The Urinary System

When cells in the body break down proteins into forms they can utilize, they produce ammonia wastes that the liver turns into urea (a chemical compound of carbon, hydrogen, nitrogen, and oxygen). When cells break down carbohydrates, they produce water and carbon dioxide as waste products. If these useless waste products were allowed to accumulate in the body, they would become dangerous to the body's health. The body eliminates these wastes (and solid wastes, also) in a process known as excretion. The body system most responsible for waste excretion is the urinary system, which eliminates water, urea, and other waste products from the body in the form of urine. Because of this main function, it is often referred to as the excretory system.

DESIGN: PARTS OF THE URINARY SYSTEM

The main organs of the urinary system are the kidneys, which form urine. The other parts of the system—the ureters, the urinary bladder, and the urethra—neither form urine nor change its composition. They are merely structures that transport urine from the kidneys to the outside of the body.

The kidneys

The kidneys are kidney bean-shaped organs located on either side of the vertebral column or spine near the small of the back (the beans were so named because they resembled small kidneys). The left kidney usually sits slightly higher than the right one. The size of an adult kidney is approximately 4 inches (10 centimeters) long, 2.5 inches (6 centimeters) wide, and 1 inch (2.5 centimeters) thick. The upper portions of the purplish-brown kidneys rest on the lower surface of the diaphragm, a membrane of muscle separating the thoracic or chest cavity from the abdominal cavity. The lower portion of the rib cage encloses and protects the kidneys. The kidneys are held in place by the abdominal lining and supporting connective tissue.

On the side of each kidney facing the vertebral column is an indentation called the hilus. Through the hilus the renal artery enters and the renal vein and ureter exit (the adjective renal comes from the Latin word renes, meaning "kidneys"). The renal artery brings blood to the kidneys from the abdominal

The Urinary System: Words to Know

Aldosterone (al-DOS-te-rone):

Hormone secreted by the adrenal cortex that controls the salt and water balance in the body.

Antidiuretic hormone (an-tee-die-yu-REH-tik HOR-mone):

Hormone produced by the hypothalamus and stored in the posterior pituitary that increases the absorption of water by the kidneys.

Bowman's capsule (BOW-manz KAP-sul):

Cupshaped end of a nephron that encloses a glomerulus.

Calyces (KAY-li-seez):

Cup-shaped extensions of the renal pelvis that enclose the tips of the renal pyramids and collect urine.

Creatinine (kree-AT-i-neen):

Waste product in urine produced by the breakdown of creatine.

Filtration (fill-TRAY-shun):

Movement of water and dissolved materials through a membrane from an area of higher pressure to an area of lower pressure.

Glomerulus (glow-MER-yoo-lus):

Network of capillaries enclosed by a Bowman's capsule.

Henle's loop (HEN-leez LOOP):

Looped portion of a renal tubule.

Hilus (HIGH-lus):

Indentation or depression on the surface of an organ such as a kidney.

Micturition (mik-tu-RISH-un):

Urination, or the elimination or voiding of urine from the urinary bladder.

Nephrons (NEFF-ronz):

Urine-forming structures in the kidneys.

Peristalsis (per-i-STALL-sis):

Series of wavelike muscular contractions that move material in one direction through a hollow organ.

Renal corpuscle (REE-nul KOR-pus-el):

Part of a nephron that consists of a glomerulus enclosed by a Bowman's capsule.

Renal cortex (REE-nul KOR-tex):

Outermost layer of the kidney.

Renal medulla (REE-nul muh-DUH-luh):

Middle layer of a kidney.

Renal filtrate (REE-nul FILL-trait):

Fluid formed in a Bowman's capsule from blood plasma by the process of filtration in the renal corpuscle.

Renal pelvis (REE-nul PELL-vis):

A cavity at the innermost area of a kidney that connects to the ureter.

Renal pyramids (REE-nul PEER-ah-mids):

Triangular or pie-shaped segments of the renal medulla in which urine production occurs.

Renal tubule (REE-nal TOO-byool):

Twisting, narrow tube leading from the Bowman's capsule in a nephron.

Renin (REE-nin):

Enzyme secreted by the cells of renal tubules that helps to raise blood pressure.

Urea (yoo-REE-ah):

Main nitrogen-containing waste excreted in the urine, produced when the liver combines ammonia and carbon dioxide.

