During pulmonary gas exchange, oxygen and carbon dioxide always move:
A. into the alveoli
B. into the blood
C. from high to low concentration
D. out of the blood
Choose A, B, C or D as your solution.
The correct answer is C. Pulmonary gas exchange is always based on simple diffusion. Diffusion allows for the movement of a substance from an area of high concentration of the substance to an area of low concentration of the substance. Depending on the concentrations, oxygen and carbon dioxide will move in variable directions. Choices $\mathrm{A}, \mathrm{B}$, and $\mathrm{D}$ indicate that the movement of gases always occurs in a fixed direction, which is incorrect.