Final answer:
In a spontaneously breathing, anesthetized patient placed in the lateral decubitus position, the lowermost lung may be compressed and less ventilated, affecting ventilation and gas exchange due to the effect of gravity.
Step-by-step explanation:
In a spontaneously breathing, anesthetized patient placed in the lateral decubitus position, the patient's body is positioned on its side with the lowermost lung in an uppermost position. This can affect ventilation and gas exchange due to the effect of gravity on the lungs. The lung in the dependent side may be compressed and less ventilated, leading to a decrease in oxygenation.