Ureter (you-REE-ter):

Muscular tube that carries urine from the renal pelvis in a kidney to the urinary bladder.

Urethra (yoo-REE-thrah):

Thin-walled tube that carries urine from the urinary bladder to the out-side of the body.

Uric acid (YUR-ik AS-id):

Waste product in urine formed by the breakdown of nucleic acids.

Urinary bladder

(YER-i-nair-ee BLA-der) Hollow, collapsible, muscular sac that stores urine temporarily.

Urine (YUR-in):

Fluid formed by the kidneys from blood plasma.

aorta. The renal vein returns blood to the inferior vena cava. Both the renal artery and renal vein have a right and left branch, each connected to a corresponding kidney.

Each kidney is covered with a thin yet tough capsule of fibrous connective tissue. The transparent capsule gives a kidney a glistening appearance. Inside, a kidney can be divided into three layers. The outer layer, light in color, is called the renal cortex. The next layer, a darker reddish-brown, is called the renal medulla. It contains six to eighteen pie-shaped structures called renal pyramids, the tips of which face toward the center of a kidney.

Attached to the tips of the renal pyramids are cup-shaped tubes called calyces (singular: calyx), which collect urine from structures in the renal pyramids. The smaller calyces unite to form larger calyces, which in turn unite to form the renal pelvis, a cavity at the innermost part of a kidney. The renal pelvis collects urine from the calyces and funnels it to the ureter, to which it is attached.

NEPHRONS. The urine-forming structures in the kidneys are called nephrons. There are approximately 1.25 million nephrons in each kidney. Almost 0.5 inch (1.2 centimeters) in length, each nephron begins in the lower portion of the renal cortex, then twists and coils down into a renal pyramid in the renal medulla. A nephron has two major portions: a renal corpuscle and a renal tubule.

The renal corpuscle is a knotted network of fine capillaries surrounded by a cup-shaped chamber. The knot of capillaries is called a glomerulus (plural: glomeruli). The cup-shaped chamber is called the Bowman's capsule, and it is filled with fluid and has an inner and outer wall. The inner wall encloses the glomerulus and has many pores that make it permeable or able to allow fluids or material to pass easily through it. Its outer wall has no pores and is, thus, not permeable.

GETTING BY ON ONE KIDNEY

It is not necessary for an individual to have two kidneys in order to live. In fact, one kidney can easily perform the job of two, filtering wastes from the body and creating urine. People who have only one kidney because of an illness or because they donated a kidney for transplantation can and do lead healthy lives. When only one kidney is functioning in the body, it usually increases in size by 50 percent in order to perform well.

The renal tubule is a long passageway that continues from the Bowman's capsule. It twists and turns down through a renal pyramid before forming a loop (called Henle's loop) and ascending back through the renal pyramid. As it ascends, it becomes slightly larger. This larger section of the renal tubule twists and coils across the top of the renal pyramid before descending once again. It then empties into a collecting duct that serves several renal tubules. Several collecting ducts then unite to form larger ducts that empty urine into a calyx at the tip of the renal pyramid.

The ureters

The ureters are muscular tubes that carry urine from the renal pelvis in each kidney to the urinary bladder. Each ureter measures about 10 to 12 inches (25 to 30 centimeters) in length, extending from the hilus of a kidney to the rear wall of the urinary bladder, where it enters at an angle. The extremely thin ureters only measure about 0.25 inch (0.64 centimeter) in diameter.

Urine drains through the ureters to the urinary bladder by gravity, but the smooth muscular walls of the ureters also help propel urine along. They compress in a series of wavelike contractions (an action known as peristalsis) that move the urine through the ureters in only one direction. When urine has entered the urinary bladder, it is prevented from flowing back into the ureters by small, valvelike folds of membrane that flap over the ureter openings.

The urinary bladder

The urinary bladder is a hollow, collapsible, muscular sac that stores urine temporarily. It is located in the pelvis behind the pelvic bones, and is held in place by ligaments. In women, the bladder is behind the uterus; in men, it is above the prostate gland.

The size of the urinary bladder varies depending on the amount of urine it contains. When empty, it is normally no longer than 2 to 3 inches (5 to 7.6 centimeters). During this state, its walls are thick and heavily folded. As it begins collecting urine, the urinary bladder's muscular walls stretch and expand, and it rises in the abdominal cavity. A urinary bladder that is moderately full measures about 5 inches (12.7 centimeters) in length and holds just over 1 pint (500 millimeters) of urine. When completely full, the urinary bladder can contain over 2 pints (1 liter) of urine.

