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Renal, a term used in medical contexts to denote relations to the kidneys, describes both the organ system and its functions. The kidneys are a pair of bean-shaped organs located in the retroperitoneal space on either side of the spine. They filter blood, remove waste, balance fluids and electrolytes, and contribute to blood pressure regulation and acid-base balance.

Each kidney contains about a million nephrons, the microscopic filtering units. The nephron consists of a glomerulus, a surrounding Bowman's capsule, and a tubular system that includes the proximal tubule, loop of Henle, distal tubule, and collecting duct. Blood arrives via the renal arteries, and urine exits through the renal pelvis to the ureter.

Urine formation combines filtration, reabsorption, and secretion. Filtration occurs at the glomerulus, producing a filtrate that becomes urine after selective reabsorption of water and solutes and secretion of waste products. The kidneys regulate fluid and electrolyte balance, acid-base status, and blood pressure through hormones and signaling pathways, including the renin-angiotensin-aldosterone system, erythropoietin production, and vitamin D activation.

Common kidney diseases include chronic kidney disease, acute kidney injury, nephrolithiasis (kidney stones), pyelonephritis, and glomerulonephritis. Evaluation relies on blood tests for creatinine and estimated glomerular filtration rate, urinalysis, imaging, and sometimes kidney biopsy. Treatment ranges from lifestyle and medication to dialysis and kidney transplantation in advanced disease.