Medical Physiology, 3rd Edition

Carriage of CO2

Blood carries “total CO2” mainly as image

The blood carries CO2 and related compounds in five forms: imageN29-6

N29-6

Total CO2

Contributed by Walter Boron

eTable 29-1 below complements Figure 29-8.

eTABLE 29-1

Components of “Total CO2

COMPONENT

ARTERIAL BLOOD (image = 40 mm Hg)

MIXED-VENOUS BLOOD (image = 46 mm Hg)

CONCENTRATION

CONTRIBUTION TO TOTAL CO2 (mL/dL)

FRACTION OF TOTAL CO2 (%)

CONCENTRATION

CONTRIBUTION TO TOTAL CO2 (mL/dL)

FRACTION OF TOTAL CO2 (%)

Physically dissolved carbon dioxide

CO2

1.2 mM

2.4

5

1.4 mM

2.8

5.3

Carbonic acid

H2CO3

3 µM

~0

~0

~3.5 µM

~0

~0

Bicarbonate

image

24 mM

43.2

90

25.6 mM

46.0

88.5

Carbonate

image

30 µM

~0

~0

30 µM

~0

~0

Carbamino compounds

R-NH-COO

1.2 mM

2.4

5

1.6 mM

3.2

6.2

Total

26.4 mM

48

100

28.6 mM

52

100

1. Dissolved carbon dioxide. [CO2]Dis follows Henry's law (see p. 593), and it is in the millimolar range in both blood plasma and blood cells. It makes up only ~5% of the total CO2 of arterial blood (gold portion of leftmost bar in Fig. 29-8).

image

FIGURE 29-8 Constituents of “total CO2 in blood.” The left bar (a) represents arterial blood; the middle bar (image), mixed-venous blood; and the right bar, the incremental CO2 that the blood picks up in the systemic capillaries.

2. Carbonic acid. H2CO3 can form either from CO2 and H2O, or from H+ and image (see p. 630). Because the equilibrium constant governing the reaction CO2 + H2O ⇄ H2CO3 is ~0.0025, [H2CO3] is only 1/400th as large as [CO2]. Thus, H2CO3 is not quantitatively important for CO2 carriage.

3. Bicarbonate. image can form in three ways. First, H2CO3 can dissociate into image and H+. Second, CO2 can combine directly with OH to form image, the reaction catalyzed by carbonic anhydrase. imageN18-3 Third, image forms when carbonate combines with H+. In arterial blood, image is ~24 mM, so that image represents ~90% of total CO2 (purple portion of leftmost bar in Fig. 29-8).

4. Carbonate. image forms from the dissociation of bicarbonate: image. Because the pK of this reaction is so high (~10.3), [image] is only ~1/1000th as high as image at pH 7.4. Thus, like H2CO3image is not quantitatively important for CO2 carriage.

5. Carbamino compounds. By far the most important carbamino compound is carbamino hemoglobin (Hb-NH-COO), which forms rapidly and reversibly as CO2 reacts with free amino groups on Hb (see p. 653). In arterial blood, carbamino compounds account for ~5% of total CO2 (blue) portion of leftmost bar in Fig. 29-8).

The reason we group together the previously listed five CO2-related compounds under the term total CO2 is that the method Van Slyke imageN29-7 introduced in the 1920s—which remains the basis for assaying blood image in modern clinical laboratories—cannot distinguish among the five.

N29-7

Van Slyke's Manometric Method for Determining Total CO2

Contributed by Emile Boulpaep, Walter Boron

In 1924, Van Slyke and Neill introduced a technique for determining what is today known as the “total CO2” of blood imageN29-6 and other solutions. This technique quickly became the standard for determining total CO2 in clinical chemistry laboratories. Moreover, the Van Slyke approach is the foundation even for modern, automated approaches for determining total CO2.