The muscular walls of the urinary bladder contract to expel urine out of the bladder into the urethra. A sphincter or ring of muscle surrounding the opening to the urethra, called the internal urethral sphincter, controls the flow of urine. This is an involuntary sphincter, meaning a person cannot consciously control its workings.

The urethra

The urethra is a thin-walled tube that carries urine from the urinary bladder to the outside of the body. Its length and function differ in females and males.

In females, the urethra measures about 1 to 1.5 inches (2.5 to 3.8 centimeters) in length. Its external opening lies in front of the vaginal opening. The only purpose of the urethra in females is to conduct urine to the out-side of the body.

In males, the urethra serves a dual purpose. It transports semen and urine to the body exterior, but never at the same time. Thus, it serves both the reproductive and urinary systems. In men, the urethra extends from the urinary bladder through the prostate gland to the tip of the penis, a distance of 6 to 8 inches (15 to 20 centimeters).

In both sexes, the urethra contains a ring of skeletal muscle that forms the external urethral sphincter as the urethra passes through the floor of the pelvis. A person is normally able to control the opening and closing of this sphincter. When the sphincter is voluntarily relaxed, urine flows into the urethra, emptying the urinary bladder. However, when the bladder fills with urine and becomes stretched beyond normal, voluntary control of the sphincter becomes no longer possible.

WORKINGS: HOW THE URINARY SYSTEM FUNCTIONS

The urinary system is not the sole system in the body concerned with excretion. Other systems and organs also play a part. The respiratory system eliminates water vapor and carbon dioxide through exhalation (the process of breathing out). The digestive system removes feces, the solid undigested wastes of digestion, by a process called defecation or elimination. The skin (the integumentary system; see chapter 4) also acts as an organ of excretion by removing water and small amounts of urea and salts (as sweat).

Through its primary role of forming and eliminating urine, the urinary system also helps regulate blood volume and pressure. In addition, it regulates the concentrations of sodium, potassium, calcium, chloride, and other mineral ions (an ion is an atom or group of atoms that has an electrical charge) in the blood. These combined actions by the urinary system help the body maintain homeostasis or the balanced state of its internal functions.

Formation of urine

Urine is the fluid waste excreted by the kidneys. It can range in color from pale straw to amber (the deeper the color, the more concentrated the urine). Fresh urine is sterile (meaning it contains no bacteria) and has very little odor. Some drugs, vegetables (such as asparagus), and various diseases alter the normal smell of urine. Water forms approximately 95 percent of urine; the remaining 5 percent is made up of urea, creatinine, uric acid, and various salts.

Urea, creatinine, and uric acid are nitrogen-containing compounds produced as wastes during cellular activity. When cells break down amino acids, they produce ammonia as a waste product. Ammonia is very toxic to the body's cells, so the liver combines ammonia with carbon dioxide to create the less toxic urea, the most abundant of the nitrogen-containing wastes. Creatinine is produced when skeletal muscle cells break down the compound creatine to generate energy for muscle contraction. Uric acid, which forms only a small portion of the urine, is a normal waste product of the breakdown of nucleic acids (complex organic molecules found in living cells).

Urine is formed in the kidneys as a result of three processes: filtration, reabsorption, and secretion. Filtration takes place in the renal corpuscles; reabsorption and secretion take place in the renal tubules.

FILTRATION. Filtration is the movement of water and dissolved materials through a membrane from an area of higher pressure to an area of lower

pressure. In the body, the pressure of blood in the capillaries is higher than the pressure of the interstitial fluid, or the fluid surrounding the body's cells. Thus, through filtration, blood plasma (fluid portion of blood) and nutrients such as amino acids, glucose, and vitamins are forced through the capillary walls into the surrounding interstitial fluid to be used by the cells.

The pressure of the blood in the glomeruli is higher than in other types of capillaries in the body. This high pressure forces plasma, dissolved waste substances, and small proteins out of the glomeruli and into the Bowman's capsules. The process is called glomerular filtration. Blood cells and larger proteins are too large to be forced out of the glomeruli, so they remain in the blood. The pressure in a Bowman's capsule is low and its inner membrane is permeable, so the material that filters out of a glomerulus passes into the capsule. The fluid and material in a Bowman's capsule is referred to as renal filtrate, which is very much like blood plasma, except it contains very little protein and no blood cells.

