Describe double circulation in human beings.
(a) Why is it necessary ?
(b) How are arteries different from veins ?

Explanation

### Step-by-Step Solution

**1. Understanding Double Circulation:**
- Double circulation refers to the process in which blood passes through the heart twice during one complete circuit of the body. In humans, this involves two separate pathways: one for oxygenated blood and another for deoxygenated blood.

**2. The Pathway of Blood:**
- **Deoxygenated Blood:** Blood that is low in oxygen and high in carbon dioxide returns to the heart from the body. It enters the right atrium, moves to the right ventricle, and is then pumped to the lungs via the pulmonary artery.
- **Oxygenated Blood:** In the lungs, carbon dioxide is exchanged for oxygen. The oxygen-rich blood returns to the heart, entering the left atrium, moving to the left ventricle, and is then pumped out to the rest of the body through the aorta.
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