Transport of Oxygen in Blood

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Table 15-7 summarizes the oxygen content of systemic arterial blood (we shall henceforth refer to systemic arterial blood simply as arterial blood). Each liter normally contains the number of oxygen molecules equivalent to 200 ml of pure gaseous oxygen at atmospheric pressure. The oxygen is present in two forms: (1) dissolved in the plasma and erythrocyte water and (2) re-versibly combined with hemoglobin molecules in the erythrocytes.

As predicted by Henry's law, the amount of oxygen dissolved in blood is directly proportional to the PO2 of the blood. Because oxygen is relatively insoluble in water, only 3 ml can be dissolved in 1 L of blood at the normal arterial PO2 of 100 mmHg. The other 197 ml of oxygen in a liter of arterial blood, more than 98 percent of the oxygen content in the liter, is transported in the erythrocytes reversibly combined with hemoglobin.

Each hemoglobin molecule is a protein made up of four subunits bound together. Each subunit consists of a molecular group known as heme and a polypeptide attached to the heme. [Hemoglobin is not the only

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

TABLE 15-7 Oxygen Content of Systemic Arterial Blood at Sea Level

1 liter (L) arterial blood contains

3 ml O2 physically dissolved (1.5%) 197 ml O2 bound to hemoglobin (98.5%) Total 200 ml O2 Cardiac output = 5 L/min

02 carried to tissues/min = 5 L/min x 200 ml O2/L

= 1000 ml O2/min heme-containing protein in the body; others are the cytochromes and myoglobin (Chapters 4 and 11).] The four polypeptides of a hemoglobin molecule are collectively called globin. Each of the four heme groups in a hemoglobin molecule (Figure 15-22) contains one atom of iron (Fe), to which oxygen binds. Since each iron atom can bind one molecule of oxygen, a single hemoglobin molecule can bind four molecules of oxygen. However, for simplicity, the equation for the reaction between oxygen and hemoglobin is usually written in terms of a single polypeptide-heme chain of a hemoglobin molecule:

HbO,

Thus this chain can exist in one of two forms— deoxyhemoglobin (Hb) and oxyhemoglobin (HbO2).

In a blood sample containing many hemoglobin molecules, the fraction of all the hemoglobin in the form of oxyhemoglobin is expressed as the percent hemoglobin saturation:

Percent saturation :

O2 bound to Hb

Maximal capacity of Hb to bind O2

For example, if the amount of oxygen bound to hemoglobin is 40 percent of the maximal capacity of hemoglobin to bind oxygen, the sample is said to be 40 percent saturated. The denominator in this equation is also termed the oxygen-carrying capacity of the blood.

What factors determine the percent hemoglobin saturation? By far the most important is the blood PO2. Before turning to this subject, however, it must be stressed that the total amount of oxygen carried by hemoglobin in blood depends not only on the percent saturation of hemoglobin but also on how much hemoglobin there is in each liter of blood. For example, if a person's blood contained only half as much hemoglobin per liter as normal, then at any given percent saturation the oxygen content of the blood would be only half as much.

Effect of PO2 on Hemoglobin Saturation

From inspection of Equation 15-7 and the law of mass action, one can see that raising the blood PO2 should increase the combination of oxygen with hemoglobin. The experimentally determined quantitative relationship between these variables is shown in Figure 15-23,

CH CH2

COOH

Globin polypeptide

CH CH2

Role Iron The Body

COOH

COOH

FIGURE 15-22

Heme. Oxygen binds to the iron atom (Fe). Heme attaches to a polypeptide chain by a nitrogen atom to form one subunit of hemoglobin. Four of these subunits bind to each other to make a single hemoglobin molecule.

