Urine formation and excretion
Filtration, reabsorption and secretion leave one to two litres of urine a day, carried by the ureters to the bladder and released under voluntary control.
Urine is mostly water carrying urea, creatinine, uric acid, salts and any drug metabolites the body needs to lose. Its colour reflects concentration; its composition, read by a dipstick, reveals infection (leucocytes, nitrites), blood, protein, glucose or ketones.
Peristalsis in the ureters delivers urine to the bladder, which stretches to hold 400 to 600 ml. Stretch receptors signal fullness at around 200 ml; the micturition reflex contracts the detrusor and relaxes the internal sphincter, while the external sphincter is released voluntarily. Children learn this control between two and four years; strokes, spinal injury and prostate enlargement disturb it.
Kidney stones form when salts crystallise in concentrated urine; keeping urine dilute is the simplest prevention.
Key facts
| Fact | Value |
|---|---|
| Bladder capacity | 400–600 ml |
| First urge to void | ≈200 ml |
| Urea excreted daily | ≈25–30 g |