**Question:** CO2 primarily transported in the arterial blood as-

A. Carbonic acid
B. Bicarbonate ion
C. Bound to hemoglobin
D. Mixed gas

**Core Concept:**
Carbon dioxide (CO2) is a waste product of cellular respiration and needs to be transported to the lungs for exhalation. In the blood, CO2 is primarily transported in the form of bicarbonate ion (HCO3-) and carbonic acid (H2CO3). These forms are derived from the buffering system maintained by the kidneys.

**Why the Correct Answer is Right:**
CO2 is mainly transported as bicarbonate ion (HCO3-) and carbonic acid (H2CO3) because these forms are derived from the buffering system maintained by the kidneys. The kidneys regulate the pH of the blood by converting CO2 into HCO3- through the reaction:

H2CO3 + H2O HCO3- + H+ + H2O

HCO3- is then combined with H+ ions to form H2CO3, which is further processed by the kidneys to maintain blood pH homeostasis.

**Why Each Wrong Option is Incorrect:**

A. Carbonic acid (H2CO3) is not the primary transport form because it is a reactive species, rapidly converted into bicarbonate ion by the kidneys.

B. Bicarbonate ion (HCO3-) is not the primary transport form as it is the final product of the buffering process involving carbonic acid (H2CO3).

C. Hemoglobin binds CO2, but the primary mode of CO2 transportation in the blood is through the buffering system mentioned above rather than direct binding to hemoglobin.

D. Mixed gas refers to the combined presence of different gases, not CO2 forms. Mixed gas would include oxygen, carbon dioxide, and nitrogen, not specific transport forms like bicarbonate ion (HCO3-) and carbonic acid (H2CO3).

**Clinical Pearls:**

1. The primary transport of CO2 in the blood is through the buffering system maintained by the kidneys, ensuring efficient CO2 removal and maintaining blood pH homeostasis.
2. Adequate renal function is essential for maintaining blood pH and CO2 homeostasis. Impaired renal function can lead to respiratory alkalosis and respiratory acidosis.
3. The buffering system involves the interaction between CO2, bicarbonate ion (HCO3-), and hydrogen ions (H+) to maintain blood pH homeostasis.