Amount of O2

-unloaded in

-

tissue

-

capillaries

-

Systemic

Systemic

/

venous

arterial

PO2

PO2

i

t

Po2 (mmHg)

FIGURE 15-23

Oxygen-hemoglobin dissociation curve. This curve applies to blood at 37°C and a normal arterial hydrogen-ion concentration. At any given blood hemoglobin concentration, the vertical axis could also have plotted oxygen content, in milliliters of oxygen. At 100 percent saturation, the amount of hemoglobin in normal blood carries 200 ml of oxygen.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Respiration CHAPTER FIFTEEN

Respiration CHAPTER FIFTEEN

which is called an oxygen-hemoglobin dissociation curve. (The term "dissociate" means "to separate," in this case oxygen from hemoglobin; it could just as well have been called an oxygen-hemoglobin association curve.) The curve is S-shaped because, as stated earlier, each hemoglobin molecule contains four subunits; each subunit can combine with one molecule of oxygen, and the reactions of the four subunits occur sequentially, with each combination facilitating the next one. (This combination of oxygen with hemoglobin is an example of cooperativity, as described in Chapter 4. The explanation in this case is as follows. The globin units of deoxyhemoglobin are tightly held by electrostatic bonds in a conformation with a relatively low affinity for oxygen. The binding of oxygen to a heme molecule breaks some of these bonds between the globin units, leading to a conformation change such that the remaining oxygen-binding sites are more exposed. Thus, the binding of one oxygen molecule to deoxy-hemoglobin increases the affinity of the remaining sites on the same hemoglobin molecule, and so on.)

Note that the curve has a steep slope between 10 and 60 mmHg PO2 and a relatively flat portion (or plateau) between 70 and 100 mmHg PO2. Thus, the extent to which oxygen combines with hemoglobin increases very rapidly as the PO2 increases from 10 to 60 mmHg, so that at a PO2 of 60 mmHg, 90 percent of the total hemoglobin is combined with oxygen. From this point on, a further increase in PO2 produces only a small increase in oxygen binding.

The importance of this plateau at higher PO2 values is as follows. Many situations, including high altitude and pulmonary disease, are characterized by a moderate reduction in alveolar and therefore arterial PO2. Even if the PO2 fell from the normal value of 100 to 60 mmHg, the total quantity of oxygen carried by hemoglobin would decrease by only 10 percent since hemoglobin saturation is still close to 90 percent at a PO2 of 60 mmHg. The plateau therefore provides an excellent safety factor in the supply of oxygen to the tissues.

The plateau also explains another fact: In a normal person at sea level, raising the alveolar (and therefore the arterial) PO2 either by hyperventilating or by breathing 100 percent oxygen adds very little additional oxygen to the blood. A small additional amount dissolves, but because hemoglobin is already almost completely saturated with oxygen at the normal arterial PO2 of 100 mmHg, it simply cannot pick up any more oxygen when the PO2 is elevated beyond this point. But this applies only to normal people at sea level. If the person initially has a low arterial PO2 because of lung disease or high altitude, then there would be a great deal of deoxyhemoglobin initially present in the arterial blood. Therefore, raising the alveolar and thereby the arterial PO2 would result in significantly more oxygen transport.

We now retrace our steps and reconsider the movement of oxygen across the various membranes, this time including hemoglobin in our analysis. It is essential to recognize that the oxygen bound to hemoglobin does not contribute directly to the PO2 of the blood. Only dissolved oxygen does so. Therefore, oxygen diffusion is governed only by the dissolved portion, a fact that permitted us to ignore hemoglobin in discussing transmembrane partial pressure gradients. However, the presence of hemoglobin plays a critical role in determining the total amount of oxygen that will diffuse, as illustrated by a simple example (Figure 15-24).

Two solutions separated by a semipermeable membrane contain equal quantities of oxygen, the gas pressures are equal, and no net diffusion occurs. Addition of hemoglobin to compartment B destroys this equilibrium because much of the oxygen combines with hemoglobin. Despite the fact that the total quantity of oxygen in compartment B is still the same, the number of dissolved oxygen molecules has decreased. Therefore, the PO2 of compartment B is less than that of A, and so there is a net diffusion of oxygen from A to B. At the new equilibrium, the oxygen pressures are once again equal, but almost all the oxygen is in compartment B and is combined with hemoglobin.

