Urinary System Structures and Functions

What happens in the urinary system?

After we eat, our body takes the nutrients from food and uses them to maintain bodily functions including energy and self-repair. When the body has taken all the necessary nutrients it needs, waste products are left behind in the blood or in the bowel.
Image of the urinary tract consisting of the kidneys, ureters, bladder and urethra. Left kidney, an enlarged view.

The urinary system works with other organs such as the lungs, skin and the intestines in keeping chemicals and water balanced inside the body. Normally, adults eliminate about a quart and a half of urine each day. The amount of urine released each day depends on how much fluid and food a person consumes and how much fluid is lost through sweat and breathing. Certain medications may also influence the amount of urine that is eliminated.

When we consume foods containing protein, such as meat and poultry, a waste product called urea is produced. Urea, along with excess water and other waste products, is normally released from the body in the form of urine.

Parts of the urinary system and their functions:
Illustration of the urinary tract or system consisting of the kidneys, ureters, bladder, and urethra.
The urinary system includes two kidneys, two ureters, the bladder, two sphincter muscles, and the urethra. It also includes the nerves that supply these urinary organs.


The kidneys are two bean-shaped organs about the size of a clenched fist. They are located below the rib cage, near the middle of the back.

Their vital functions include:

  • Removal of extra water and waste products from the blood in the form of urine.

  • Keeping a stable balance of salts and other substances in the body.

  • Releasing hormones that help maintain the normal number of red blood cells and regulate blood pressure.

To know more about the kidneys and the actual structures that create urine, visit our Kidney Anatomy Page.


The two ureters, about 8 to 10 inches long, are narrow tubes that carry urine from the kidneys to the bladder.

Constant tightening and relaxing of muscles in the ureter wall forces urine downward away from the kidneys. These muscles help to maintain a one-way flow of urine from the kidneys to the bladder. If urine is allowed to stand still, or to flow back into the kidneys, a kidney infection may develop.

Bladder: Also called the urinary bladder

The bladder is a balloon-shaped, hollow, muscular organ inside the pelvis (hip bones). It is held in place by ligaments attached to other orgas and the pelvic bones.

The bladder stores urine until it is released when you urinate. As urine flows into the bladder, it becomes larger and the shape becomes more rounded or spherical. When it is empty, its size becomes smaller. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds.

A normal adult bladder can store up to 2 cups of urine for as long as 2 to 5 hours.

Sphincter Muscles

Sphincter muscles are two circular muscles around the opening of the bladder into the urethra. They help keep urine from leaking into the urethra.


The urethra is the tube that allows urine to pass outside the body. The male and female urethra are different in some ways. In females, their urethra are much shorter than in males. In addition to urine, the male urethra is also where semen passes through.

Nerves in the bladder

They inform a person when it is time to urinate, or empty the bladder.

What causes the feeling of need or urge to urinate?

The urge to urinate is caused by urine filling you bladder.

Initially, as your bladder first fills with urine, you may notice a feeling that you need to urinate. The feeling becomes stronger as your bladder continues to fill and reaches its limit.

When your bladder is full, the nerves from your bladder send a message to your brain that your bladder is full. At that point, your urge to empty your bladder intensifies.

When it is time to urinate, your brain signals your bladder muscles to tighten, squeezing urine out of the bladder. At the same time, your brain signals the sphincter muscles to relax. As these muscles relax, urine exits your bladder through your urethra. Normal urination occurs when all of the signals occur in the correct order.

The urge to urinate may be controlled for some time. But when the bladder has reached its maximum limit, a person may not be able to control the passage of urine.

Kidney and Kidney Stones website has provided this material for your information. It is not intended to substitute for the medical expertise and advice of your primary health care provider. We encourage you to discuss any decisions about treatment or care with your health care provider. The mention of any product, service, or therapy is not an endorsement by Kidney and Kidney Stones (KKS).

Article by:
David Mangusan Jr., BSPT, PTRP

Image Credit: National Kidney and Urologic Diseases Information Clearinghouse.