What is the urinary bladder main function?

Urine storage and elimination.

After urine is produced by the kidneys, it travels down the ureters to the urinary bladder, where it is stored until it is excreted from the body. The bladder is a muscular organ that can expand to hold up to 2 cups of urine. When the bladder is full, it sends signals to the brain that it needs to be emptied. The brain then triggers the muscles in the bladder to contract and expel the urine through the urethra.