What spreads from blood into the air space?

Carbon dioxide is the correct answer because it is a byproduct of cellular respiration that is produced in the tissues and then transported through the bloodstream to the lungs. In the lungs, carbon dioxide diffuses from the blood into the air spaces (alveoli) due to differences in partial pressure between the blood and the air in the alveoli. This process allows for carbon dioxide to be exhaled from the body.

Oxygen, while also involved in the respiratory process, moves in the opposite direction— it diffuses from the air in the alveoli into the bloodstream. Hydrogen and nitrogen are not typically involved in this exchange at the alveolar level; hydrogen is a component of metabolic processes but is not directly exchanged in gas form in the respiratory process, and nitrogen makes up a large portion of the air we breathe but does not readily participate in the gas exchange necessary for respiration.