In gas exchange in the alveoli, how is an oxygen concentration gradient maintained between alveolar air (paO2 = 13.0 kPa) and blood in the pulmonary capillary (points A, B, C shown with paO2 values 11.7, 11.4, and 5.7 kPa)? Describe two ways the oxygen gradient is maintained.

What this question is asking

You need two physiological processes that keep alveolar oxygen higher than blood oxygen so diffusion can keep happening from alveoli into the capillary.

Way 1: Ventilation keeps alveolar $pO_2$ high

Breathing in and out refreshes the air in the alveoli. This brings in air with a higher oxygen concentration and removes air that has lost oxygen to the blood, so alveolar $pO_2$ stays relatively high (given as $13.0\ \text{kPa}$).

Way 2: Perfusion keeps blood $pO_2$ lower

Continuous blood flow through pulmonary capillaries delivers deoxygenated blood (low $pO_2$) to the alveoli and removes oxygenated blood. Because oxygen is carried away as it diffuses in, the blood near the alveolus does not quickly “catch up” to alveolar $pO_2$, so the diffusion gradient is maintained along the capillary.

Linking to the values in the source

At point C, blood $pO_2$ is much lower ($5.7\ \text{kPa}$) than alveolar $pO_2$ ($13.0\ \text{kPa}$), so diffusion of $O_2$ into the blood is strongly favored. Ventilation keeps the alveolar value high, and perfusion keeps bringing in blood with a lower value, maintaining that difference.