A portal system is a system in which

Options

A. A vein begins from an organ and ends in heart
B. An artery breaks up in an organ & restarts by the union of its capillaries
C. The blood from gut is brought in to kidneys before it is poured in to heart
D. A vein breaks up in an organ in to capillaries & restarts by their union as a new vein in the same organ

Answer

To answer the question "A portal system is a system in which," we need to analyze the options provided and understand the definition of a portal system in human physiology.

### Step-by-Step Solution:

1. **Understanding the Definition of a Portal System**:
- A portal system is a unique arrangement of blood vessels where blood flows through two sets of capillaries before returning to the heart. This system allows for the direct transport of substances from one organ to another.

2. **Analyzing the Options**:
- **Option A**: "A vein begins from an organ and ends in heart."
- This is incorrect because in a portal system, blood does not go directly to the heart.

- **Option B**: "An artery breaks up in an organ and restarts by the union of its capillaries."
- This is also incorrect because it describes an artery, while a portal system involves veins.

- **Option C**: "The blood from gut is brought into kidneys before it is poured into heart."
- This is misleading. While the hepatic portal system does transport blood from the gut to the liver, it does not involve the kidneys in the same way described here.

- **Option D**: "A vein breaks up in an organ into capillaries and restarts by their union as a new vein in the same organ."
- This is the correct description of a portal system. In a portal system, a vein collects blood from an organ, which then breaks into capillaries and eventually forms a new vein within that organ.

3. **Conclusion**:
- Based on the analysis, the correct answer is **Option D**: "A vein breaks up in an organ into capillaries and restarts by their union as a new vein in the same organ."