**Question:** Normal diffusion of CO2 at rest

**Core Concept:** Diffusion is the passive movement of molecules across a selectively permeable membrane from an area of high concentration to an area of low concentration. Carbon dioxide (CO2) is a small, polar molecule that readily diffuses across biological membranes.

**Why the Correct Answer is Right:** In humans, the normal diffusion of carbon dioxide at rest occurs primarily in the alveoli of the lungs, where the concentration gradient is the most favorable for CO2 diffusion. The alveoli have a high CO2 concentration, while the surrounding capillaries have a low CO2 concentration. This passive movement of CO2 leads to its clearance from the blood and deposition in the alveoli.

**Why Each Wrong Option is Incorrect:**

A. This option does not specifically address the correct diffusion process in relation to the alveoli and capillaries.
B. This option discusses the importance of diffusion in maintaining pH balance, but it does not address the specific CO2 diffusion at rest in the alveoli and capillaries.
C. This option talks about the role of exhalation in removing CO2, but does not explain the passive diffusion process within the respiratory system.
D. This option discusses the role of the respiratory system in CO2 removal, but does not specifically address the normal diffusion process at rest in the alveoli and capillaries.

**Clinical Pearl / High-Yield Fact:** Normal diffusion of CO2 at rest is crucial for maintaining acid-base balance in the body. As CO2 diffuses from the capillaries into the alveoli, it lowers the pH of the blood, promoting the excretion of H+ ions via the kidneys to maintain a stable pH level. Adequate CO2 diffusion ensures efficient gas exchange within the lungs and helps regulate pH, a vital parameter for cellular function and overall organ health.