The kidney regulates electrolyte and fluid balance in the body through its main functions of filtering blood to form urine and reabsorbing necessary water and ions back into circulation. The functional unit of the kidney is the nephron, which contains a glomerulus for blood filtration and a tubule system for reabsorption and secretion. As fluid moves through the nephron's tubules, complex mechanisms involving countercurrent exchange and active transport work to concentrate urine by returning water to the blood while retaining waste products.