2.3 Gas transport

Based on the fact that the air in the lungs is only refreshed for each breath, and that the oxygen (O₂) and carbon dioxide (CO₂) are diffusing from the alveoli and blood respectively (see below), the gas composition in the alveoli is not identical to the air surrounding us (Fig. 10).

•  In the air pO₂ is approximately 159 mmHg and pCO₂ 0.3 mmHg.
•  In the alveoli the pO₂ is approximately 104 mmHg and pCO₂ approximately 40 mmHg.

Blood arriving at the alveoli has a pO₂ of about 40 mm Hg (Fig. 10). This means there is a much higher oxygen pressure in the alveoli than in the blood. Therefore O₂ diffuses from the alveoli into the blood down the pressure gradient from high to low concentration until the pressures are equal to 104 mmHg. The diffusion takes about 0.25 seconds. This corresponds to approximately one third of the time the blood is in the capillaries in normal, resting conditions. This therefore means that the blood velocity can increase three times, and the blood will still be in the capillaries for long enough for the maximum amount of O₂ to diffuse across from the lungs.

In the body’s various tissues pO₂ will be approximately 40 mmHg due to continuous consumption in the mitochondria. In other words, the oxygen diffuse from the blood into the tissues, whereas when the blood leaves the tissue pO₂ is back to about 40 mmHg.

The partial pressure of carbon dioxide in the alveoli is about 40 mmHg (Fig. 10). In venous blood pCO₂ is about 45 mmHg. This means that CO₂ diffuses from the blood into the alveoli and that pCO₂ in the blood leaving the lungs is 40 mmHg. In the various tissues CO₂ is produced as a by-product of metabolism and energy (ATP) production.Therefore the  PCO₂ in the tissues is 45 mmHg. Thus, CO₂ diffuses into the blood down the pressure gradient where pCO₂ is lower (about 40 mmHg).

Fig.10