Chapter 14 — Chapter 14 — Human Urinary System

Excretion is the process by which an organism eliminates metabolic wastes — nitrogenous wastes, extra water and extra salts. The urinary system performs three roles: nitrogenous waste removal (filtering out urea, made from protein breakdown in the liver), osmoregulation (maintaining the water-to-salt balance of blood) and pH and ion regulation (balancing Na⁺, K⁺ and H⁺ to keep blood slightly basic, about pH 7.4). It consists of two kidneys, two ureters, one urinary bladder and one urethra. Kidneys lie high in the abdominal cavity on each side of the vertebral column, between the 12th thoracic and 3rd lumbar vertebrae; the right kidney is slightly lower and smaller due to the liver. Ureters (about 25 cm) carry urine from the renal pelvis to the bladder, a hollow distensible muscular reservoir in the pelvic cavity; the urethra carries urine outside.