Although the original approach requires painstaking precision in the laboratory in order to obtain reliable results, the fundamental principle is a rather simple two-step process. First, one uses an acid (Van Slyke and Neill used 1 N lactic acid) to titrate virtually all of the carbamino Hb, carbonate (image), and bicarbonate (image) to carbon dioxide (CO2), which enters the gaseous phase. Second, one uses a manometer to measure the pressure (P) of a known volume (V) of this CO2 gas. The ideal gas law tells us that

image (NE 29-2)

Here, n is the number of molecules, R is the universal gas constant, and T is the absolute temperature. Thus, with the proper corrections for nonideality, it is possible to compute the number of CO2molecules.

We will now examine the titration reactions in a little more detail. For the titration of carbamino Hb by the H+ in lactic acid, two reactions occur in series:

image (NE 29-3)

These reactions are the reverse of the ones shown in Equation 29-10.

The titration of image by the H+ in lactic acid yields image:

image (NE 29-4)

Finally, the titration of this newly formed image—as well as the pre-existing image (which is a far larger amount under physiological conditions)—by the H+ in lactic acid yields carbonic acid (H2CO3), which in turn yields CO2 and H2O:

image (NE 29-5)

Because the pK governing the equilibrium CO2 + H2O ⇄ H2CO3 is ~2.6, >99.7% of the newly formed H2CO3 goes on to form CO2.

In practice, the technique used to generate the CO2 gas also extracts a variable amount of O2 from the solution. In other words, the CO2 in the gas phase is mixed with an amount of O2 that is difficult to predict. Therefore, Van Slyke and Neill introduced an additional step to their analysis: they added 1 N NaOH to convert all total CO2 to image, thereby removing the CO2 from the gas phase. This last step allowed them to obtain, with precision, the amount of CO2 gas in the gas phase and thus to calculate the amount of total CO2 that had been in their sample.

References

Simoni RD, Hill RL, Vaughan M. The determination of gases in blood and other solutions by vacuum extraction and manometric measurement. I. [Classics: A paper in a series reprinted to celebrate the centenary of the JBC in 2005.]. J Biol Chem. 2002;277:e16.

Van Slyke DD, Neill JM. The determination of gases in blood and other solutions by vacuum extraction and manometric measurement. I. J Biol Chem. 1924;61:523–573 [Available online at the JBC website by accessing the commentary in the reference above.].

CO2 transport depends critically on carbonic anhydrase, the Cl-HCO3 exchanger, and Hb

The total CO2 concentration of arterial blood is ~26 mM, or ~48 mL of CO2 gas/dL (measured at STP). image constitutes ~90% of this 48 mL/dL, with CO2 and carbamino compounds contributing ~5% each (bar a in Fig. 29-8). As blood courses through the systemic capillary beds, it picks up ~4 mL/dL of CO2, so that the total CO2 of mixed-venous blood is ~52 mL/dL (bar image in Fig. 29-8). In what forms does blood carry this incremental 4 mL/dL of CO2 to the lungs? About 10% of the incremental CO2 moves as dissolved CO2, ~69% as image, and ~21% as carbamino compounds (rightmost two bars in Fig. 29-8). Therefore, dissolved CO2 and carbamino CO2 are far more important for carrying incremental CO2 to the lungs than we might have surmised, given their contribution to total CO2 in arterial blood.

Figure 29-9 summarizes the events that occur as incremental CO2 enters systemic capillaries. As fast as biological oxidations in the mitochondria produce CO2, this gas diffuses out of cells, through the extracellular space, across the capillary endothelium, and into the blood plasma. Some of the incremental CO2 (~11%) remains in blood plasma throughout its journey to the lungs, but most (~89%), at least initially, enters the RBCs.

image

FIGURE 29-9 Carriage of CO2 from systemic capillaries to the lungs.

The ~11% of the incremental CO2 in plasma travels in three forms:

1. Dissolved CO2. About 6% of incremental CO2 remains dissolved in blood plasma (assuming a hematocrit of 40%).

2. Carbamino compounds. An insignificant amount forms carbamino compounds with plasma proteins.

3. Bicarbonate. About 5% of incremental CO2 forms image in the plasma and remains in the plasma: CO2 + H2O → H2CO3 → H+ + image. The amount of image that follows this path depends critically on non-image buffering power (see pp. 637–638), which is very low in plasma (~5 mM/pH unit).

