A red blood cell, entering the right side of the heart passes by or through the following structures
(1) Atriventricular valves
(2) Semilunar valves
(3) Right atrium
(4) Right ventricle
(5) SAN
Which of the following options represents the correct sequence ?

Options

A. `2 rarr 3 rarr 1 rarr 4 rarr 5`
B. `3 rarr 1 rarr 5 rarr 2 rarr 4`
C. `3 rarr 5 rarr 1 rarr 2 rarr 4`
D. `5 rarr 3 rarr 1 rarr 4 rarr 2`

Explanation

To determine the correct sequence a red blood cell passes through when entering the right side of the heart, we need to understand the flow of blood through the heart's structures. Here’s the step-by-step solution:

1. **Sinoatrial Node (SAN)**: The red blood cell does not actually pass through the SAN, but it is important to note that the SAN is responsible for initiating the heartbeat. However, for the purpose of this question, we will consider it as the starting point for the blood flow.

2. **Right Atrium**: The red blood cell enters the right atrium after being collected from the body via the superior and inferior vena cavae. This chamber receives deoxygenated blood.

3. **Atrioventricular Valve (Tricuspid Valve)**: From the right atrium, the blood flows through the tricuspid valve (the right atrioventricular valve) into the right ventricle. This valve prevents backflow into the atrium when the ventricle contracts.

4. **Right Ventricle**: The blood now enters the right ventricle, where it is pumped out of the heart.

5. **Semilunar Valve (Pulmonary Valve)**: Finally, the blood moves through the pulmonary semilunar valve into the pulmonary artery, which carries the deoxygenated blood to the lungs for oxygenation.

Putting this all together, the correct sequence of structures that the red blood cell passes through is:
**5 (SAN) → 3 (Right Atrium) → 1 (Atrioventricular Valve) → 4 (Right Ventricle) → 2 (Semilunar Valve)**.

Thus, the final answer is **5, 3, 1, 4, 2**.