Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 36Gas Transport & pH in the Lung 613

this reason, plus the fact that a small amount of fluid in the
arterial blood returns via the lymphatics rather than the veins,
the hematocrit of venous blood is normally 3% greater than
that of the arterial blood. In the lungs, the Cl– moves out of
the cells and they shrink.

SUMMARY OF CARBON

DIOXIDE TRANSPORT

For convenience, the various fates of CO 2 in the plasma and
red cells are summarized in Table 36–2. The extent to which
they increase the capacity of the blood to carry CO 2 is indicat-
ed by the difference between the lines indicating the dissolved
CO 2 and the total CO 2 in the dissociation curves for CO 2
shown in Figure 36–7.
Of the approximately 49 mL of CO 2 in each deciliter of
arterial blood (Table 36–1), 2.6 mL is dissolved, 2.6 mL is in
carbamino compounds, and 43.8 mL is in HCO 3 –. In the tis-
sues, 3.7 mL of CO 2 per deciliter of blood is added; 0.4 mL
stays in solution, 0.8 mL forms carbamino compounds, and

2.5 mL forms HCO 3 –. The pH of the blood drops from 7.40 to

7.36. In the lungs, the processes are reversed, and the 3.7 mL

of CO 2 is discharged into the alveoli. In this fashion, 200 mL

of CO 2 per minute at rest and much larger amounts during

exercise are transported from the tissues to the lungs and

excreted. It is worth noting that this amount of CO 2 is equiva-

lent in 24 hours to over 12,500 mEq of H+.

ACID–BASE BALANCE & GAS TRANSPORT

The major source of acids in the blood under normal condi-

tions is through cellular metabolism. The CO 2 formed by me-

tabolism in the tissues is in large part hydrated to H 2 CO 3 , and

the total H+ load from this source is over 12,500 mEq/d. How-

ever, most of the CO 2 is excreted in the lungs, and the small

quantities of the remaining H+ are excreted by the kidneys.

FIGURE 36–5 Dissociation curve of hemoglobin and
myoglobin. The myoglobin binding curve (B) lacks the sigmoidal shape
of the hemoglobin binding curve (A) because of the single O 2 binding
site in each molecule. Myoglobin also has greater affinity for O 2 than he-
moglobin (curve shifted left) and thus can store O 2 in muscle.

FIGURE 36–6 Fate of CO 2 in the red blood cell. Upon entering
the red blood cell, CO 2 is rapidly hydrated to H 2 CO 3 by carbonic anhy-
drase. H 2 CO 3 is in equilibrium with H+ and its conjugate base, HCO 3 –.
H+ can interact with deoxyhemoglobin, whereas HCO 3 – can be trans-
ported outside of the cell via AE1 (Band 3). In effect, for each CO 2
molecule that enters the red cell, there is an additional HCO 3 – or Cl– in
the cell.

100

80

60

40

20

04080 120

B

A

PO 2 (mm Hg)

O

saturation (%) 2

A = Hemoglobin

B = Myoglobin

CO 2

CI−

CO 2 + H 2 OH 2 CO 3

Carbonic

anhydrase HHb H+ + Hb−

H+ + HCO 3 −

TABLE 36–2 Fate of CO 2 in blood.

In plasma

1. Dissolved


2. Formation of carbamino compounds with plasma protein


3. Hydration, H+ buffered, HCO 3 – in plasma
   In red blood cells


4. Dissolved


5. Formation of carbamino-Hb


6. Hydration, H+ buffered, 70% of HCO 3 – enters the plasma


7. Cl– shifts into cells; mOsm in cells increases

FIGURE 36–7 CO 2 dissociation curves. The arterial point (a)

and the venous point (v) indicate the total CO 2 content found in arterial

blood and venous blood of normal resting humans. Note the low

amount of CO 2 that is dissolved (orange trace) compared to that which

can be carried by other means (Table 36–2). (Modified and reproduced with

permission from Schmidt RF, Thews G [editors]: Human Physiology. Springer, 1983.)

70

60

50

40

30

20

10

01020 30 40 50 60 70

5

10

15

20

25

30

v

Deoxygenated blood

Oxygenated blood

Dissolved CO 2

CO

concentration (mL/dL) 2

CO

concentration (mmol/L) 2

a

PCO 2 (mm Hg)