The remaining ~89% of incremental CO2 enters the RBCs, predominantly through two “gas channels,” aquaporin 1 (AQP1) and the Rh complex. imageN29-8 This CO2 also has three fates:

N29-8

Gas Channels

Contributed by Walter Boron

The traditional view had been that all gases cross all cell membranes by simply dissolving in the lipid phase of the membrane. The first evidence that the gas dogma was in need of refinement was the observation that apical membranes of gastric gland cells are impermeable to CO2 and NH3 (Waisbren etal, 1994) and that apical membranes of colonic crypts are impermeable to NH3 (Singh etal, 1995). Low permeabilities to gases and H2O may be a general property of membranes facing inhospitable environments (Cooper etal, 2002), including the mechanical stresses experienced by RBCs and capillary endothelial cells. Cells exposed to such hostile environments may have evolved specialized, robust membranes with unique lipid composition or protein or oligosaccharide coatings at the membrane surface.

The second observation that upset the gas dogma was the discovery that CO2 passes through the water channel AQP1 (Cooper etal, 1998Nakhoul etal, 1998). Peter Agre shared the 2003 Nobel Prize in Chemistry for discovering AQPs and their H2O permeability. imageN5-7 The physiological role of AQPs transcends water. For example, AQP7 and AQP9 transport glycerol. Might AQP1's CO2 permeability be physiologically relevant? The first such evidence was the demonstration that an AQP plays a critical role in CO2 uptake for photosynthesis by tobacco leaves (Uehlein etal, 2003). Endeward and colleagues (2008) demonstrated that approximately half of the CO2 entering human RBCs passes through the AQP1 channel.

The AQPs form homotetramers, and each monomer has a pore for H2O. Molecular dynamics simulations suggest that CO2 could pass single file with H2O through each of the four aquapores as well as through the central pore between the four monomers (Wang etal, 2007). Preliminary inhibitor studies from the Boron laboratory confirm this prediction, indicating that ~40% of the CO2 that passes through AQP1 moves through the four aquapores, whereas the remaining 60% passes through the central pore. The molecular dynamics simulations suggest that virtually all of the O2 passes through the central pore.

A second family of gas channels is the Rh proteins. The bacterial Rh homolog AmtB forms a homotrimer, and each monomer appears to have a pore for NH3. The RhAG protein from the human Rh complex in RBCs is permeable both to NH3 and to CO2 (Endeward etal, 2008). Preliminary work from the Boron laboratory indicates that NH3 moves exclusively through one of the three ammonia pores, whereas CO2moves mainly through the central pore of both AmtB and RhAG.

Interestingly, gas channels exhibit selectivity for gas in much the same way as ion channels exhibit selectivity for ions. For example, AQP4 (heavily expressed at the blood-brain barrier) and AQP5 (heavily expressed in alveolar type I pneumocytes) are virtually totally selective for CO2 over NH3. AQP1 (heavily expressed in RBCs) is intermediate in its CO2/NH3 selectivity, AmtB is shifted more toward NH3selectivity, and RhAG (heavily expressed in RBCs) is even more shifted toward selectivity for NH3 over CO2. This work is the first evidence for gas selectivity by a protein channel (Musa-Aziz etal, 2009).

References

Cooper GJ, Boron WF. Effect of PCMBS on CO2 permeability of Xenopus oocytes expressing aquaporin 1 or its C189S mutant. Am J Physiol. 1998;275:C1481–C1486.

Cooper GJ, Zhou Y, Bouyer P, et al. Transport of volatile solutes through AQP1. J Physiol. 2002;542:17–29.

Endeward V, Cartron JP, Ripoche P, Gros G. RhAG protein of the Rhesus complex is a CO2 channel in the human red cell membrane. FASEB J. 2008;22:64–73.

Endeward V, Musa-Aziz R, Cooper GJ, et al. Evidence that aquaporin 1 is the major pathway for CO2 transport in the human erythrocyte membrane. FASEB J. 2006;20:1974–1981.

