The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. The urinary bladder is a triangular, hollow and distensible (or elastic) organ that is located in the lower abdominal region. Urine enters the bladder via the ureters and exits via the urethra. Bladders occur throughout much of the animal kingdom, but they are very diverse in form and in some cases are not homologous with the urinary bladder in humans. The human urinary bladder is derived in embryo from the urogenital sinus and, it is initially continuous with the allantois. In males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to the prostate, and separated from the rectum by the rectovesical excavation. In females, the bladder sits inferior to the uterus and anterior to the vagina; thus, its maximum capacity is lower than in males. It is separated from the uterus by the vesicouterine excavation. In infants and young children, the urinary bladder is in the abdomen even when empty. The sphincter muscles which are located at the opening of the bladder into the urethra, prevent the leakage of urine from the bladder. The nerves present in the bladder control the process of urination or micturition. When the bladder is full and it is time for urination, after the nerves of the bladder transmit this information to the brain, the brain signals the bladder muscles to contract and the sphincter muscles to relax, so as to facilitate urination.