In the alveoli of the lungs, air crosses into and out of the capillaries by what process?

• A. Osmosis
• B. Simple diffusion
• C. Facilitated diffusion
• D. Active transport

Answer: (B)

Verified step by step guidance

1. Understand the context: The alveoli are tiny air sacs in the lungs where gas exchange occurs. Oxygen from inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses out into the alveoli to be exhaled.
2. Identify the process: Gas exchange in the alveoli occurs due to the concentration gradient of gases. Oxygen moves from an area of high concentration in the alveoli to an area of low concentration in the blood, and carbon dioxide moves in the opposite direction.
3. Eliminate incorrect options: Osmosis involves the movement of water, not gases, so it is not applicable here. Facilitated diffusion requires the assistance of transport proteins, which is not the case for gas exchange in the alveoli. Active transport requires energy (ATP), but gas exchange in the alveoli is a passive process.
4. Focus on the correct process: Simple diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration without the use of energy or transport proteins. This is the mechanism by which oxygen and carbon dioxide cross the alveolar and capillary membranes.
5. Conclude: The correct answer is 'Simple diffusion,' as it explains how gases move across the alveolar and capillary membranes driven by their concentration gradients.