For Air to Flow into or Out of the Lungs

Multiple Choice

For air to flow into or out of the lungs, there must be

A) a volume gradient established between the atmosphere and the alveoli.
B) a temperature gradient between the atmosphere and the alveoli.
C) a pressure gradient established between the atmosphere and the alveoli.
D) a diffusion gradient between the atmosphere and the alveoli.
E) None of the choices are correct.