The urinary system has an upper tract — the paired kidneys and ureters, lying retroperitoneally on the posterior abdominal wall — and a lower tract: the urinary bladder in the pelvis and the urethra that drains it to the exterior. Urine is made continuously by the kidneys, propelled down the ureters by peristalsis, stored at low pressure in the bladder and expelled intermittently and voluntarily (micturition). The urinary tract is lined throughout from the renal calyces to the proximal urethra by urothelium (transitional epithelium), a stretchable, highly impermeable barrier.
The functional unit of the kidney is the nephron (≈1 million per kidney at birth; no new nephrons form after ≈36 weeks of gestation). Each nephron begins as a renal corpuscle (glomerulus + glomerular capsule) where plasma is ultrafiltered across a three-layer filtration barrier, and continues as a tubule — proximal convoluted tubule, nephron loop (of Henle), distal convoluted tubule — that empties into a collecting duct. Urine formation is the net result of three processes: glomerular filtration (≈125 mL/min, ≈180 L/day), tubular reabsorption (>99% of filtered water and Na⁺, essentially all glucose and amino acids) and tubular secretion (H⁺, K⁺, NH₄⁺, organic acids and bases, many drugs). Excretion = filtration − reabsorption + secretion.
The kidneys receive ≈20–25% of the resting cardiac output (≈1.1–1.2 L/min renal blood flow) despite weighing <0.5% of body mass, because their job is to process plasma, not merely to be perfused. Their microcirculation is unique: two capillary beds in series (glomerular capillaries, then peritubular capillaries or vasa recta) separated by the efferent arteriole, a portal arrangement that allows high pressure for filtration in the first bed and low pressure favouring reabsorption in the second. Renal blood flow and GFR are autoregulated over a mean arterial pressure of ≈80–180 mmHg by the myogenic response and tubuloglomerular feedback.
Topographically the nephron is organised in a strict pattern: glomeruli, convoluted tubules and cortical collecting ducts lie in the cortex, while loops of Henle, vasa recta and medullary collecting ducts run radially in the medulla. This architecture generates a corticomedullary osmotic gradient (≈300 → ≈1200 mOsm/kg H₂O) by countercurrent multiplication, which — under the control of antidiuretic hormone — lets the kidney excrete urine as dilute as ≈50 or as concentrated as ≈1200 mOsm/kg.
The kidney integrates many homeostatic loops: ECF volume and blood pressure (RAAS, pressure natriuresis, ANP), plasma osmolality (ADH and thirst), K⁺, Ca²⁺, Mg²⁺ and phosphate balance (aldosterone, PTH, FGF23, calcitriol), and acid–base balance (bicarbonate reclamation and new HCO₃⁻ generation by titratable acid and ammonium excretion). Its endocrine outputs — erythropoietin (red cell production), calcitriol (active vitamin D) and renin — explain why chronic kidney disease causes anaemia, renal bone disease and hypertension.
Development
The urinary system develops from intermediate mesoderm in three overlapping, craniocaudal generations: the pronephros (week 4; rudimentary, regresses), the mesonephros (functions briefly; its duct persists as the mesonephric/Wolffian duct, which forms the male genital ducts) and the definitive metanephros, which begins in week 5. The metanephric kidney arises by reciprocal induction between two components: the ureteric bud, an outgrowth of the mesonephric duct, branches to form the ureter, renal pelvis, major and minor calyces and collecting ducts; the metanephric mesenchyme (blastema) it invades forms the nephrons from the glomerular capsule to the distal convoluted tubule (signalling via GDNF–RET, WT1, PAX2). Urine production begins at ≈10–12 weeks and fetal urine forms most of the amniotic fluid in the second half of pregnancy. Nephrogenesis is complete by ≈34–36 weeks.
The kidneys form in the pelvis and ascend to the lumbar region (weeks 6–9), rotating ≈90° so the hilum faces anteromedially and acquiring successively higher arterial supplies — persistent lower vessels explain accessory (polar) renal arteries. If the lower poles fuse, the resulting horseshoe kidney is trapped beneath the inferior mesenteric artery; failure of ascent leaves a pelvic kidney; premature splitting of the ureteric bud gives a duplex/bifid ureter. Bilateral renal agenesis (or severe obstruction) causes oligohydramnios and the Potter sequence: pulmonary hypoplasia (the usual cause of death), flattened facies, limb deformities. The cloaca is divided by the urorectal septum; its anterior part, the urogenital sinus, forms the urinary bladder and urethra. The allantois becomes the urachus → median umbilical ligament (a patent urachus leaks urine from the umbilicus). The trigone is classically described as derived from the incorporated ends of the mesonephric ducts.