In plasma, carbon-di-oxide is mainly transported as: (Repeat)
## Core Concept
Carbon dioxide (CO2) transport in the blood is a critical process for removing CO2, a waste product of metabolism, from tissues to the lungs for exhalation. In plasma, CO2 is transported in several forms, but one form is predominant. This process involves various chemical reactions and interactions with blood components.
## Why the Correct Answer is Right
The correct answer, **Bicarbonate ions (HCO3-)**, is the main form in which CO2 is transported in the plasma. Here's why: When CO2 enters the blood from tissues, it reacts with water (H2O) in the presence of the enzyme **carbonic anhydrase** (CA) within red blood cells to form **carbonic acid (H2CO3)**, which then quickly dissociates into **bicarbonate ions (HCO3-)** and **hydrogen ions (H+)**. Although this reaction primarily occurs inside red blood cells, the bicarbonate ions can move out of the red blood cells into the plasma. In the plasma, bicarbonate ions serve as a buffer and are the primary form in which CO2 is transported back to the lungs.
## Why Each Wrong Option is Incorrect
- **Option A:** *Dissolved CO2* is incorrect because, although some CO2 does dissolve directly in blood plasma, this form accounts for a small fraction (about 5-10%) of the total CO2 transported in the blood.
- **Option C:** *Carbaminohemoglobin* is incorrect because it refers to a compound formed when CO2 binds directly to hemoglobin in red blood cells, not a primary method of CO2 transport in plasma.
- **Option D:** *Carbonic acid* is incorrect because, while carbonic acid is an intermediate in the formation of bicarbonate ions, it is not the main form in which CO2 is transported in the plasma.
## Clinical Pearl / High-Yield Fact
A key point to remember is that the bicarbonate buffering system is crucial for maintaining acid-base balance in the body. The reaction of CO2 with H2O to form H2CO3, which then dissociates into HCO3- and H+, is reversible and depends on the concentration of CO2, which is efficiently regulated by respiratory and renal mechanisms.
## Correct Answer: B. Bicarbonate ions.