The kidney controls the production of urine through three main processes: 1) Glomerular filtration - The glomerulus filters blood in the kidneys, allowing fluid and dissolved substances to pass into the tubules. 2) Reabsorption - Most of the fluid and useful substances like water and glucose are reabsorbed back into the blood from the tubules. 3) Tubular secretion - The tubules actively secrete some substances like acids and drugs directly into the urine. Together these processes determine the final composition and concentration of urine to regulate water and electrolyte balance.