CBSE Class 10 Answered

The kidneys are the main organs of homeostasis because they maintain the acid base balance and the water salt balance of the blood. Nephron is the basic structural and functional unit of the kidney. Nephrons eliminate wastes from the body and regulate water and soluble substances by filtering the blood, reabsorbing what is needed and excreting the rest as urine.  

 Urine is formed in three steps: Filtration, Reabsorption, and Secretion.

1. Blood enters the afferent arteriole and flows into the glomerulus. Blood in the glomerulus has both filterable blood components and non-filterable blood components. Filterable blood components include  water, nitrogenous waste, nutrients and salts (ions). In this step, filtration of water and dissolved substances occurs out of the blood in the glomeruli and into Bowman's capsule.
2. Next reabsorption of water and dissolved substances out of the kidney tubules back into the blood (note that this process prevents substances needed by the body from being lost in the urine);
3. Secretion of hydrogen ions (H⁺), potassium ions (K⁺), ammonia (NH₃), and certain drugs occurs out of the blood and into the kidney tubules, where they are eventually eliminated in the urine.