The Urinary System

The organs, tubes, muscles, and nerves that work together to create, store, and carry urine are referred to as the urinary system, which is another name for the renal system. The renal system filters the plasma of blood and regulates blood volume by excreting excess water in the form of urine. Urine transport follows a path through the kidneys, ureters, bladder, and urethra, which are collectively known as the urinary tract.

Urine Transport

Urine is essentially water, ions, and secreted molecules that leave the collecting duct of the many nephrons of the kidney and flow into the ureters. The ureters are two tubes that drain urine from the kidneys to the bladder. 

Each ureter is a muscular tube that drains into the bladder. Smooth muscle contractions in the walls of the ureters, over time, send the urine in small spurts into the bladder, the organ where urine is stored before it can be eliminated.

Urine Storage

The bladder is a hollow muscular organ shaped like a balloon. It sits in the pelvis and is held in place by ligaments attached to other organs and the pelvic bones. The bladder stores urine until enough of it accumulates for removal from the body. It swells into a round shape when it is full and gets smaller when empty. 

If the urinary system is healthy, the bladder can hold up to 16 ounces (2 cups) of urine comfortably for 2 to 5 hours. Circular muscles called sphincters help keep urine from leaking. The sphincter muscles close tightly, like a rubber band, around the opening of the bladder into the urethra, the tube that allows urine to pass outside the body. 

Urine Elimination

Nerves in the bladder are stimulated as the bladder fills with urine and becomes larger, which in turn stimulates the need to urinate. When you urinate, the brain signals the bladder muscles to tighten, squeezing urine out of the bladder. 

At the same time, the brain signals the sphincter muscles to relax. As these muscles relax, urine exits the bladder through the urethra, and leaves the body through an opening in the genital region that contains the urethra. When all the signals occur in the correct order, normal urination occurs, removing urine from the body.

Urinary tract

The transport and removal of urine from the body follows the urinary tract.

A schematic depiction of the urinary tract. The transport and removal of urine from the body follows the urinary tract—from a kidney, to a ureter, to the bladder, to the urethra.