Step 1: Understanding the Concept:
The human kidney is the organ responsible for filtering blood to remove nitrogenous wastes like urea, and for maintaining the water-electrolyte balance (osmoregulation).
The vast complexity of these tasks is managed by millions of identical, microscopic structural and functional modules.
Step 2: Detailed Explanation:
The Nephron is the microscopic unit that performs the actual filtration of blood and formation of urine.
Each human kidney contains roughly $1,000,000$ to $1,200,000$ nephrons.
A nephron consists of several specialized segments:
1. Renal Corpuscle: Comprising the Glomerulus (a high-pressure capillary bed) and Bowman's capsule. This is the site of ultrafiltration.
2. Proximal Convoluted Tubule (PCT): Where $70-80%$ of electrolytes and water, and almost all nutrients (glucose, amino acids) are reabsorbed.
3. Loop of Henle: A U-shaped tube that plays a critical role in concentrating urine by creating an osmotic gradient in the medulla.
4. Distal Convoluted Tubule (DCT): Site for conditional reabsorption of sodium and water, and secretion of ions to maintain pH and ionic balance.
Why other options are incorrect:
(a) Neuron: The functional unit of the nervous system.
(b) Alveolus: The functional unit of the respiratory system (lungs).
(d) Glomerulus: It is only a part of the nephron (the filtration head). It cannot function as a unit on its own without the tubule system to process the filtrate.
Step 3: Final Answer:
The Nephron is the structural and functional unit of the kidney, containing all the machinery required to produce urine from blood.