Which mechanism increases efficiency of high-altitude avian flight due to specialized system of air sacs?

💡 Explanation

At high altitudes resulting in decreased oxygen partial pressure, crosscurrent flow in avian air sacs maximizes oxygen extraction from inhaled air because blood capillaries flow perpendicularly to air flow yielding greatest oxygen gradient per unit length. Therefore enhanced oxygen extraction results, not bone density, muscle output, or heat loss, these rely on distinct biological processes.

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