REABSORPTION. In an average twenty-four-hour period, the kidneys form 160 to 190 quarts (150 to 180 liters) of renal filtrate. Normal urinary output in that same time frame is only about 1.1 to 2.1 quarts (1 to 2 liters). Many factors (such as increased water intake or increased sweating) can significantly alter that output amount. Nonetheless, it is quite obvious that most of the renal filtrate does not become urine, but is reabsorbed or taken back into the blood. This is important because renal filtrate contains many useful substances—water, glucose, amino acids, and mineral ions—that are needed by the body.

Reabsorption is the return of water and other substances from the filtrate to the blood. The process begins as soon as the filtrate enters the renal tubule. Cells lining the tubule actively take up useful materials (such as glucose, amino acids, vitamins, proteins, and certain ions), move them through their cell bodies, then release them into the interstitial fluid outside the tubule.

As these materials collect in the interstitial fluid, water in the tubules is drawn out through the process of osmosis. Osmosis is the diffusion of water through a semipermeable membrane from an area where it is abundant to an area where it is scarce or less abundant. Once in the interstitial fluid, the materials and water then diffuse into or enter nearby capillaries, which empty into the renal vein.

The reabsorption process is selective. The cells of the renal tubules have been "programmed" to retain substances that are useful to the body, not those substances that are of no use (such as urea and uric acid). Also, the amount of a substance that is reabsorbed is dependent on its concentration in the blood. If it exists in a low concentration in the blood, a large amount of it will be reabsorbed from the renal tubules. Conversely, if it already exists in a high concentration, very little of it will be reabsorbed into the blood.

SECRETION. Secretion is the transport of materials from the interstitial fluid into the renal filtrate. It is essentially reabsorption in reverse. The process is important for getting rid of substances not already in the filtrate. Waste products such as ammonia, some creatinine, and the end products of medications move from the blood in the capillaries around the renal tubules into the interstitial fluid. They are then taken in by the cells of the tubules and deposited into the renal filtrate to be eliminated in the urine.

Hormones and the composition of urine

Hormones are chemical "messengers" secreted by endocrine glands that control or coordinate the activities of other tissues, organs, and organ systems in the body. Each type of hormone affects only specific tissue cells or organs, called target cells or target organs. Most hormones are composed of amino acids, the building blocks of proteins. The smaller class of hormones are steroids, which are built from molecules of cholesterol (fatlike substance produced by the liver).

The hormones that affect the urinary system help it regulate the amount of water and mineral ions in urine. By extension, this action also regulates the pressure in the bloodstream and the concentration of mineral ions in the blood.

Excessive water loss in the urine is controlled by the antidiuretic hormone (ADH), which is released by the pituitary gland (a small gland lying at the base of the skull). If an individual perspires a lot, fails to drink enough water, or loses water through diarrhea, special nerve cells in the hypothalamus (a region of the brain controlling body temperature, hunger, and thirst) detect the low water concentration in the blood. They then signal the pituitary gland to release ADH into the blood, where it travels to the kidneys. With ADH present, the renal tubules are stimulated to reabsorb more water from the renal filtrate and return it to the blood. The volume of water in the urine is thus reduced, and the urine becomes more concentrated. Harmful substances are still eliminated from the body, but necessary water is not.

THE FIRST SUCCESSFUL ORGAN TRANSPLANT

In 1950, Ruth Tucker, a forty-nine-year-old American woman, was dying from chronic kidney failure. American surgeon Richard Lawler of Loyola University in Chicago transplanted a kidney from a cadaver (dead body) into Tucker. Although her body rejected the new kidney after only a short time, Tucker became the first human to survive an organ transplantation.

With the development of immunosuppressant drugs (those that hinder the body's immune response to "foreign" tissue), the success of kidney transplants has risen. Today, individuals who receive a kidney transplant from a living donor (who is often a close relative) have a survival rate of 80 percent.

The action of ADH also controls blood volume and pressure. As more water is removed from the urine and transported into the bloodstream, blood volume and pressure increase. This is an important safeguard against low blood volume and pressure, which might be brought about by an injury.

On the other hand, if an individual takes in too much water, production of ADH decreases. The renal tubules do not reabsorb as much water, and the volume of water in the urine is increased. Alcohol and liquids containing caffeine (coffee, tea, and cola drinks) inhibit ADH production (these substances are called diuretics). Large amounts of urine are consequently excreted from the body and blood pressure decreases. If that fluid is not replaced, an individual may feel dizzy due to low blood pressure.

