Describe double circulation of blood in human beings. Why is it necessary?

Explanation

### Step-by-Step Solution:

1. **Definition of Double Circulation**:
- Double circulation refers to the process in which blood flows through the heart two times during one complete cycle of circulation. This system is unique to humans and some other mammals.

2. **Components of Double Circulation**:
- There are two main types of circulation involved in double circulation:
- **Pulmonary Circulation**:
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