The urinary bladder is a storage organ (elastic in nature) of the excretory system of the liquid waste in the form of urine. It is located behind the pubic bone in the pelvis. The bladder muscles contract while urination. This contraction of muscles opens two sphincters or valves at the end of the bladder. Through these valves urine flows out. During pregnancy the uterus takes up significantly more space and severely limits the expansion of the urinary bladder.
The urinary bladder ends into the urethra, a hollow tube. The urethra is a tube through which collected urine is passed out of the body. The opening of the urethra is kept closed by the urethral sphincter. It is a valve which is voluntarily controlled by a human being for urination. The act of passing the urine is called micturation. The act of micturation is a dual process: contraction of bladder muscles and relaxation of the sphincter muscles.