Another hormone that helps to control blood volume by acting on the kidneys is aldosterone. Aldosterone is a steroid hormone secreted by the adrenal cortex (the outer layer of an adrenal gland, which sits like a cap on top of a kidney). A decrease in blood pressure or volume, a decrease in the sodium ion level in blood, and an increase in the potassium ion level in blood all stimulate the secretion of aldosterone. Once released, aldosterone spurs the cells of the renal tubules to reabsorb sodium from the urine and to excrete potassium instead. Sodium is then returned to the bloodstream. When sodium is reabsorbed into the blood, water in the body follows it, thus increasing blood volume and pressure.

The kidneys themselves play a role in regulating blood pressure. When blood pressure around the kidneys decreases below normal, the cells of the renal tubules react by secreting the enzyme renin (an enzyme is a protein that speeds up the rate of chemical reactions). Renin, in turn, stimulates an inactive blood protein to change into a hormone that causes blood vessels to constrict or narrow, which immediately raises blood pressure.

Elimination of urine

The process of eliminating urine from the urinary bladder is known as urination. It is also called micturition or voiding. The sensation of having to urinate usually occurs after the urinary bladder has filled with about 7 ounces (200 milliliters) of urine. The bladder has stretched enough at this point to activate receptors within its walls. Once activated, these receptors send impulses to the spinal cord, which sends impulses back to the bladder, causing it to contract. The internal urethral sphincter (surrounding the opening of the urethra) relaxes and urine is forced into the upper portion of the urethra. This initiates the "feeling" of having to urinate.

Urination occurs when an individual voluntarily relaxes his or her external urethral sphincter. Urine flows through the urethra and the bladder is emptied. Babies do not exert control over their external urethral sphincter and urine is automatically forced out of the body when it reaches a certain level. As children mature, they learn to control their external urethral sphincter, retaining their urine until it is appropriate to urinate.

If the external urethral sphincter is not relaxed, urine will continue to accumulate in the bladder. When it reaches a volume over 1 pint (475 milliliters), the pressure created will cause pain and the external urethral sphincter eventually will be forced open, regardless of an individual's desire.

AILMENTS: WHAT CAN GO WRONG WITH THE URINARY SYSTEM

Most urinary system problems are associated with age. As individuals grow older, the functioning of their kidneys declines. An average seventy-five-year-old person has half the original number of nephrons in their kidneys. Because of this, the kidneys lose some of their ability to concentrate urine. The urinary bladder also shrinks with age, leading to a need to urinate more frequently. In some older people, the ability to control urination is lost, a condition known as urinary incontinence (see discussion below).

KIDNEY DIALYSIS

Dialysis is a process by which small molecules in a solution are separated from large molecules. The process has come to play a crucial role in the health of humans. For some people, the term dialysis refers to a specific kind of medical treatment in which a machine (the dialysis machine) takes on the functions of a human kidney. Dialysis machines have made possible the survival of thousands of people who would otherwise have died as a result of kidney failure.

The kidney dialysis machine was invented by Dutch-American surgeon Willem Johan Kolff in 1945. Since that time, many improvements have been made to the machine and to the procedure of removing wastes from the blood of people whose kidneys have ceased to function.

The most common dialysis treatment prescribed in the United States is known as hemodialysis (he-moe-die-AL-i-sis). In short, during this procedure, two needles atached to tubes are inserted into veins in an individual's arm. Blood is drawn out of the person's body through one tube and pumped through the dialysis machine.

Inside the machine, the blood is circulated on one side of a semipermeable membrane. This means that the membrane allows the passage of certain sized molecules (such as waste products) across it, but prevents the passage of other, larger molecules (such as blood cells). A special dialysis fluid containing mineral ions and other substances necessary to the body circulates on the other side of the membrane.

As blood circulates in the machine, wastes and other unneeded substances in the blood are drawn out through the membrane. At the same time, the mineral ions and other chemicals in the dialysis fluid cross the membrane into the blood. The "cleansed" and chemically-balanced blood is then returned to the person's body through the second tube.

Most hemodialysis patients require treatment two to three times a week, and each treatment can last several hours.

