Kidneys filter blood into urine that bladder and urethra expel
The urinary tract of humans and placental mammals pairs kidneys, ureters, bladder, and urethra to clear urea-rich urine while tuning blood volume, pressure, electrolytes, metabolites, and pH. Healthy adults typically make about eight hundred to two thousand millilitres daily.
Nephrons begin urine formation; fluid then converges through collecting ducts into minor and major calyces, the renal pelvis, and ureters that fill the bladder. Male and female bladder and urethral anatomy differ. Renal arteries supply the kidneys richly as they sit retroperitoneal on both sides of the spine.
Urothelium—a transitional epithelium—lines pelvis, ureters, and bladder so the tract can stretch flat then rebound. How much flows depends on hydration, activity, weight and health; polyuria exceeds roughly two and a half litres, while oliguria below about four hundred millilitres signals trouble.
Filtering starts with a big slice of the heart's output: the kidneys receive 12 to 30 percent of it, about 20 percent or roughly 1.25 litres a minute on average. In each nephron, Bowman's capsule lets water, small molecules and ions cross a semipermeable membrane, driven by hydrostatic and osmotic gradients, while proteins and blood cells stay behind; the tubules then reclaim what the body still needs. The kidneys also help produce red blood cells and synthesize calcitriol, the active form of vitamin D. Hormones such as antidiuretic hormone, aldosterone and parathyroid hormone steer the whole operation, and anuria, under 100 millilitres a day, marks the extreme low.
Beyond waste removal, the system is a continuous chemical governor of the internal sea. When filters, tubes, or storage fail, volume and acid–base chaos follow quickly—proof that excretion is also regulation.
Source: Urinary system
