Oxygen and carbon dioxide are transported between the blood and interstitial fluid by way of:

• A. endocytosis and exocytosis
• B. facilitated diffusion through protein channels
• C. simple diffusion across capillary walls
• D. active transport via membrane pumps

Answer: (C)

Verified step by step guidance

1. Understand the key concept: Gas exchange (oxygen and carbon dioxide) occurs between the blood and interstitial fluid primarily through passive processes, as these gases are small, nonpolar molecules that can easily cross cell membranes.
2. Recall the mechanism of simple diffusion: Simple diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration without the need for energy or transport proteins. This is how oxygen and carbon dioxide move across capillary walls.
3. Eliminate incorrect options: Endocytosis and exocytosis involve vesicle formation and are used for larger molecules, not gases. Facilitated diffusion requires protein channels, which are not necessary for small, nonpolar gases like oxygen and carbon dioxide. Active transport requires energy (ATP) and is used to move substances against their concentration gradient, which is not the case here.
4. Focus on the correct answer: Simple diffusion across capillary walls is the correct mechanism because it aligns with the properties of oxygen and carbon dioxide and the concentration gradients present in the blood and interstitial fluid.
5. Apply this understanding: In the lungs, oxygen diffuses from the alveoli (high concentration) into the blood (low concentration), while carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration). Similarly, at the tissues, oxygen diffuses from the blood into the interstitial fluid, and carbon dioxide diffuses in the opposite direction.