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

Explanation

### Step-by-Step Solution:

**Step 1: Understanding Double Circulation**
- Double circulation refers to the two separate pathways through which blood circulates in the human body. In humans, the heart is divided into four chambers: the right atrium, right ventricle, left atrium, and left ventricle.

**Step 2: Pathway of Deoxygenated Blood**
- Deoxygenated blood (blood low in oxygen and high in carbon dioxide) returns from the body to the right atrium of the heart.
- From the right atrium, the blood flows into the right ventricle.
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