Disorders and diseases of the urinary system do not affect only the elderly, however. The systems in children and adults through late middle age can also be affected, mainly by bacterial infections that cause inflammation. If not treated properly, many of these infections can lead to serious, even life-threatening, conditions.

Infections and diseases that strike elsewhere in the body can eventually impair the functioning of the kidneys. Acute (short-term) kidney failure appears most frequently as a complication of a serious illness such as heart failure, liver failure, dehydration, severe burns, and excessive bleeding. Acute kidney failure is a temporary condition that can be reversed with proper and timely treatment. Chronic kidney failure, which is long-term and irreversible, can be triggered by diabetes, hypertension, glomerulonephritis (see below), and sickle cell anemia, among other conditions. Without proper treatment to remove wastes from the bloodstream, chronic kidney failure is fatal.

The following are just a few of the many diseases and disorders that can affect the urinary system or its parts.

Bladder cancer

Bladder cancer develops when cells lining the urinary bladder become abnormal and grow uncontrollably, forming tumors. It is the fifth most common cancer in the United States. The disease is three times more common in men than woman. Most cases of bladder cancer are found in people who are fifty to seventy years old.

The exact cause of bladder cancer is unknown. However, smokers are twice as likely as nonsmokers to get the disease. Workers who are exposed to certain chemicals in the dye, rubber, leather, textile, and paint industries are also believed to be at a higher risk for developing bladder cancer.

One of the first warning signs of bladder cancer is blood in the urine. Painful urination, increased frequency of urination, and a feeling of having to urinate but not being able to are additional signs of bladder cancer.

URINARY SYSTEM DISORDERS

Cystitis (sis-TIE-tis): Inflammation of the urinary bladder caused by a bacterial infection.

Glomerulonephritis (glah-mer-u-lo-ne-FRY-tis): Inflammation of the glomeruli in the renal corpuscles of the kidneys.

Kidney stones: Large accumulations of calcium salt crystals from urine that may form in the kidneys.

Pyelonephritis (pie-e-low-ne-FRY-tis): Inflammation of the kidneys caused by a bacterial infection.

Urethritis (yer-i-THRY-tis): Inflammation of the urethra caused by a bacterial infection.

Urinary incontinence (YER-i-nair-ee in-KON-tinence): Involuntary and unintentional passage or urine.

If bladder cancer is diagnosed, the three standard methods of treatment are surgery, radiation therapy, and chemotherapy. During surgery, surgeons may remove the tumor, part of the bladder containing the tumor, or the entire bladder and adjoining organs (the prostate gland in men; the uterus, ovaries, and fallopian tubes in women). Radiation therapy (using X rays or other high-energy rays to kill any remaining cancer cells and shrink any tumors) is generally used after surgery. Chemotherapy (using a combination of drugs to kill any remaining cancer cells and shrink any tumors) may also be given after surgery.

When detected early and treated appropriately, bladder cancer can be cured. In those people who have sought early treatment, at least 94 percent survive five years or more. However, when the disease has spread to nearby tissues, the survival rate drops below 50 percent.

Glomerulonephritis

Glomerulonephritis is the inflammation of the glomeruli in the renal corpuscles. It is generally caused by a bacterial infection elsewhere in the body, mostly in the throat or skin. In children, it is mostly associated with an upper respiratory infection, tonsillitis, or scarlet fever.

During a bacterial infection, the body produces antibodies or substances that help protect the body against foreign invaders. Glomerulonephritis develops when antibodies and the bacteria they attach to accumulate in the glomeruli, producing inflammation. If left untreated, the glomeruli are soon replaced by fibrous tissue and waste products cannot be effectively filtered from the blood. The kidneys become enlarged, fatty, and congested.

Symptoms of severe cases of glomerulonephritis include fatigue, nausea, vomiting, shortness of breath, high blood pressure, blood in the urine, and swelling in the face, hands, feet, and ankles.

Treatment for glomerulonephritis includes bed rest to maintain adequate blood flow to the kidneys and antibiotics to rid the body of the infection. If too much fluid has accumulated in the body, diuretics may be given to increase urine output. Sodium and protein intake may also be decreased to help rest the kidneys. Symptoms of glomerulonephritis usually disappear in two weeks to several months. Ninety percent of children recover without complications. Adults often recover more slowly.

Kidney cancer

Kidney cancer develops when cells in certain tissues in the kidneys become abnormal and grow uncontrollably, forming tumors. Kidney cancer accounts for 3 percent of cancer cases in the United States. The disease occurs most often in men over the age of forty. Men are twice as likely as women to suffer from this type of cancer.

