Oxygen and CO2 cascade

Vivian Imbriotis | Oct. 11, 2025

These describe the series of concentration gradients that drive the diffusion of oxygen from the atmosphere into the mitochondria, and the diffusion of CO2 from the mitochondria to the atmosphere.

Atmosphere. Here the total pressure is 760mmHg at sea level, and \(760 \cdot FiO_2 = 760 \cdot 0.21 = 160\text{mmHg}\)


Isothermic saturation boundary. Here the gas is heated to 37% and 100% relative humidity; here 47mmHg of H2O dilutes atmospheric gas, leaving \((760-47) \cdot 0.21 = 713 \cdot 0.21 = 150\text{mmHg}\)


Alveolus. Here the gas is diluted by CO2 diffusing out of the blood. Via the alveolar gas equation:

$$P_AO_2 = (760 - P_AH_2O) \cdot FiO_2 - \frac{P_aCO_2}{RQ}$$

$$P_AO_2 = 150 - P_aCO_2 \cdot 1.25$$

$$P_AO_2 = 150 - 50 = 100$$


Arterial blood. Here the oxygen tension is diluted by venous admixture (from thebesian and bronchial veins). The difference between this and the PAO2 is the A-a gradient (normal is 7 in youth, 14 in age), so PaO2 = 92


Tissue. Tension drop due to diffusion distance, PO2 10-30mmHg


Mitochondria. Tension drop due to diffusion distance, PO2 1-10mmHg

Mitochondria. Highly variable depending on tissue metabolic rate. PCO2 20-100mmHg


Tissue. Tiny drop in tension due to diffusion distance.


Capillary blood. Highly variable depending on metabolic activity of tissue.


Mixed venous blood. \(P\bar{v}CO_2\) is 6mmHg higher than PaCO2, or about 46mmHg.


Alveolar capillary blood. Here oxygen charges in, flicking the haemoglobin into R state and displacing bound CO2 - the "reverse Haldane" effect. This spikes the PCO2 to... say, 50mmHg [citation needed].


Alveolar gas. Identical to alveolar capillary blood due to high diffusivity of CO2


Atmosphere. PCO2 of 0.3mmHg.