Musa-Aziz R, Chen LM, Pelletier MF, Boron WF. Relative CO2/NH3 selectivities of AQP1, AQP4, AQP5, AmtB, and RhAG. Proc Natl Acad Sci U S A. 2009;106:5406–5411.

Nakhoul NL, Davis BA, Romero MF, Boron WF. Effect of expressing the water channel aquaporin-1 on the CO2 permeability of Xenopus oocytes. Am J Physiol. 1998;274:C543–C548.

Singh SK, Binder HJ, Geibel JP, Boron WF. An apical permeability barrier to image in isolated, perfused colonic crypts. Proc Natl Acad Sci U S A. 1995;92:11573–11577.

Uehlein N, Lovisolo C, Siefritz F, Kaldenhoff R. The tobacco aquaporin NAQP1 is a membrane CO2 pore with physiological functions. Nature. 2003;425:734–737.

Waisbren SJ, Geibel JP, Modlin IM, Boron WF. Unusual permeability properties of gastric gland cells. Nature. 1994;368:332–335.

Wang Y, Cohen J, Boron WF, et al. Exploring gas permeability of cellular membranes and membrane channels with molecular dynamics. J Struct Biol. 2007;157:534–544.

1. Dissolved CO2. About 4% of incremental CO2 remains dissolved inside the RBC.

2. Carbamino compounds. About 21% of incremental CO2 forms carbamino compounds with Hb. Why does so much CO2 travel as carbamino compounds inside the RBC, whereas so little does in the blood plasma? First, the Hb concentration inside RBCs (~33 g/dL) is far higher than that of albumin and globulins in plasma (~7 g/dL). Second, Hb forms carbamino compounds far more easily than do major plasma proteins. Moreover, Hb forms carbamino compounds even more easily as it loses O2 in the systemic capillaries (reverse of CO2-Bohr effect). Finally, Hb is a far better buffer than the plasma proteins for the H+formed as a byproduct in carbamino formation, and becomes an even better buffer as it loses O2 in the systemic capillaries (reverse of the pH-Bohr effect).

3. Bicarbonate. About 64% of the incremental CO2 forms image. Why does so much more CO2 form image in the erythrocyte than in plasma? First, erythrocytes contain a high level of carbonic anhydrases (see p. 434), imageN18-3 which greatly accelerates the conversion of CO2 to image. In the absence of enzyme, hardly any image would form inside erythrocytes during the brief time that the cells spend during their passage through a typical systemic capillary. Second, the Cl-HCO3 exchanger AE1 (see pp. 124–125 and 434–435) transports some of the newly formed image out of the cell, promoting formation of more image. This uptake of Cl in exchange for image is known as the chloride or Hamburger shift. Third, the buffering of H+ by Hb (see point 2 above) also pulls the reaction to the right.

The combined effects of the described intracellular and extracellular events is that ~10% of incremental CO2 formed in systemic tissues moves to the lungs as dissolved CO2, 6% in plasma and 4% inside erythrocytes (gold portion of rightmost bar in Fig. 29-8). About 21% moves as carbamino compounds, almost exclusively inside erythrocytes as carbamino Hb (blue portion of rightmost bar in Fig. 29-8). Finally, ~69% of incremental CO2 moves as image, 5% that forms in plasma and 64% that forms inside the RBC (purple portion of rightmost bar in Fig. 29-8). Because H2O enters the cell during image formation, erythrocytes swell as they pass through systemic capillaries.

When mixed-venous blood (with a image of ~46 mm Hg) reaches the pulmonary capillaries (surrounded by alveoli with a image of only ~40 mm Hg), CO2 moves from the erythrocytes and blood plasma into the alveolar air space. All of the reactions discussed above reverse. In the process, Cl and H2O leave the erythrocytes, and the cells shrink.