The exact causes of kidney cancer are unknown. However, there is a strong connection between cigarette smoking and kidney cancer; smokers are twice as likely as nonsmokers to get the disease. Obesity may be another risk for kidney cancer.

The most common symptom of kidney cancer is blood in the urine. Other symptoms include painful urination, pain in the lower back or sides, abdominal pain, a lump or hard mass that can be felt in the kidney area, unexplained weight loss, fever, weakness, and high blood pressure.

The primary treatment for kidney cancer that has not spread to other parts of the body is surgical removal of the diseased kidney. Because most cancers affect only one kidney, an individual can function well on the one remaining. Radiation therapy (using X rays or other high-energy rays to kill cancer cells and shrink any tumors) may be used when the cancer is inoperable, but it is has not proven to be of much use in destroying kidney cancer cells. Chemotherapy (using a combination of drugs to kill any cancer cells and shrink any tumors) has also not produced good results.

Because kidney cancer is often caught early and sometimes progresses slowly, the chances of a surgical cure are good.

Kidney stones

Kidney stones are solid accumulations or material that form in the tubal system of the kidneys. Kidney stones cause problems when they block the flow of urine through or out of the kidneys. When the stones move along the ureter, they cause severe pain.

Kidney stones are most common among white males over the age of thirty. The stones can be composed of a variety of substances, but the majority (about 80 percent) are formed from calcium salts that have separated from the urine to form crystals that combine to form larger stones. Some may grow as big as golf balls.

Increased blood levels of calcium caused by a diet heavy in meat, fish, and poultry can lead to the formation of kidney stones. Certain diseases—hyperthyroidism and some types of cancer—can also increased blood calcium levels.

Individuals who have kidneys stones usually do not have symptoms until the stones pass into the ureter. Prior to this, some people may notice blood in their urine. Once the stone is in the ureter, however, most people will experience severe bouts of crampy pain that usually begins in the area between the lower ribs and the hip bone. Nausea, vomiting, and extremely frequent and painful urination may then occur.

Although most kidney stones will pass out of the body on their own, some will not. If a stone is too large to pass or is causing a serious obstruction, surgical removal of the stone may be necessary. In the past, open surgery to remove the stone was common. Now, however, physicians may use a machine to aim shock waves at the stone, either from outside or inside the body. The shock waves often crush the stone into smaller fragments, which may then pass on their own or may be removed surgically. In most cases, individuals with uncomplicated kidney stones will recover very well.

Urinary incontinence

Urinary incontinence is the involuntary and unintentional passage of urine. Approximately 13 million Americans suffer from this disorder. Women are affected more frequently than men; approximately one out of every ten women under the age of sixty-five are affected. Older people, too, are more prone to the condition. Twenty percent of Americans over the age of sixty-five are incontinent.

The inability to control urination can be caused by a wide variety of physical conditions. Any blockage at the bladder outlet that permits only small amounts of urine to pass; irritation of the bladder due to an infection; undue pressure placed on the bladder (such as in obese individuals); and the loss of muscle tone in the pelvic muscles, the bladder, or the urethral sphincter muscles—these are all just a few of the many causes of urinary incontinence.

Left untreated, incontinence can cause physical and emotional harm. Those people with long-term incontinence suffer from urinary tract infections and skin rashes. Incontinence can also affect their self-esteem, causing depression and so cial withdrawal.

There are numerous treatment options for urinary incontinence, depending on the cause. The condition may not be stopped, but it can at least be improved. If weakened pelvic muscles are to blame, exercises to tone them can be performed. In certain people, especially older women, medications may help tighten pelvic muscle tone or the urethral sphincters. A balloonlike device may be inserted into a woman's urethra and inflated to prevent urine leakage. Surgery to raise and support the bladder neck and urethra may also be undertaken.

Urinary tract infections

Urinary tract infections (UTIs) are inflammations of the urinary tract caused by a bacterial infection. UTIs have specific names, depending on the location of the inflammation. Inflammation of the urethra is known as urethritis. Inflammation of the urinary bladder is known as cystitis. When the bacterial infection spreads to the kidneys, the condition is known as pyelonephritis.

