The iron-containing protein that carries oxygen in the blood is:

A. Carbon dioxide does diffuse into the blood from the alveoli, but oxygen diffuses into the blood from the alveoli, not into the alveoli.
B. Oxygen diffuses from the alveoli into the blood where it binds to hemoglobin in red blood cells, while carbon dioxide diffuses from the blood into the alveoli to be exhaled.
C. Both oxygen and carbon dioxide do not diffuse from the alveoli into the blood at the same time. Only oxygen moves into the blood, and carbon dioxide moves in the opposite direction, from the blood to the alveoli.
D. While carbon dioxide moves from the blood into the alveoli, oxygen moves in the opposite direction, from the alveoli into the blood, so this answer is incorrect.

A. Bicarbonate is a buffer in the blood that helps maintain pH balance, but it is not a gas and does not directly regulate breathing.
B. Carbon dioxide (CO₂) is the primary regulator of respiration. Elevated CO₂ levels in the blood lower pH (making it more acidic), which stimulates chemoreceptors to increase the rate and depth of breathing to expel more CO₂ and restore balance.
C. Nitrogen makes up the majority of the air we breathe, but it is inert and plays no active role in regulating respiration.
D. Oxygen does influence breathing, especially at very low levels, but it is not the primary driver. In healthy individuals, CO₂ levels are the main factor controlling respiratory rate.