Final answer:
Urine is expelled from the bladder through urination, which is controlled by a series of urinary structures such as the bladder, sphincter muscles, and the urethra. The kidneys filter blood to create urine which then moves to the bladder to be stored until expelled.
Step-by-step explanation:
Urine is expelled from the urinary bladder by a process known as urination, not filtration. This process involves multiple urinary system components, including the kidneys, the ureters, the bladder, and the urethra.
The kidneys perform the initial step of filtering blood, creating urine by removing waste and excess substances from the bloodstream. This urine, a combination of water, salts, and nitrogen-containing molecules, collects in each kidney and drains to the bladder via the ureters. The bladder acts as a storage reservoir for urine until it is ready to be expelled.
During urination, the bladder contracts and sends a nerve impulse to a sphincter muscle at the base of the bladder, which then relaxes, allowing urine to flow into the urethra. The urethra is a muscular tube that carries urine out of the body through the external urethral opening. Urine excretion is typically under voluntary control, allowing individuals to decide when to urinate.