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How does the urinary system work?

Your body takes nutrients from food and uses them to maintain all bodily functions including energy
and self-repair. After your body has taken what it needs from the food, waste products are left
behind in the blood and in the bowel. The urinary system works with the lungs, skin, and
intestines—all of which also excrete wastes—to keep the chemicals and water in your body balanced.
Adults eliminate about a quart and a half of urine each day. The amount depends on many factors,
especially the amounts of fluid and food a person consumes and how much fluid is lost through sweat
and breathing. Certain types of medications can also affect the amount of urine eliminated.

The urinary system removes a type of waste called urea from your blood. Urea is produced when foods
containing protein, such as meat, poultry, and certain vegetables, are broken down in the body.
Urea is carried in the bloodstream to the kidneys.

The kidneys are bean-shaped organs about the size of your fists. They are near the middle of the
back, just below the rib cage. The kidneys remove urea from the blood through tiny filtering units
called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a
glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste
substances, forms the urine as it passes through the nephrons and down the renal tubules of the
kidney.

From the kidneys, urine travels down two thin tubes called ureters to the bladder. The ureters are
about 8 to 10 inches long. Muscles in the ureter walls constantly tighten and relax to force urine
downward away from the kidneys. If urine is allowed to stand still, or back up, a kidney infection
can develop. Small amounts of urine are emptied into the bladder from the ureters about every 10 to
15 seconds.

The bladder is a hollow muscular organ shaped like a balloon. It sits in your pelvis and is held in
place by ligaments attached to other organs and the pelvic bones. The bladder stores urine until
you are ready to go to the bathroom to empty it. 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.

Nerves in the bladder tell you when it is time to urinate, or empty your bladder. As the bladder
first fills with urine, you may notice a feeling that you need to urinate. The sensation to urinate
becomes stronger as the bladder continues to fill and reaches its limit. At that point, nerves from
the bladder send a message to the brain that the bladder is full, and your urge to empty your
bladder intensifies.

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. When all the signals occur in the correct
order, normal urination occurs.