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Blood enters the heart because muscles o...

Blood enters the heart because muscles of the

A

Atria relax

B

Ventricle contract

C

Ventricle relax

D

Atria contract

Text Solution

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The correct Answer is:
### Step-by-Step Solution: 1. **Understanding Heart Function**: The heart has four chambers: two atria (upper chambers) and two ventricles (lower chambers). Blood enters the heart through the atria. 2. **Muscle Action in the Heart**: The muscles of the heart can either contract or relax. When they contract, it is referred to as "systole," and when they relax, it is referred to as "diastole." 3. **Atria Relaxation (Atrial Diastole)**: Blood enters the heart primarily when the atria are in a state of relaxation (diastole). During this phase, the pressure within the atria decreases. 4. **Pressure Dynamics**: As the atria relax, the pressure inside them drops, creating a pressure gradient. This allows blood from the veins (which have higher pressure) to flow into the atria. 5. **Conclusion**: Therefore, blood enters the heart because the muscles of the atria relax. This is the correct answer to the question posed. ### Final Answer: Blood enters the heart because the muscles of the **atria relax**. ---
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