The role of the bladder in the excretory system is to store urine before it is excreted from the body. The bladder is a muscular sac that can expand as it fills with urine from the kidneys, where it is produced through filtration. When the bladder reaches capacity, nerve signals prompt the urge to urinate, leading to voluntary relaxation of the sphincter muscles, allowing urine to flow through the urethra. This storage function is crucial for maintaining fluid balance and avoiding continuous urination, allowing for more controlled waste elimination.