Kidneys produce urine → ureters transport it → urinary bladder stores it → urethra carries it out. Kidneys are retroperitoneal on the posterior abdominal wall.
The nephron is the functional unit. Blood is filtered at the renal corpuscle (glomerulus + Bowman's/glomerular capsule), then tubular processes modify the filtrate through reabsorption and secretion.
Filtration: water/small solutes move from glomerular blood into capsule. Reabsorption: needed substances/water return from tubule to blood. Secretion: selected substances move from blood into tubule. What remains becomes urine.
Kidneys regulate fluid volume, electrolytes, acid-base balance and blood pressure-related systems. They produce erythropoietin and participate in vitamin D activation. ADH promotes water reabsorption in collecting ducts; aldosterone promotes sodium retention/potassium secretion in distal nephron regions.
Do not claim massage “flushes toxins through the kidneys.” The kidneys continuously regulate blood chemistry. Acute kidney infection, severe flank pain, systemic illness, dialysis access, edema of uncertain cause and advanced disease require appropriate screening/modification/referral.