Each kidney is constituted of numerous __which are the functional units of excretory system.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the name of the functional units of the excretory system found in each kidney.

2. **Identifying the Functional Unit**: In the human body, the kidneys are responsible for filtering blood and producing urine. The functional units that carry out these processes are called nephrons.

3. **Defining Nephrons**: Nephrons are the microscopic structures in the kidneys that filter blood, remove waste, and regulate the balance of fluids and electrolytes. They play a crucial role in urine production.

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