The nephron, the functional unit of the kidney, performs filtration at the glomerulus, selective reabsorption along the tubular system, and secretion to maintain fluid, electrolyte, and acid-base homeostasis. Glomerular filtration rate (GFR), normally 90-120 mL/min/1.73m², is determined by the balance of hydrostatic and oncotic pressures across the glomerular capillary membrane, regulated by afferent and efferent arteriolar tone. The proximal convoluted tubule reabsorbs approximately 65% of filtered sodium and water, all filtered glucose and amino acids, and most bicarbonate through sodium-hydrogen exchange. The loop of Henle establishes the medullary concentration gradient through countercurrent multiplication, while the collecting duct responds to antidiuretic hormone (ADH) by inserting aquaporin-2 channels for water reabsorption.