Blood enters the heart because muscles of the

Options

A. atrium contracts
B. atrium relaxes
C. ventricles relaxes
D. ventricle contracts

Answer

To answer the question, "Blood enters the heart because muscles of the...," we can break down the process step by step:

### Step-by-Step Solution:

1. **Understanding Heart Structure**:
- The heart consists of four chambers: two atria (upper chambers) and two ventricles (lower chambers).
- The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs.

2. **Blood Flow into the Heart**:
- Deoxygenated blood from the body returns to the heart through two major veins: the superior vena cava and the inferior vena cava.
- This blood enters the right atrium.

3. **Role of Atria**:
- The atria are responsible for receiving blood. They contract and relax during the cardiac cycle.
- When the atrium relaxes, it creates a low-pressure environment that allows blood to flow in from the vena cava.

4. **Pressure Difference**:
- The pressure in the vena cava (where the blood is coming from) is higher than the pressure in the relaxed atrium.
- This pressure difference facilitates the movement of blood into the atrium.

5. **Conclusion**:
- Therefore, blood enters the heart primarily because the muscles of the atrium relax, allowing blood to flow in due to the pressure difference.

### Final Answer:
Blood enters the heart because the muscles of the **atrium relax**.

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