A : Elevated levels of `Na^(+)` increases the heart rate and contractility.
R : Elevated `Na^(+)` level increase the excitability of SA node.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statements, then mark (4).

Answer

To solve the assertion and reason question, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
The assertion states: "Elevated levels of Na⁺ increase the heart rate and contractility."

- Elevated sodium levels can lead to increased blood volume due to osmotic effects.
- Increased blood volume can raise blood pressure.
- However, increased blood pressure can lead to a reflex bradycardia (decrease in heart rate) as the body tries to maintain homeostasis.
- Thus, the assertion that elevated Na⁺ increases heart rate and contractility is **incorrect**.

### Step 2: Analyze the Reason
The reason states: "Elevated Na⁺ level increases the excitability of the SA node."

- The SA node is responsible for initiating the heartbeat.
- Elevated sodium levels can increase the excitability of the SA node by affecting the membrane potential.
- Increased sodium influx can lead to depolarization, making the SA node more excitable and potentially increasing heart rate.
- Therefore, the reason is **correct**.

### Step 3: Conclusion
- The assertion is false, and the reason is true.
- Therefore, the correct answer is that the assertion is incorrect, and the reason is correct.

### Final Answer
The answer to the question is option 4: "If the assertion is false and the reason is true."

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