Oxygen-rich blood from the lungs comes to the thin-walled upper chamber of the heart on the left. The left upper chamber (A) then relaxes. It then contracts and the blood is allowed to enter the next chamber (B), as it expands. When the muscular left lower chamber of heart contracts the blood is pumped out to the body via aorta.
Deoxygenated blood reaches from the body to the upper chamber on the right side of heart (C) and it expands. As this part contracts, the corresponding lower chamber (D) dilates. This transfers the blood to right ventricle, which in turn pumps it to the lungs for oxygenated.
What prevents backflow of blood inside the heart during contraction?

Options

A. Valves in heart
B. Thick muscular walls of ventricles
C. Thin walls of atria
D. All of the above

Explanation

### Step-by-Step Solution:

1. **Understanding Blood Flow in the Heart**:
- Oxygen-rich blood from the lungs enters the left atrium (upper chamber A) of the heart.
- This blood then moves to the left ventricle (lower chamber B) when the left atrium contracts.

2. **Pumping Blood to the Body**:
- The left ventricle contracts, pumping oxygen-rich blood into the aorta, which distributes it throughout the body.

3. **Deoxygenated Blood Return**:
- Deoxygenated blood from the body returns to the right atrium (upper chamber C) of the heart.
- The right atrium expands and then contracts, sending blood into the right ventricle (lower chamber D).

4. **Pumping Blood to the Lungs**:
- The right ventricle then contracts, pumping deoxygenated blood to the lungs for oxygenation.

5. **Preventing Backflow of Blood**:
- The heart has valves that prevent the backflow of blood during contraction.
- Specifically, the tricuspid valve is located between the right atrium and right ventricle, and the bicuspid (or mitral) valve is located between the left atrium and left ventricle.

6. **Conclusion**:
- The valves in the heart (tricuspid and bicuspid) are responsible for preventing backflow of blood during the contraction of the heart chambers.