UTIs are much more common in women than in men, probably due to anatomy. In women, bacteria from fecal matter and vaginal discharges can enter the urethra because its opening is very close to the vaginal opening and the anus. Once an infection occurs in the urethra of a woman, the relative shortness of the urethra makes it easy for bacteria to gain entry to the bladder and multiply. In men who are not circumcised, the foreskin can harbor bacteria that can enter the urethra and cause UTIs. UTIs can also be sexually transmitted.

Sometimes, a UTI has no symptoms. When symptoms appear, they include pain or a burning sensation when urinating, frequent urination, or blood in the urine. In pyelonephritis, additional symptoms include fever and chills, aching pain on one or both sides of the lower back or abdomen, fatigue, nausea, vomiting, and diarrhea. If left untreated, pyelonephritis can last for months or years. Scarring of the kidneys and the possible loss of kidney function may result.

Typical treatment for all three types of UTIs is a course of antibiotics. An individual suffering from pyelonephritis may also require hospitalization if the disorder is severe. Given the appropriate antibiotic, UTIs usually go away quickly. Drinking plenty of fluids at the first sign of a UTI may help ward it off by diluting the bacteria present and flushing the urinary system. Drinking unsweetened cranberry juice may also help. The juice seems to contain a compound that can prevent bacteria from sticking to and thus growing in the urinary tract.

TAKING CARE: KEEPING THE URINARY SYSTEM HEALTHY

It is well known that aging taxes the urinary system. However, the many problems than can arise are not the inevitable consequence of aging and can be prevented or at least minimized. A person can lessen the effects of aging on the urinary system (like every other system in the body) by following a healthy lifestyle. This includes getting adequate rest, reducing stress, drinking healthy amounts of good-quality drinking water, not smoking, drinking moderate amounts of alcohol (or not drinking at all), following a proper diet, and exercising regularly.

A healthy diet is important in maintaining the health of the urinary system. A poor diet—one high in fats and meats—can cause kidney stones to develop. People who are obese or overweight place undue pressure on the organs of the urinary system, which can lead to further medical problems.

Drinking plenty of water is a necessity in keeping the urinary system healthy. The amount of water in the body helps the urinary system determine how many mineral ions, such as sodium, should be eliminated. The urinary bladder should be emptied every few hours during the day, and drinking enough water should allow an individual to produce a large enough amount of urine to accomplish this. Producing large amounts of urine also helps to flush the urinary system, washing bacteria out of the normally sterile urinary tract.

To further prevent infection of the urinary system by bacteria, it is important to practice good hygiene by keeping the genital area clean. This is vital in both sexes, but is of special concern to women because of the structure of their anatomy. In women, the urethral opening is very close to the vaginal opening and the anus. After urinating or defecating, women should wipe their genital area from the front to the back to avoid introducing fecal matter into the urethral opening.

Good hygiene after sexual intercourse is also important. During intercourse, bacteria from the vagina or from a man's penis may be introduced into a woman's urethra. If left unchecked, the bacteria may spread and create conditions such as urethritis and cystitis. Washing the genital area and urinating after intercourse can help flush out any bacteria from the urethra.

FOR MORE INFORMATION

Books

Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. The Urinary System. New York: Twenty-First Century Books, 1994.

WWW Sites

Cyber Anatomy: Excretory System
http://tqd.advanced.org/11965/html/cyber-anatomy_excboth.html
Geared for students in grades 6 through 12, site provides a broad discussion of the urinary system. Also includes a diagram of the system with the major parts identified.

Excretory System
http://gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookEXCRET.html
Site presents a detailed chapter (with extensive images) on the excretory (urinary) system from the On-Line Biology textbook.

Nephron Information Center
http://www.nephron.com
Site provides information about the kidneys and links for further information.

The Urinary System
http://www.msms.doe.k12.ms.us/biology/anatomy/urinary/urinary.html
Site provides a diagram of the urinary system and links that give information on the four main parts of the system, including larger diagrams of each organ.

What Kids Need to Know About Kidneys
http://kidshealth.org/kid/normal/kidneys.html
Site presents information explaining how kidneys clean wastes out of the body and how they maintain a balance of fluids and minerals in the body.

Your Urinary System and How It Works
http://www.niddk.nih.gov/health/urolog/pubs/yrurinar/index.htm
The site, a service of the National Institute of Diabetes and Digestive and Kidney Diseases (part of the National Institutes of Health), provides information on how the urinary system works, what can cause problems with the system, and what are some disorders of the system, among other items.