Which one of the following statement is not correct ?

Options

A. The walls of ventricles are much thicker than that of the atria
B. The SAN can generate the maximum number of action potential
C. Action potential (atrial systole) increases the flow of blood into the ventricles by about 60 percent
D. Stroke volume is approximately 70 ml

Answer

To solve the question of which statement is not correct, we will analyze each of the provided options one by one.

### Step-by-Step Solution:

1. **Understanding the Question**:
We need to identify which of the four statements is incorrect regarding the human heart and its functions.

2. **Analyzing Option 1**:
- Statement: "The walls of ventricles are much thicker than that of atria."
- Explanation: In the human heart, the ventricles (especially the left ventricle) have thicker walls compared to the atria. This is because ventricles need to pump blood with greater force and over longer distances.
- Conclusion: This statement is **correct**.

3. **Analyzing Option 2**:
- Statement: "The S.A.N. (Sinoatrial Node) is the pacemaker of the heart and generates the maximum number of action potentials."
- Explanation: The Sinoatrial Node (S.A.N.) is indeed the natural pacemaker of the heart and is responsible for initiating the heartbeat by generating action potentials.
- Conclusion: This statement is **correct**.

4. **Analyzing Option 3**:
- Statement: "Atrial systole increases the flow of blood into the ventricle by about 60%."
- Explanation: During atrial systole, the atria contract and push blood into the ventricles. However, approximately 70% of the blood enters the ventricles passively before atrial contraction, and only about 30% is actively pushed in during atrial systole. Therefore, stating that it increases the flow by 60% is incorrect.
- Conclusion: This statement is **incorrect**.

5. **Analyzing Option 4**:
- Statement: "Stroke volume is approximately 70 ml."
- Explanation: Stroke volume refers to the amount of blood pumped by each ventricle per heartbeat, and it is approximately 70 ml in a healthy adult.
- Conclusion: This statement is **correct**.

6. **Final Conclusion**:
After analyzing all the options, we find that **Option 3** is the incorrect statement regarding the flow of blood during atrial systole.

### Answer:
**Option 3 is the incorrect statement.**