How does the blood take up oxygen and release carbon dioxide? The key to understanding this gas exchange is a concept called partial pressure. Partial pressure is the pressure exerted by one gaseous component in a mixture.

You probably know that the pressure of the atmosphere at sea level is measured as the height of a column of mercury at that altitude. At sea level, that height is 760 millimeters of mercury. The amount of oxygen in the air is about 21% by volume, so the partial pressure of oxygen is 0.21 times 760, or about 160 millimeters of mercury.

The partial pressure of carbon dioxide at sea level is much lower—only 0.23 millimeters of mercury.

The partial pressure of oxygen is designated as PO2 (P sub O-2); the partial pressure of carbon dioxide is designated as PCO2 (P sub C-O-2).

Gas exchange depends on diffusion. The tendency of both gases and liquids is to equalize concentration. A gas always diffuses from a region of higher concentration or partial pressure to a region of lower concentration or partial pressure.

Once equilibrium is reached, the movement of molecules stops.

Each alveolus is surrounded by a network of capillaries, which bring inhaled air close to the gases dissolved in the blood. Recall that blood arriving at the alveoli comes from the heart via the pulmonary artery. The tissues of the body have already absorbed much of the oxygen for use in cellular respiration and have returned carbon dioxide to the blood.

Therefore, blood arriving at the alveoli has a lower partial pressure of oxygen and a higher partial pressure of carbon dioxide than the air in the alveoli.

To establish equilibrium, oxygen from the air dissolves in the fluid that coats the alveoli and diffuses across them into the capillaries. Similarly, carbon dioxide dissolved in the blood diffuses from the capillaries across the same membrane into the alveoli.

Blood leaving the lungs through the pulmonary vein has a raised partial pressure of oxygen and a lowered partial pressure of carbon dioxide.

See if you understand the relationship between the partial pressures of oxygen and carbon dioxide in the body tissues and in the lungs.

Drag boxes to each part of the body to show the relative partial pressures of each gas.

Click Submit to see if you're correct.

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All: In the body tissues, oxygen diffuses from the blood to the tissues themselves, so the partial pressure of oxygen in the blood is lowered. Conversely, the partial pressure of carbon dioxide in the tissues is raised, because carbon dioxide diffuses from the body tissues to the blood.

In the lungs, atmospheric oxygen diffuses into the blood across the alveoli, so the partial pressure of oxygen in the blood is higher than it is in the body tissues. Conversely, the partial pressure of carbon dioxide is lower in the lungs, as carbon dioxide diffuses across the alveoli to be passed back in to the atmosphere.

Now that we've seen what drives gases to move in and out of the blood, we still have one question to answer: How do oxygen and carbon dioxide move around the body?

We’ll examine the mechanisms involved in gas transport next.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education