Let us now apply this analysis to capillaries of the lungs and tissues (Figure 15-25). The plasma and erythrocytes entering the lungs have a PO2 of 40 mmHg. As we can see from Figure 15-23, hemoglobin

Po2

Po2

A

B

X f \

^ X / f

Pure H2O with O2

A

B

9

X

Po2

PO2

A

B

X

t

Pure H2O with O2

Add Hb to right side

New equilibrium

FIGURE 15-24

Effect of added hemoglobin on oxygen distribution between two compartments containing a fixed number of oxygen molecules and separated by a semipermeable membrane. At the new equilibrium, the PO2 values are again equal to each other but lower than before the hemoglobin was added. However, the total oxygen, in other words, that dissolved plus that combined with hemoglobin, is now much higher on the right side of the membrane.

Adapted from Comroe.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

<Begin> Atmosphere

Inspired O2

In pulmonary capillaries

Alveoli

Dissolved O2

Plasma

Less than 1% remains dissolved in plasma

Dissolved O2

Dissolved O2 +

Erythrocytes

Less than 1% remains dissolved in erythrocytes

HbO2

In tissue capillaries

Dissolved O2 +

Cells used (in mitochondria)

Interstitial fluid

Dissolved O2

lla pil

Plasma

Dissolved O2

Erythrocytes

Dissolved O2 + Hb

HbO2

FIGURE 15-25

Oxygen movement in the lungs and tissues. Movement of inspired air into the alveoli is by bulk-flow; all movements across membranes are by diffusion. %

saturation at this PO2 is 75 percent. The alveolar PO2— 105 mmHg—is higher than the blood PO2 and so oxygen diffuses from the alveoli into the plasma. This increases plasma PO2 and induces diffusion of oxygen into the erythrocytes, elevating erythrocyte PO2 and causing increased combination of oxygen and hemoglobin. The vast preponderance of the oxygen diffusing into the blood from the alveoli does not remain dissolved but combines with hemoglobin. Therefore, the blood PO2 normally remains less than the alveolar PO2 until hemoglobin is virtually 100 percent saturated. Thus the diffusion gradient favoring oxygen movement into the blood is maintained despite the very large transfer of oxygen.

In the tissue capillaries, the procedure is reversed. Because the mitochondria of the cells all over the body are utilizing oxygen, the cellular PO2 is less than the PO2 of the surrounding interstitial fluid. Therefore, oxygen is continuously diffusing into the cells. This causes the interstitial fluid PO2 to always be less than the PO2 of the blood flowing through the tissue capillaries, and so net diffusion of oxygen occurs from the plasma within the capillary into the interstitial fluid. Accordingly, plasma PO2 becomes lower than erythrocyte PO2, and oxygen diffuses out of the erythrocyte into the plasma. The lowering of erythrocyte PO2 causes the dissociation of oxygen from hemoglobin, thereby liberating oxygen, which then diffuses out of the erythrocyte. The net result is a transfer, purely by diffusion, of large quantities of oxygen from hemo globin to plasma to interstitial fluid to the mitochondria of tissue cells.

To repeat, in most tissues under resting conditions, hemoglobin is still 75 percent saturated as the blood leaves the tissue capillaries. This fact underlies an important automatic mechanism by which cells can obtain more oxygen whenever they increase their activity. An exercising muscle consumes more oxygen, thereby lowering its tissue POr This increases the blood-to-tissue PO2 gradient and hence the diffusion of oxygen from blood to cell. In turn, the resulting reduction in erythrocyte PO2 causes additional dissociation of hemoglobin and oxygen. In this manner, an exercising muscle can extract almost all the oxygen from its blood supply, not just the usual 25 percent. Of course, an increased blood flow to the muscles (active hyperemia) also contributes greatly to the increased oxygen supply.

