Q.Why are kidneys called 'retroperitoneal'?

The human excretory system consists of a pair of bean-shaped kidneys (lying retroperitoneally — behind the peritoneum — roughly between the 12th thoracic and 3rd lumbar vertebrae, about 10 cm long each, with a notch called the hilus on the concave inner surface through which the renal artery enters and the renal vein and ureter leave), a pair of ureters, a urinary bladder, and a urethra.

Each ureter is a long muscular tube (25-30 cm) carrying urine from the kidney to the bladder; it enters the bladder wall obliquely and through a valveless opening, an arrangement that prevents urine reflux when the filling bladder compresses the ureter. The urinary bladder is a hollow, muscular sac in the pelvic cavity that stores urine temporarily (capacity around 700 ml); its base has a triangular area called the trigone (receiving the two ureteric openings at its base corners and opening into the urethra at its apex), its wall is built of a three-layered smooth-muscle detrusor covered externally by peritoneum, and its lining is a stretch-friendly transitional epithelium. The urethra carries urine from the bladder to the exterior (and, in males, also carries semen); two sphincters guard it — an internal (inv …