissues.

image Decreases in temperature. Decreases in temperature cause the opposite effect of increases in temperature—the curve shifts to the left. When tissue metabolism decreases, less heat is produced and less O2is unloaded in the tissues.

image Decreases in 2,3-DPG concentration. Decreases in 2,3-DPG concentration also reflect decreased tissue metabolism, causing a left shift of the curve and less O2 to be unloaded in the tissues.

image Hemoglobin F. As previously described, hemoglobin F is the fetal variant of hemoglobin. The β chains of adult hemoglobin (hemoglobin A) are replaced by γ chains in hemoglobin F. This modification results in increased affinity of hemoglobin for O2, a left shift of the O2-hemoglobin dissociation curve, and decreased P50.

  The mechanism of the left shift is based on the binding of 2,3-DPG. 2,3-DPG does not bind as avidly to the γ chains of hemoglobin F as it binds to the β chains of hemoglobin A. When less 2,3-DPG is bound, the affinity for O2increases. This increased affinity is beneficial to the fetus, whose image is low (approximately 40 mm Hg).

Carbon Monoxide

All the effects on the O2-hemoglobin dissociation curve discussed thus far have involved right or left shifts. The effect of CO is different: It decreases O2 bound to hemoglobin and also causes a left shift of the O2-hemoglobin dissociation curve (Fig. 5-23).

image

Figure 5–23 Effect of carbon monoxide on the O2-hemoglobin dissociation curve. Carbon monoxide reduces the number of sites available for O2 binding to hemoglobin and causes a shift of the O2-hemoglobin dissociation curve to the left.

CO binds to hemoglobin with an affinity that is 250 times that of O2 to form carboxyhemoglobin. In other words, when the partial pressure of CO is only 1/250 that of O2, equal amounts of CO and O2 will bind to hemoglobin! Because O2 cannot bind to heme groups that are bound to CO, the presence of CO decreases the number of O2-binding sites available on hemoglobin. In the example shown in Figure 5-23, hemoglobin bound to O2 is reduced to 50%, which means that one half the binding sites would be bound to CO and one half the binding sites would be available for O2. The implications for O2 transport are obvious: This effect alone would reduce O2 content of blood and O2delivery to tissues by 50%.

CO also causes a left shift of the O2-hemoglobin dissociation curve: Those heme groups not bound to CO have an increased affinity for O2. Thus, P50 is decreased, making it more difficult for O2 to be unloaded in the tissues.

Together, these two effects of CO on O2 binding to hemoglobin are catastrophic for O2 delivery to tissues. Not only is there reduced O2-binding capacity of hemoglobin, but the remaining heme sites bind O2more tightly (Box 5-1).

BOX 5–1 Clinical Physiology: Carbon Monoxide Poisoning

DESCRIPTION OF CASE. On a cold February morning in Boston, a 55-year-old man decides to warm his car in the garage. While the car is warming, he waits in a workshop adjoining the garage. About 30 minutes later, his wife finds him tinkering at his workbench, confused and breathing rapidly. He is taken to a nearby emergency department and given 100% O2 to breathe. The following arterial blood values are measured:

image

image

image

image

EXPLANATION OF CASE. The man inhaled the exhaust fumes from his automobile and is suffering from acute carbon monoxide (CO) poisoning. The arterial blood values obtained can be explained by the effects of CO-binding to hemoglobin.

CO binds avidly to hemoglobin, with an affinity that is 250 times that of O2-binding to hemoglobin. Thus, heme groups that normally are bound to O2 now are bound to CO. The percent saturation of hemoglobin with O2 is measured as 60%, so 40% of the sites must be occupied by CO. Because O2-hemoglobin is the major form of O2 transport to the tissues, the first detrimental effect of CO poisoning is the decreased O2-carrying capacity of blood. The second detrimental effect of CO poisoning is a shift of the O2-hemoglobin dissociation curve to the left, which reduces P50 and increases the affinity of hemoglobin for what little O2 is bound. As a result, it is more difficult to unload O2 to the tissues. Together, these two effects of CO poisoning can result in death caused by a failure to deliver sufficient O2 to critical tissues such as the brain.

TREATMENT. Treatment of this patient consists of having him breathe 100% O2 in an effort to rapidly displace as much CO from hemoglobin as possible.

Notice the strikingly high value of image at 660 mm Hg. Is this value plausible? Assuming that there is no image defect, image should be equal to image because there is equilibration of pulmonary capillary blood with alveolar gas. Therefore, a better question is Why isimage660 mm Hg? The expected value for image can be calculated from the alveolar gas equation, if values are known for the PO2 of inspired air, image, and the respiratory quotient. image can be calculated from the barometric pressure (corrected for water vapor) and the percent of O2 in inspired air (100%). image is equal to image, which is given. The respiratory quotient is assumed to be 0.8. Thus,

image

image

Again, assuming that systemic arterial blood has the same PO2 as alveolar gas, and assuming that image ratios are normal, the measured value for image of 660 mm Hg is consistent with the expectedimage value of 668 mm Hg, calculated with the alveolar gas equation. This extremely high image does little to improve O2 delivery to the tissues because the solubility of O2 in blood is so low (0.003 mL O2/100 mL blood/mm Hg). Thus, at a image of 660 mm Hg, the dissolved O2 content is only 1.98 mL O2/100 mL blood.

Erythropoietin

Erythropoietin (EPO) is a glycoprotein growth factor that is synthesized in the kidneys (and to a lesser extent in the liver) and serves as the major stimulus for erythropoiesis by promoting the differentiation of proerythroblasts into red blood cells.

EPO synthesis is induced in the kidney in response to hypoxia in the following steps (Fig. 5-24).

image

Figure 5–24 Hypoxia induces synthesis of erythropoietin. The circled numbers correspond to the numbered steps in the text. EPO, Erythropoietin; mRNA, messenger RNA.

1.          When there is decreased O2 delivery to the kidneys (hypoxia), either due to decreased hemoglobin concentration or decreased image, there is increased production of the alpha subunit of hypoxia-inducible factor 1 (hypoxia-inducible factor 1α).

2.          Hypoxia-inducible factor 1α acts on fibroblasts in the renal cortex and medulla to cause synthesis of the mRNA for EPO.

3.          The mRNA directs increased synthesis of EPO.

4.          EPO then acts to cause differentiation of proerythroblasts.

5.          Proerythroblasts undergo further steps in development to form mature erythrocytes (red blood cells). These further maturation steps do not require EPO.

Interestingly, the kidneys are the ideal site for EPO synthesis because they can distinguish between decreased blood flow as a cause of decreased O2 delivery and decreased O2 content of arterial blood (e.g., due to decreased hemoglobin concentration or decreased image) as a cause of decreased O2 delivery. This distinguishing ability is based on the fact that decreased renal blood flow causes decreased glomerular filtration, which leads to decreased filtration and reabsorption of Na+. Because O2 consumption in the kidneys is strongly linked to Na+ reabsorption, decreased renal blood flow results in both decreased O2delivery and decreased O2 consumption; thus, renal O2delivery and renal O2 consumption remain matched in that scenario and, as is appropriate, the kidney is not alerted to a need for more erythrocytes. If there is decreased O2 content of arterial blood, then the kidney is alerted to a need for more erythrocytes.

Anemia is a common finding in chronic renal failure because the decrease in functioning renal mass results in decreased synthesis of EPO and decreased production of erythrocytes and the accompanying decrease in hemoglobin concentration. The anemia of chronic renal failure can be treated with recombinant human EPO.