The high image in the lungs causes the blood to dump CO2

The carriage of total CO2 in the blood depends on the three blood-gas parameters—image, plasma pH, and image. The three plots in the main portion of Figure 29-10 are CO2 dissociation curves. Each plot shows how changes in image affect the total-CO2 content of blood. Although pH per se does not appear in this diagram, pH decreases as image increases along the x-axis (i.e., respiratory acidosis; see p. 833). The blue plot is the CO2 dissociation curve when image is zero (image ≅ 0% Hb). The next two plots are CO2 dissociation curves for image values of 40 mm Hg (image ≅ 75%; purple curve) and 100 mm Hg (image ≅ 97.5%; red curve). The green line at the bottom of Figure 29-10 shows that the dissolved component of total CO2 rises only slightly with increases in image.

image

FIGURE 29-10 CO2 dissociation curves (Haldane effect).

Three features of the CO2 dissociation curves in Figure 29-10 are noteworthy:

1. Near-linear relationship in the physiological range of image and image values (see Fig. 29-10, inset). In contrast, the O2 dissociation curve is highly nonlinear in its physiological range (i.e., 40 to 100 mm Hg).

2. Up-shift of curve with decreasing image. At any image, total CO2 content rises as image (or Hb saturation) falls—the Haldane effect.imageN29-9 Thus, as blood enters systemic capillaries and releases O2, the CO2-carrying capacity rises so that blood picks up extra CO2. Conversely, as blood enters the pulmonary capillaries and binds O2, the CO2-carrying capacity falls so that blood dumps extra CO2 (Table 29-4). The Haldane effect is the flip side of the coin from the pH-Bohr and CO2-Bohr effects. First, just as H+ binding lowers the O2 affinity of Hb (see Equation 29-9), O2 binding destabilizes protonated hemoglobin (Hb-H+), promoting H+ release. By mass action, this H+ reduces CO2-carrying capacity by favoring the formation of CO2 from both carbamino Hb and image (see Fig. 29-9). Second, just as carbamino formation lowers the O2 affinity of Hb (see Equation 29-11), O2 binding destabilizes carbamino Hb (Hb-NH-COO), promoting CO2 release.

TABLE 29-4

Factors Affecting the Amount of Total CO2 Carried by Blood

PARAMETER

EFFECTS OF INCREASING THE PARAMETER

image

Increased [CO2]Dis (Henry's law).
Increased formation of image (CO2 + H2O → image + H+).
Increased formation of carbamino (CO2 + image → Hb-NH-COO + 2H+).

[Plasma protein]

Increased plasma buffering power. The increased capacity for consuming H+ indirectly promotes formation of image.

Plasma pH

Increased formation of image in plasma (Henderson-Hasselbalch equation).
Increased pH inside red cell, promoting formation of image and carbamino Hb.

[Hb]

Increased formation of carbamino Hb (direct).
Increases buffering power inside erythrocyte. The increased capacity for consuming H+ indirectly promotes formation of image and carbamino Hb.

image

Decreased buffering power of Hb (inverse of pH-Bohr effect). The decreased capacity for consuming H+ indirectly restrains formation of image and carbamino Hb.
Decreased formation of carbamino Hb (inverse of CO2-Bohr effect).

3. Steepness. Because CO2 dissociation curves (see Fig. 29-10) are much steeper than O2 dissociation curves (see Fig. 29-3), image must increase from 40 mm Hg in arterial blood to only 46 mm Hg in mixed-venous blood to increase the CO2 content by the ~4 mL/dL of CO2 required to remove CO2 as fast as the mitochondria produce it. In contrast, image must decrease from 100 to 40 mm Hg to dump enough O2 to meet metabolic demands.

N29-9

John Scott Haldane

Contributed by Emile Boulpaep, Walter Boron

John Scott Haldane (1860–1936) was born in Edinburgh, where he received his medical degree in 1884. He began his academic career at Queen's College, Dundee, before moving to Oxford. Haldane is credited with several notable discoveries:

• He showed that high blood levels of CO2 are a more powerful stimulus for breathing than low levels of O2 (1905).

• He developed a method for the staged decompression of deep-sea divers to avoid the bends on the return of the divers to the surface. He published the first diving decompression tables (1908).

