The primary function of the urinary bladder is to store urine produced by the kidneys before it is excreted from the body. The bladder is a muscular sac that can expand and contract, allowing it to hold varying amounts of urine. When it reaches a certain level of fullness, nerve signals are sent to the brain, prompting the urge to urinate. This whole system helps maintain the body's balance of water and electrolytes, allowing for the efficient excretion of waste products and toxins filtered from the bloodstream by the kidneys.