**Question:** All of the following statements concerning CO2 transport are TRUE, EXCEPT:

A. Plasma CO2 is transported bound to hemoglobin (Hb) in erythrocytes.
B. HCO3- ions are the primary carriers of CO2 in the blood.
C. CO2 is transported as a gas in the alveoli.
D. Diffusion is the primary means of CO2 transport in the body.

**Correct Answer:** **D.** Diffusion is the primary means of CO2 transport in the body.

**Core Concept:**
Carbon dioxide (CO2) is a waste product of cellular respiration and needs to be transported and eliminated from the body. In humans, CO2 is transported primarily through two mechanisms: diffusion and binding to hemoglobin (Hb).

**Why the Correct Answer is Right:**

Diffusion is the movement of substances from an area of high concentration to an area of low concentration until equilibrium is reached. In the context of CO2 transport, diffusion occurs between the alveoli (air sacs in the lungs) and the capillaries. In the alveoli, CO2 concentration is high (around 40-50 mmHg), while in the blood, CO2 concentration is lower (around 20 mmHg). As a result, CO2 diffuses from the alveoli into the blood, maintaining a gradient that promotes further diffusion.

**Why Each Wrong Option is Incorrect:**

A. Plasma CO2 is transported bound to hemoglobin (Hb) in erythrocytes (red blood cells). This is correct, as CO2 binds to Hb in erythrocytes and is transported through the bloodstream to the lungs for exhalation.

B. HCO3- ions are the primary carriers of CO2 in the blood. While HCO3- ions are involved in the formation of bicarbonate (HCO3-) and carbonic acid (H2CO3) in the kidneys, they are not directly responsible for CO2 transport. CO2 binds to Hb, as explained above.

C. CO2 is transported as a gas in the alveoli. While CO2 exists as a gas in the alveoli, this option is incorrect because CO2 is transported through the bloodstream, not just as a gas.

D. Diffusion is the primary means of CO2 transport in the body. As mentioned earlier, diffusion occurs between the alveoli and capillaries, but not as the primary mode of transport for CO2. CO2 binding to Hb and its transport through the bloodstream is the primary mode of CO2 elimination.

**Clinical Pearls:**

1. The primary role of erythrocytes (red blood cells) is to transport oxygen (O2) and carbon dioxide (CO2) throughout the body.
2. O2 binds to Hb in the lungs, and CO2 binds to Hb in the lungs and red blood cells.
3. CO2 is transported primarily bound to Hb in erythrocytes, not just as a gas.

**Core Concept Explanation:**

CO2 is primarily transported in the body via binding to hemoglobin (Hb) within erythrocytes (