• He—along with Christiansen and Douglas—showed that the CO2 content of the blood decreases with increasing image, known as the Haldane effect (1914). This effect is responsible for about half of the CO2exchange in the blood.

For more information about Haldane, visit the following websites:

1. http://www.geo.ed.ac.uk/scotgaz/people/famousfirst1349.html.

2. http://www.diegoweb.com/diving/cards/page2.html.

Reference

Christiansen J, Douglas CG, Haldane JS. The absorption and dissociation of carbon dioxide by human blood. J Physiol. 1914;48:244–277.

In the inset to Figure 29-10, point a on the red curve represents arterial blood, with a image of 40 mm Hg and a image of 100 mm Hg (image ≅ 97.5%). Point image on the purple curve represents mixed-venous blood, with a image of 46 mm Hg but a image of only 40 mm Hg (image ≅ 75%). The difference between the total CO2 contents represented by the two points (i.e., 52 versus 48 mL/dL) represents the 4 mL/dL of CO2 the blood takes up as it passes through systemic capillaries. If it were not for the Haldane effect, the blood would remain on the red curve, and the image increase would cause the CO2 content to increase by only ~2.7 mL/dL. Thus, at a image of 46 mm Hg, the fall in image that occurs as blood flows through systemic capillaries allows the blood to pick up ~50% more CO2 (i.e., 4 versus 2.7 mL/dL). Viewed differently, if it were not for the Haldane effect, mixed-venous image would have to increase to ~49 mm Hg for blood to carry 4 mL/dL of CO2Table 29-4 summarizes how changes in blood parameters can influence the amount of total CO2 that the blood is able to carry.

The O2-CO2 diagram describes the interaction of image and image in the blood

We have seen that Hb plays a key role in transporting O2 from the lungs to peripheral tissues, transporting CO2 in the opposite direction, and buffering H+. These functions are all intimately interrelated: image and pH influence the O2-Hb dissociation curve (Bohr effects; see Fig. 29-5), and image influences the CO2 dissociation curve (Haldane effect; see Fig. 29-10). A useful way of illustrating this mutual dependence is the O2-CO2 diagram, which we will revisit on page 691 to understand regional differences between alveolar image and image.

On a coordinate system with image on the y-axis and image on the x-axis, each blue curve in Figure 29-11 represents an isopleth of identical O2 content in whole blood (from the Greek isos [equal] + plethein [to be full]). For example, arterial blood (point a) lies on the isopleth for an O2 content of 20.0 mL/dL, with coordinates of image = 100 mm Hg and image = 40 mm Hg. Following this isopleth from a image of 40 mm Hg (point a) to, say, 46 mm Hg, we see that the blood could carry the same 20.0 mL/dL of O2 only if we increase image from 100 to nearly 105 mm Hg. Thus, as image increases, the O2 content of blood decreases (Bohr effect). If it were not for the Bohr effect, all the blue curves would be vertical lines. Mixed-venous blood (point image) is on the O2-content isopleth for 15.3 mL/dL, at a image of 40 mm Hg and a image of 46 mm Hg. If blood were equilibrated with inspired air (point I), it would have a image of 150 mm Hg and a image of zero.

image

FIGURE 29-11 O2-CO2 diagram.

Each red curve is an isopleth of identical CO2 content. Arterial blood (point a) lies on the isopleth for 48 mL/dL. Similarly, mixed-venous blood (point image) lies on the isopleth for 52 mL/dL. Following this 52-mL/dL isopleth from a image of 40 mm Hg (point image) to, say, 100 mm Hg, we see that the blood could carry the same 52 mL/dL of CO2 only if we increase image from 46 to nearly 50 mm Hg. Thus, as the image increases, the CO2 content of blood decreases (Haldane effect). If it were not for the Haldane effect, all the red curves would be horizontal lines. Blood equilibrated with inspired air (point I) would have a CO2 content of zero.

In Figure 29-11, the green curve connecting the points image, a, and I represents all possible combinations of image and image in normal lungs.