How does the urinary system work?

The body takes nutrients from food and changes them to energy. After the body has taken the food components that it needs, waste products are left behind in the bowel and in the blood.

The kidney and urinary systems help the body to get rid of liquid waste called urea. They also help to keep chemicals (such as potassium and sodium) and water in balance. Urea is made when foods containing protein (such as meat, poultry, and certain vegetables) are broken down in the body. Urea is carried in the blood to the kidneys. This is where it is removed, along with water and other wastes in the form of urine.

The kidneys have other important functions. They control blood pressure and make the hormone erythropoietin. This hormone controls red blood cell production in the bone marrow. The kidneys also control the acid-base balance and conserve fluids.

Kidney and urinary system parts and their functions

• Two kidneys. This pair of purplish-brown organs is located below the ribs toward the middle of the back. They:

  • Remove waste products and medicines from the body

  • Balance the body's fluids

  • Balance a variety of electrolytes

  • Release hormones to control blood pressure

  • Release a hormone to control red blood cell production

  • Help with bone health by controlling calcium and phosphorus

The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries (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.

• Two ureters. These narrow tubes carry urine from the kidneys to the bladder. Muscles in the ureter walls keep tightening and relaxing. This forces urine downward, away from the kidneys. If urine backs up, or is allowed to stand still, a kidney infection can develop. About every 10 to 15 seconds, small amounts of urine are emptied into the bladder from the ureters.

• Bladder. This triangle-shaped, hollow organ is located in the lower belly. It's held in place by ligaments that are attached to other organs and the pelvic bones. The bladder's walls relax and expand to store urine. They contract and flatten to empty urine through the urethra. The typical healthy adult bladder can store up to 2 cups of urine for 2 to 5 hours.

• Two sphincter muscles. These circular muscles help keep urine from leaking by closing tightly like a rubber band around the opening of the bladder.

• Nerves in the bladder. The nerves alert a person when it's time to urinate, or empty the bladder.

• Urethra. This tube allows urine to pass outside the body. The brain signals the bladder muscles to tighten. This squeezes urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals happen in the correct order, normal urination happens.

Facts about urine

• Normal, healthy urine is a pale straw or clear yellow color.

• Darker yellow or honey-colored urine often means you need more water.

• A darker, brownish color may mean a liver problem or severe dehydration.

• Pinkish or red urine may mean blood in the urine.