Mammals are said to have a double circulatory system.it means that

Options

A. There are two types of blood vessels attached to every orgen, e.g,an artery and a vein.
B. There are two system, one from the heart to the lungs and back to the rest of the body via heart.
C. The blood circulates twice in the heart.
D. Both (2) and (3)

Answer

**Step-by-Step Solution:**

1. **Understanding Double Circulation**:
- Double circulation refers to the two distinct circuits through which blood flows in mammals. It consists of the pulmonary circulation (from the heart to the lungs and back) and the systemic circulation (from the heart to the rest of the body and back).

2. **Components of the Circulatory System**:
- In mammals, the heart is divided into four chambers: two atria (right and left) and two ventricles (right and left). This structure allows for efficient separation of oxygenated and deoxygenated blood.

3. **Pulmonary Circulation**:
- Deoxygenated blood returns to the right atrium from the body via the superior and inferior vena cava. It then moves to the right ventricle, which pumps it through the pulmonary artery to the lungs for oxygenation.

4. **Systemic Circulation**:
- Once the blood is oxygenated in the lungs, it returns to the left atrium via the pulmonary veins. From there, it moves to the left ventricle, which pumps the oxygenated blood through the aorta to the rest of the body.

5. **Circulation Cycle**:
- The blood circulates twice through the heart: once to receive oxygen (pulmonary circulation) and once to deliver oxygen to the body (systemic circulation). This ensures that tissues receive a constant supply of oxygen-rich blood.

6. **Conclusion**:
- Therefore, mammals have a double circulatory system, meaning blood circulates twice through the heart: once for oxygenation and once for distribution to the body. The correct answer to the question is that both the second and third statements are correct.