Final answer:
The urinary bladder is the muscular sac that stores urine and releases it through the process of urination.
Step-by-step explanation:
The muscular sac that stores urine and releases it periodically by the process of urination is the urinary bladder. Urine travels from the kidneys through the ureters to the bladder, where it is stored. When the bladder is about half full, a signal is sent to relax the sphincter muscles, allowing urine to flow from the bladder into the urethra and out of the body.
The urinary bladder's walls are stretchy and muscular, enabling it to hold varying amounts of urine without damage to the body from the urine's content.