How the Urinary System Removes Waste - dummies

How the Urinary System Removes Waste

By David Terfera, Shereen Jegtvig

The urinary system filters waste from the blood and produces urine in the kidneys; then it removes the urine from the body. Adults produce about a quart and a half of urine every day, but that amount can vary greatly depending on how much fluid and what types of foods are consumed and how much fluid is lost by sweating or breathing.

Two bean-shaped organs called the kidneys are found in the abdomen, near the back and just below the ribs. The kidneys are responsible for filtering the blood. The filtrate, as it moves through the kidney tubules, is modified to produce urine. The two kidneys lie behind the peritoneum on the posterior abdominal wall near the 12th thoracic and first three lumbar vertebrae. The right kidney is slightly lower than the left kidney. Each kidney and suprarenal gland is encased in a perinephric fat capsule, which is covered by a membranous renal fascia. Paranephric fat lies outside of the renal fascia.

Each kidney is shaped like a kidney bean (surprised?) with an anterior and posterior surface. The renal hilum is located at the medial border and allows the renal artery to enter the kidney and the renal vein to leave. It also allows the ureter to exit the kidney. The internal part of the kidney includes a section called the medulla surrounded by a covering called the cortex. The medulla contains minor calyces that merge to form major calyces. The innermost renal pelvis is formed by the merger of the major calyces.


The body needs a way to store the urine until you have a convenient time to urinate (remove urine from the body). Urine leaves the kidneys and travels down the ureters to the bladder is a thin-walled, bag-like organ that can hold up to two cups of urine.

The bladder is connected to the urethra, which is a tube that opens externally through an opening called the external meatus. Sphincter muscles keep urine from leaving the bladder until the person is ready to urinate. The female urethra is quite short, whereas the male urethra is longer and runs through the penis.

Sometimes in the process of filtering blood and forming urine, salt-and-mineral crystals form in the kidneys. These kidney stones can travel down the ureters and into the bladder. The stones vary in size, and in some cases passing kidney stones can be extremely painful. The stones can become lodged in the ureter, blocking the flow of urine and causing a back-up of urine in the kidney. Treatments include medications and the use of lithotripsy (shock waves) to reduce the size of the stone so that it is more easily passed.