Describe main functions of kidneys.

Explanation

### Step-by-Step Solution:

1. **Introduction to Kidneys**:
- The kidneys are vital organs in the human body, typically described as two bean-shaped structures. Each kidney is approximately the size of a fist and is located just below the rib cage, one on each side of the spine.

2. **Function 1: Filtration of Blood**:
- The primary function of the kidneys is to filter the blood. This process involves removing waste products and excess substances from the bloodstream, ensuring that the blood remains clean and balanced.

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