[A]: At any given `CO_2` tension, reduced blood takes up larger amount of `CO_2` than oxygenated blood.
[R]: Oxygenation of blood causes evolution of `CO_2` in lungs.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To solve the question regarding the assertion and reason statements about carbon dioxide (CO₂) and oxygen in blood, we can break it down into a step-by-step analysis:

### Step 1: Understand the Assertion
The assertion states that at any given CO₂ tension, reduced blood takes up a larger amount of CO₂ than oxygenated blood. This means that deoxygenated blood (reduced blood) has a higher capacity to absorb CO₂ compared to oxygenated blood.

**Hint:** Think about the role of hemoglobin in binding gases and how its state (oxygenated vs. deoxygenated) affects its ability to carry CO₂.

### Step 2: Analyze the Mechanism
At the lung level, the partial pressure of oxygen (pO₂) is higher due to fresh air entering the alveoli. This high pO₂ causes hemoglobin to bind with oxygen, forming oxyhemoglobin. Conversely, in tissues where metabolism occurs, CO₂ is produced, resulting in a higher partial pressure of CO₂ (pCO₂). This leads to hemoglobin binding with CO₂, forming carbaminohemoglobin (HbCO₂).

**Hint:** Consider how gas exchange occurs in the lungs versus the tissues and how partial pressures influence gas binding.

### Step 3: Understand the Reason
The reason states that the oxygenation of blood causes the evolution (release) of CO₂ in the lungs. This is indeed true because when deoxygenated blood reaches the lungs, the high pO₂ causes hemoglobin to release CO₂ and bind with O₂ instead.

**Hint:** Reflect on the process of gas exchange and how the binding and release of gases are influenced by their respective partial pressures.

### Step 4: Conclusion
Both the assertion and reason statements are true. The reason correctly explains the assertion. Therefore, the answer is that both statements are true and the reason is the correct explanation for the assertion.

**Hint:** When evaluating statements, always check if the reason logically supports the assertion.

### Final Answer
Both the assertion (A) and reason (R) are true, and R is the correct explanation of A.