The nephron is the fundamental functional unit of the kidney. Its basic function is to filter blood, reabsorb essential substances, and excrete waste products in the form of urine. This process involves three main steps: 1. Filtration: Blood enters the glomerulus, a network of capillaries, where water, salts, glucose, urea, and other small molecules are filtered out into Bowman's capsule. 2. Reabsorption: As the filtrate passes through the renal tubules (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct), essential substances like glucose, amino acids, water, and ions are reabsorbed back into the bloodstream. 3. Secretion: Waste products and excess ions that were not filtered out are actively secreted from the blood into the tubules. The remaining fluid, now concentrated urine, is then transported to the bladder for excretion.