The Cause of Urinary Tract Infections

Infections of the urinary tract are common -- only respiratory infections occur more often. Each year, urinary tract infections (UTI's) account for about 8 million doctor visits. Women are especially prone to UTI's for reasons that are poorly understood. One woman in five develops a UTI during her lifetime.

The urinary system consists of the kidneys, ureters, bladder, and urethra. The key players in the system are the kidneys, a pare of purplish-brown organs located below the ribs toward the middle of the back. The kidneys remove liquid waste from the blood in the form of urine, keep a stable balance of salts and other substances in the blood, and produce a hormone that aids the formation of red blood cells. Narrow tubes called ureters carry urine from the kidneys to the bladder, a triangle-shaped chamber in the lower abdomen. Urine is stored in the bladder and emptied through the urethra.

The average adult passes about a quart and a half of urine each day. The amount of urine varies, depending on the fluids and foods a person consumes. The volume formed at night is about half that formed in the daytime.

Causes

Normal urine is sterile. It contains fluids, salts, and waste products, but is free of bacteria, viruses, and fungi. An infection occurs when microorganisms, usually bacteria from the digestive tract, cling to the opening of the urethra and begin to multiply. Most infections arise from one type of bacteria, Escherichia coli (E. coli), which normally live in the colon.

In most cases, bacteria first begin growing in the urethra. An infection limited to the urethra is called urethritis. From there bacteria often moves on to the bladder, causing a bladder infection (cystitis). If the infection is not treated properly, bacteria may then go up the ureters to infect the kidneys (pyelonephritis).

Microorganisms called Chlamydia and Mycoplasma may also cause UTI's in both men and women, but these infections tend to remain limited to the urethra and reproductive system. Unlike E. coli, Chlamydia and Mycoplasma may be sexually transmitted, and infections require treatment of both partners.

The urinary system in structured in a way that helps ward off infection. The ureters and bladder normally prevent urine from backing up toward the kidneys, and the flow of urine from the bladder helps wash bacteria out of the body. In men, the prostate gland produces secretion that slow bacterial growth. In both sexes, immune defenses also prevent infection. Despite these safeguards, though, infections still occur.

Each year, Medical College of Wisconsin physicians care for more than 180,000 patients, representing nearly 500,000 patient visits. Medical College physicians practice at Children's Hospital of Wisconsin, Froedtert Memorial Lutheran Hospital, the Milwaukee VA Medical Center, and many other hospitals and clinics in Milwaukee and southeastern Wisconsin.