Carbon Monoxide and Oxygen Carriage Carbon monoxide is a colorless, odorless gas that is a product of the incomplete combustion of hydrocarbons, such as gasoline. It is one of the most common causes of sickness and death due to poisoning, both intentional and accidental. Its most striking pathophysio-logical characteristic is its extremely high affinity— 250 times that of oxygen—for the oxygen-binding sites in hemoglobin. For this reason, it reduces the amount of oxygen that combines with hemoglobin in pulmonary capillaries by competing for these sites. It

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Respiration CHAPTER FIFTEEN

Respiration CHAPTER FIFTEEN

exerts a second deleterious effect: It alters the hemoglobin molecule so as to shift the oxygen-hemoglobin dissociation curve to the left, thus decreasing the unloading of oxygen from hemoglobin in the tissues. As we shall see later, the situation is worsened by the fact that persons suffering from carbon monoxide poisoning do not show any reflex increase in their ventilation.

Effects of Blood PCO2, H+ Concentration, Temperature, and DPG Concentration on Hemoglobin Saturation

At any given PO2, a variety of other factors influence the degree of hemoglobin saturation: blood PCO2, H+ concentration, temperature, and the concentration of a substance—2,3-diphosphoglycerate (DPG) (also known as bisphosphoglycerate, BPG)—produced by the erythrocytes. As illustrated in Figure 15-26, an increase in any of these factors causes the dissociation curve to shift to the right, which means that, at any given PO2, hemoglobin has less affinity for oxygen. In contrast, a decrease in any of these factors causes the dissociation curve to shift to the left, which means that, at any given PO2, hemoglobin has a greater affinity for oxygen.

The effects of increased PCO2, H+ concentration, and temperature are continuously exerted on the blood in tissue capillaries, because each of these factors is higher in tissue-capillary blood than in arterial blood: The PCO2 is increased because of the carbon dioxide entering the blood from the tissues. For reasons to be described later, the H+ concentration is elevated because of the elevated PCO2 and the release of metabol-ically produced acids such as lactic acid. The temperature is increased because of the heat produced by tissue metabolism. Therefore, hemoglobin exposed to this elevated blood PCO2, H+ concentration, and temperature as it passes through the tissue capillaries has its affinity for oxygen decreased, and therefore hemoglobin gives up even more oxygen than it would have if the decreased tissue-capillary PO2 had been the only operating factor.

The more active a tissue is, the greater its PCO2, H+ concentration, and temperature. At any given PO2, this causes hemoglobin to release more oxygen during passage through the tissue's capillaries and provides the more active cells with additional oxygen. Here is another local mechanism that increases oxygen delivery to tissues that have increased metabolic activity.

What is the mechanism by which these factors influence hemoglobin's affinity for oxygen? Carbon dioxide and hydrogen ions do so by combining with the globin portion of hemoglobin and altering the conformation of the hemoglobin molecule. Thus, these effects are a form of allosteric modulation (Chapter 4).

100 r

Oxygen Transport Steps Hemoglobin Low Temperature

100 r

Low —

acidity

-Normal

arterial

acidity

I//

-High acidity

r

Effect of acidity i i

Po2 (mmHg)

Po2 (mmHg)

FIGURE 15-26

Effects of DPG concentration, temperature, and acidity on the relationship between PO2 and hemoglobin saturation. The temperature of normal blood, of course, never diverges from 37°C as much as shown in the figure, but the principle is still the same when the changes are within the physiological range.

Adapted from Comroe.

An elevated temperature also decreases hemoglobin's affinity for oxygen by altering its molecular configuration.

Erythrocytes contain large quantities of DPG, which is present in only trace amounts in other mammalian cells. DPG, which is produced by the erythrocytes

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions during glycolysis, binds reversibly with hemoglobin, al-losterically causing it to have a lower affinity for oxygen (Figure 15-26). The net result is that whenever DPG levels are increased, there is enhanced unloading of oxygen from hemoglobin as blood flows through the tissues. Such an increase in DPG concentration is triggered by a variety of conditions associated with inadequate oxygen supply to the tissues and helps to maintain oxygen delivery. Examples include anemia and exposure to high altitude.

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  • torsten lehmann
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