Micturition, also known as urination, is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans the process of urination is under voluntary control.Urination is usually stimulated by the distention ofthe bladder as it fills with urine. When the walls of thebladder contract, nerve receptors are stimulated, andthe urination reflex is triggered. The urination reflexcauses the internal urethral sphincter to open and theexternal urethral sphincter to relax. This relaxationallows the bladder to empty. The bladder can hold up to600 ml of urine. The desire to urinate may not occuruntil the bladder contains 250-300 ml.