There is a pair of bean-shaped organs P in the human body towards the back, just above the waist. A waste product Q formed by the decomposition of unused proteins in the liver is brought into organs P through blood. The numerous tiny filters R present inside the organs P clean the dirty blood by removing the waste product Q. If due to some reason, the organs P of a person stop functioning completely, the person's blood can be cleaned periodically by a process S as to save his life. What areP, Q, R and S?

Explanation

To solve the question, we need to identify the terms P, Q, R, and S based on the clues provided in the question.

### Step-by-Step Solution:

1. **Identify Organ P**:
- The question states that there is a pair of bean-shaped organs located towards the back, just above the waist.
- In the human body, the organs that fit this description are the **kidneys**.
- Therefore, **P = Kidneys**.

2. **Identify Waste Product Q**:
- The waste product is formed by the decomposition of unused proteins in the liver and is brought to organ P through the blood.
- The main nitrogenous waste produced from protein metabolism is **urea**.
- Hence, **Q = Urea**.

3. **Identify Tiny Filters R**:
- The question mentions numerous tiny filters present inside organ P (the kidneys) that clean the dirty blood by removing the waste product Q (urea).
- These tiny filters in the kidneys are known as **nephrons**.
- Thus, **R = Nephrons**.

4. **Identify Process S**:
- The question states that if organ P (the kidneys) stops functioning completely, the person's blood can be cleaned periodically by a process S to save their life.
- The medical procedure used to clean the blood when kidneys fail is called **dialysis**.
- Therefore, **S = Dialysis**.

### Final Answer:
- **P = Kidneys**
- **Q = Urea**
- **R = Nephrons**
- **S = Dialysis**