How is the majority of CO2 generated by respiring tissue transported to back to the lungs?
The correct answer and explanation is:
Correct Answer:
The majority of CO₂ generated by respiring tissue is transported back to the lungs as bicarbonate ions (HCO₃⁻) in the blood plasma.
Explanation (300 Words):
Carbon dioxide (CO₂) is a waste product produced by cells during aerobic respiration. Once generated in tissues, CO₂ must be transported to the lungs for exhalation. The human body has three main mechanisms for transporting CO₂ in the blood:
- Dissolved in plasma (7-10%): A small portion of CO₂ is transported directly dissolved in the blood plasma. Since CO₂ is somewhat soluble in water, it can dissolve in plasma, but this accounts for only a minor portion of total transport.
- Bound to hemoglobin (20-23%): CO₂ can bind directly to the amino groups on hemoglobin (not the heme group that binds oxygen), forming a compound called carbaminohemoglobin. This mechanism facilitates CO₂ transport but is limited in capacity.
- As bicarbonate ions (70%): The vast majority of CO₂ (about 70%) is converted into bicarbonate (HCO₃⁻) ions in red blood cells. This process is facilitated by the enzyme carbonic anhydrase, which catalyzes the reaction: CO2+H2O→H2CO3→H++HCO3−\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^- The resulting bicarbonate ions then diffuse into the plasma for transport to the lungs.
When the blood reaches the lungs, this reaction is reversed: bicarbonate is converted back into CO₂ and water. The CO₂ then diffuses into the alveoli and is exhaled.
This bicarbonate transport system is crucial for maintaining blood pH homeostasis. It acts as a buffer, regulating the acid-base balance of the blood. Without this mechanism, CO₂ would accumulate and acidify the blood, leading to harmful physiological effects.
In conclusion, bicarbonate ions represent the primary form in which CO₂ is carried from tissues to the lungs.