Enrico Tiepolo
Gas exchange
Gases can dissolve in fluids. Gas concentration in the fluid is proportional to its partial
pressure. The proportionality factor is the solubility coefficient for that gas in that liquid.
In arterial blood, pCO2 is 40 mmHg and pO2 is 100 mmHg; in venous blood pCO2 is 46 mmHg and
pO2 is 40 mmHg. It would seem that much more oxygen is acquired at the lungs than the release
carbon dioxide, but this is not correct, because:
1. The solubility of CO2 in plasma (57% vol/vol at 760 mmHg = 0.031 mM/mmHg) is much
higher than that of oxygen (2.4%);
2. CO2 is in equilibrium with 20 times as much bicarbonate at pH 7.4
3. Most O2 is bound to hemoglobin and from 100 to 40 mmHg in pO2 total oxygen only
declines by 25%.
O2 in fact, is not particularly soluble, and we manage to accumulate a big quantity of O2 in blood simply
because it can bind to hemoglobin.
Hemoglobin can bind four molecules of O2, in a cooperative way (i.e. the more O2 is bound to
a molecule of Hb the higher its affinity for oxygen); it is 50% saturated for 25 mmHg pO2.
A pO2 value of 40 mmHg (the typical value of venous blood) is sufficient to saturate Hb to 75%, and
normal arterial values (100 mmHg) can almost fully saturate hemoglobin.
Hemoglobin can also bind CO2. When this happens, it reduces affinity of hemoglobin for O2. This is a
mechanism that allows our body to work perfectly: hemoglobin will be very greedy for O2 at the lungs
where there is lot of oxygen (cooperative binding) and becomes very little affine for O2 at the tissue
where there is a low concentration of oxygen and high concentration of CO2 (especially if tissue is acidic).
This allows for O2 to be taken up at the lungs and to be released at the tissues.
As regards CO2, its concentration (40 mmHg ×0.031 = 1.24 mM) is in equilibrium with small amounts
(400:1) of H2CO3 and a 20-fold higher concentrations of bicarbonate (pH = 7.4, i.e. [log(20)=1.3] pH
units above [pK=6.1]); a small amount of CO2 is also bound to hemoglobin (Hb).
Thus the total concentration of CO2 + bicarbonate buffer is ≈25 mM, corresponding to a volume of
CO2 of (≈ 0.025 Z) × (24 L/mole) = 0.6, i.e. ≈60% vol/vol.
158 Body At Work II
, Enrico Tiepolo
• a pCO2 = 46 mmHg implies 46 mmHg × 0.031 = 1.43 mM [CO2]; at the venous pH (7.37)
the ratio bicarbonate to carbon dioxide will be 18, for total of 1.43×19 = 27 mM
• the change is from 60% to (24 L/mol) × (0.027 M) = 64.8%, i.e., by 4.8% vol/vol. As regards
O2, at pO2 =100 mmHg the concentration is 100/760 × (2.4% solubility) = 0.32%; however,
we have ≈15 mg/dl of Hb, with a maximum transport capacity of 1.34 ml O2 per gram of Hb
and a saturation of 97%, i.e., 15×1.34×0.97=19.5 ml/dl = 19.5% vol/vol.
• a drop to pO2 =40 mmHg implies a 60% fall in O2 concentration (0.13%) plus a decrease in
Hb saturation to 75% and of Hb bound oxygen to 15×1.34×0.75 = 15% vol/vol
• so, the overall O2 concentration in arterial blood is ≈20%, and when the PO2 drops to 40
mmHg in venous blood, it falls to ≈15.2%, i.e., a change by 4.8% vol/vol.
This indicates that the large change in pO2 (100 to 40 mmHg, 12 ml/dl) is fully compensated by
the small change in pCO2 (40 to 46 mmHg): actually, in general the ratio eliminated CO2 to
absorbed O2 (RER, respiratory exchange ratio) is between 0.7 and 1. This could be predicted,
because if carbohydrates are burned (stoichiometry Cn(H2O)n), only the carbon atoms need to be
oxidized (1 molecule of CO2 per molecule of O2, RER = 1), while if lipids are burned more oxygen is
needed to oxidize the hydrogens as well (RER < 1).
Hemoglobin
As mentioned, hemoglobin is essentially saturated (almost 100% saturation) at typical arterial pO2 values
(100 mmHg) and only releases about 25% of its oxygen at pO2 = 40 mmHg (saturation at 75%).
This aspect can be confusing:
• as regard dissolved gas, concentration is proportional to
partial pressure
• as regards HB-bound, the relation is not linear: 60%
decrease in pO2 → –25% decrease in HB-bound
amount (notice that a small further decrease in pO2
produces a much larger release of O2)
• On the other hand, below this value the curve is much
steeper, indicating that when the PO2 falls below 40
mmHg hemoglobin releases oxygen with high
efficiency.
• This is not the only favorable property of the protein.
Due to the cooperativity, the higher the concentration
of carbon dioxide the lower is the affinity for oxygen.
• Lowering pH also reduces the affinity for oxygen (Bohr effect). In a metabolically active tissue
in fact, a lot of O2 is consumed and a lot of CO2 is produced. As we saw before this CO2 is
given to the hemoglobin that either bring it to the lungs or transform it into HCO3- and give
rise to protons, lowering the pH.
