2.2 Partial pressure

Pressure is an important factor in O₂ and CO₂ exchange in the alveoli. The pressure of each individual gas in the atmosphere is described as its partial pressure.

Partial pressure is calculated by multiplying the percentage of the particular gas in the atmosphere by the total atmospheric pressure. For example, O₂ accounts for about 21% of the Earth’s atmosphere so the partial pressure of O₂ (PO₂) in the atmosphere is 0.21 × 760 mmHg = 160 mmHg. CO₂ is present only in trace amounts, so the partial pressure of CO₂ (PCO₂) in the atmosphere is roughly 0.3 mmHg.

The difference in PO₂ and PCO₂ between fresh air and the blood drives the diffusion of O₂ and CO₂ down their respective concentration gradients, as described in Video 8.

Download this video clip.Video player: Video

Show transcript|Hide transcript

Transcript: Video 8 PO2 and PCO2 in lung and tissues.

SPEAKER

Air that enters the lungs contains both oxygen and carbon dioxide that are present in the atmosphere. The partial pressure of oxygen as it enters the body, abbreviated as PO₂, is around 160 millimetres of mercury. Within the moist environment of the alveoli, the PO₂ decreases to 104 millimetres of mercury.

The partial pressure of carbon dioxide as it enters the body, abbreviated as PCO₂, is around 0.3 millimetres of mercury. The carbon dioxide delivered to the lungs from the blood raises the PCO₂ in the alveoli to about 40 millimetres of mercury. De-oxygenated blood from the systemic tissues is carried to the lungs by the pulmonary arteries and has a PO₂ of 40 and a PCO₂ of 45 millimetres of mercury.

As the blood enters the alveoli, the higher PO₂ in the lungs drives oxygen shown in red out of the alveoli and into the blood. At the same time, the slightly higher PCO₂ in the blood drives carbon dioxide, shown in blue, out of the blood and into the alveoli. Because this diffusion is fast, the PO₂ and PCO₂ of the blood rapidly matches the PO₂ and the PCO₂ of the air in the alveoli, at which point there is no more net movement of oxygen and carbon dioxide.

Oxygenated blood is now carried by the pulmonary veins to the heart where it will be pumped out to the systemic tissues. Within the tissues, metabolically active cells consume oxygen and produce carbon dioxide. This results in a PO₂ of about 20 millimetres of mercury and a PCO₂ of about 46 millimetres of mercury in the cells.

Within the surrounding tissue fluid, the PO₂ is approximately 40, and the PCO₂ is around 45 millimetres of mercury. Because the PO₂ is highest in the blood, oxygen will diffuse from the blood into the tissue fluid and then into the cells. In parallel, carbon dioxide, whose partial pressure is highest in the cells, will diffuse down its pressure gradient from the cells into the tissue fluid and then into the blood.

Again, this process occurs quickly so blood that is carried by the systemic veins has the same PO₂ and PCO₂ as that in the tissue fluid. And at this point, there is no more net movement of oxygen and carbon dioxide. De-oxygenated blood is now returned to the heart to be pumped out to the lungs, and the cycle of oxygen and carbon dioxide exchange between the tissues, blood and lungs begins again.

End transcript: Video 8 PO2 and PCO2 in lung and tissues.

Download

Video 8 PO₂ and PCO₂ in lung and tissues.