The substance that moves from the bloodstream to the air via the lungs is carbon dioxide.

Gas Exchange in the Lungs

In the lungs, specifically at the alveoli (tiny air sacs), gas exchange occurs. Oxygen from the inhaled air passes through the thin walls of the alveoli into the blood in the capillaries. At the same time, carbon dioxide—a waste product produced by cells—moves from the blood in the capillaries into the alveoli to be exhaled into the air. This exchange happens by diffusion through a thin barrier separating air and blood.

Summary

• Oxygen moves from the air in the lungs into the bloodstream.
• Carbon dioxide moves from the bloodstream into the air in the lungs for removal.
• This process is essential for respiration, allowing carbon dioxide to be expelled as a waste gas.

Thus, the substance moving from the bloodstream to the air via the lungs is carbon dioxide.