159 Body At Work II
Gas exchange
Gases can dissolve in fluids. Gas concentration in the fluid is proportional to its partial
pressure. The proportionality factor is the solubility coefficient for that gas in that liquid.
In arterial blood, pCO2 is 40 mmHg and pO2 is 100 mmHg; in venous blood pCO2 is 46 mmHg and
pO2 is 40 mmHg. It would seem that much more oxygen is acquired at the lungs than the release
carbon dioxide, but this is not correct, because:
1. The solubility of CO2 in plasma (57% vol/vol at 760 mmHg = 0.031 mM/mmHg) is much
higher than that of oxygen (2.4%);
2. CO2 is in equilibrium with 20 times as much bicarbonate at pH 7.4
3. Most O2 is bound to hemoglobin and from 100 to 40 mmHg in pO2 total oxygen only
declines by 25%.
O2 in fact, is not particularly soluble, and we manage to accumulate a big quantity of O2 in blood simply
because it can bind to hemoglobin.
Hemoglobin can bind four molecules of O2, in a cooperative way (i.e. the more O2 is bound to
a molecule of Hb the higher its affinity for oxygen); it is 50% saturated for 25 mmHg pO2.
A pO2 value of 40 mmHg (the typical value of venous blood) is sufficient to saturate Hb to 75%, and
normal arterial values (100 mmHg) can almost fully saturate hemoglobin.
Hemoglobin can also bind CO2. When this happens, it reduces affinity of hemoglobin for O2. This is a
mechanism that allows our body to work perfectly: hemoglobin will be very greedy for O2 at the lungs
where there is lot of oxygen (cooperative binding) and becomes very little affine for O2 at the tissue
where there is a low concentration of oxygen and high concentration of CO2 (especially if tissue is acidic).
This allows for O2 to be taken up at the lungs and to be released at the tissues.
As regards CO2, its concentration (40 mmHg ×0.031 = 1.24 mM) is in equilibrium with small amounts
(400:1) of H2CO3 and a 20-fold higher concentrations of bicarbonate (pH = 7.4, i.e. [log(20)=1.3] pH
units above [pK=6.1]); a small amount of CO2 is also bound to hemoglobin (Hb).
Thus the total concentration of CO2 + bicarbonate buffer is ≈25 mM, corresponding to a volume of
CO2 of (≈ 0.025 Z) × (24 L/mole) = 0.6, i.e. ≈60% vol/vol.
158 Body At Work II
, Enrico Tiepolo
• a pCO2 = 46 mmHg implies 46 mmHg × 0.031 = 1.43 mM [CO2]; at the venous pH (7.37)
the ratio bicarbonate to carbon dioxide will be 18, for total of 1.43×19 = 27 mM
• the change is from 60% to (24 L/mol) × (0.027 M) = 64.8%, i.e., by 4.8% vol/vol. As regards
O2, at pO2 =100 mmHg the concentration is 100/760 × (2.4% solubility) = 0.32%; however,
we have ≈15 mg/dl of Hb, with a maximum transport capacity of 1.34 ml O2 per gram of Hb
and a saturation of 97%, i.e., 15×1.34×0.97=19.5 ml/dl = 19.5% vol/vol.
• a drop to pO2 =40 mmHg implies a 60% fall in O2 concentration (0.13%) plus a decrease in
Hb saturation to 75% and of Hb bound oxygen to 15×1.34×0.75 = 15% vol/vol
• so, the overall O2 concentration in arterial blood is ≈20%, and when the PO2 drops to 40
mmHg in venous blood, it falls to ≈15.2%, i.e., a change by 4.8% vol/vol.
This indicates that the large change in pO2 (100 to 40 mmHg, 12 ml/dl) is fully compensated by
the small change in pCO2 (40 to 46 mmHg): actually, in general the ratio eliminated CO2 to
absorbed O2 (RER, respiratory exchange ratio) is between 0.7 and 1. This could be predicted,
because if carbohydrates are burned (stoichiometry Cn(H2O)n), only the carbon atoms need to be
oxidized (1 molecule of CO2 per molecule of O2, RER = 1), while if lipids are burned more oxygen is
needed to oxidize the hydrogens as well (RER < 1).
Hemoglobin
As mentioned, hemoglobin is essentially saturated (almost 100% saturation) at typical arterial pO2 values
(100 mmHg) and only releases about 25% of its oxygen at pO2 = 40 mmHg (saturation at 75%).
This aspect can be confusing:
• as regard dissolved gas, concentration is proportional to
partial pressure
• as regards HB-bound, the relation is not linear: 60%
decrease in pO2 → –25% decrease in HB-bound
amount (notice that a small further decrease in pO2
produces a much larger release of O2)
• On the other hand, below this value the curve is much
steeper, indicating that when the PO2 falls below 40
mmHg hemoglobin releases oxygen with high
efficiency.
• This is not the only favorable property of the protein.
Due to the cooperativity, the higher the concentration
of carbon dioxide the lower is the affinity for oxygen.
• Lowering pH also reduces the affinity for oxygen (Bohr effect). In a metabolically active tissue
in fact, a lot of O2 is consumed and a lot of CO2 is produced. As we saw before this CO2 is
given to the hemoglobin that either bring it to the lungs or transform it into HCO3- and give
rise to protons, lowering the pH.
159 Body At Work II
