Gas Exchange General Information

A. Dalton's law of partial pressures

- can be expressed by the following equation:

Partial pressure = Total pressure x Fractional gas concentration

1. In dry inspired air, the partial pressure of 02 can be calculated as follows. Assume that total pressure is atmospheric and the fractional concentration of 02 is 0.21.

2. In humidified tracheal air at 37°C, the calculation is modified to correct for the partial pressure of H20 = 47 mm Hg.

B. Partial pressures of 02 and C02 in inspired air, alveolar air, and blood (Table 4-2)

- Approximately 2% of the systemic cardiac output bypasses the pulmonary circulation ("physiologic shunt"). The resulting admixture of venous blood with oxygenated arterial blood makes the Po2 of arterial blood slightly lower than that of alveolar air.

C. Dissolved gases

- The amount of gas dissolved in a solution (such as blood) is proportional to its partial pressure. The units of concentration for a dissolved gas are ml gas/100 ml blood.

Table 4-2. Partial Pressures of 02 and C02 (mm Hg)

Inspired Air

Po, 160

Pco2 0



Addition of H20

decreases Po2

Alveolar Air

02 has diffused from alveolar air into pulmonary capillary blood, decreasing the Po2 of alveolar air

C02 has been added from pulmonary capillary blood into alveolar air


Arterial Blood


Blood has equilibrated with alveolar air (is "arterialized")

Blood has equilibrated with alveolar air

Mixed Venous


02 has diffused from arterial blood into tissues, decreasing the Po2 of venous blood

C02 has diffused from the tissues into venous blood, increasing the Pco2 of venous blood

* Actually, slightly < 100 mm Hg because of "physiologic shunts."

- The following calculation uses 02 in arterial blood as an example:

[02] = Po2 x Solubility of 02 in blood

= 100 mm Hg x 0.03 ml 0,/L/mm Hg = 0.3 ml 02/100 ml blood where:

[02] = 02 concentration in blood Po2 = Partial pressure of 02 in blood 0.03 ml (VL/mm Hg = Solubility of 02 in blood

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