The kidneys are bean-shaped organs found in the lower back below the ribcage. They filter waste out of the blood and produce urine to carry waste and excess water out of the body. Urine flows from the kidneys through the ureters and into the bladder.
The bladder is a hollow, muscular organ that stores urine for a short period of time. The bladder stretches as urine accumulates and, at a certain point, it signals the body to relieve the growing pressure. A muscular valve at the opening of the bladder relaxes and the bladder contracts to send urine through the urethra and out of the body.
The blanket term UTI is frequently used, but a urinary tract infection may also be identified by the specific part of the urinary tract that is affected.
- Urethritis is an inflammation and/or infection of the urethra.
- A bladder infection is called cystitis.
- Infection of one or both kidneys is called pyelonephritis.
Urine does not normally contain microbes. Urinary tract infections develop when bacteria enter the opening of the urethra. The bacteria stick to the walls of the urethra, multiplying and moving up the urethra to the bladder. Most UTIs remain in the lower urinary tract (urethra or bladder), where they cause symptoms such as urinary urgency and a burning sensation during urination. Typically, these infections are considered uncomplicated and are easily treated, but if they are not addressed, the infection may spread through the ureters and into the kidneys.
A kidney infection is more dangerous than a lower urinary tract infection and can lead to permanent kidney damage. In some cases, a UTI may lead to an infection in the bloodstream (sepsis, septicemia) that can be life-threatening. Rarely, a bloodstream infection may infect the kidneys.
Although a variety of bacteria can cause UTIs, most are due to Escherichia coli, bacteria that are common in the digestive system and are routinely found in stool and around the anus. Other bacteria that may cause UTIs include species of Proteus, Klebsiella, Enterococcus, and Staphylococcus. Occasionally, UTIs may be caused by a fungus (yeast), such as Candida albicans.
Urethritis may be a symptom of a sexually transmitted disease (STD) such as herpes, chlamydia, or gonorrhea. The treatment and prevention of STDs differs from that of typical UTIs. For more information, see the Sexually Transmitted Diseases article.
Urinary tract infections are common. According to the American Urological Association Foundation, UTIs result in more than 8.1 million visits to doctor's offices each year. Although they can affect anyone at any age, women are much more likely than men to have UTIs, with about 40% of women and 12% of men having at least one UTI in their lifetime.
It is thought that a woman's increased susceptibility is partly anatomical because the female urethra is relatively short compared to a male's, so bacteria do not have as far to travel to reach the bladder. Additionally, the short distance between the female urethra and the vagina and anus increases the risk of exposure to bacteria.
Anything that slows or blocks the passage of the urine or introduces bacteria into the urinary tract can increase a person's risk of having a UTI. Activities and conditions that can lead to the development of UTIs include:
- Sexual intercourse with the same, new or multiple sex partners
- Spermicide use, especially if combined with diaphragm or condom use
- A history of previous UTIs
- Anatomical problems, such as narrowing of the urethra or ureters
- Urine retention (the bladder does not empty completely)
- Abnormal flow of urine from the bladder back to the ureters (vesicoureteral reflux)
- Kidney stones
- Bladder catheterization, especially long-term
- Spinal cord injuries or other nerve damage involving the bladder
- Diabetes, which causes changes to the immune system, damage to the kidneys, and often results in glucose (sugar) in the urine that promotes the growth of bacteria
- Kidney disease or kidney transplant
- Any condition that suppresses the immune system
- In men, an enlarged prostate, which may reduce the flow of urine
- In women, menopause leads to changes in the lining of the vagina and the loss of the protective effects of estrogen
In most cases, UTIs are acute and uncomplicated. They are treated and the symptoms subside within a day or two. UTIs that spread to the kidneys, however, may cause permanent kidney damage, especially in the elderly or very young. Conditions or diseases that result in chronic or recurrent UTIs may also damage the kidneys and, in some cases, cause renal failure and septicemia, which can be life-threatening conditions. They require immediate treatment, which often involves hospitalization. In pregnant women, UTIs can lead to premature labor and delivery and cause high blood pressure. In men, UTIs can cause prostate infection and inflammation, which can be difficult to treat.
Recurrent UTIs are a problem for about 1 in 5 women after having their first infection. Most recurrent UTIs are caused by the same bacteria that caused previous infections. With each UTI a woman has, her risk of continuing to have more increases. Risk factors for recurrent UTIs in women include:
- Frequent sexual intercourse
- Having a first UTI at an early age
- Spermicide use
- Having a maternal history of UTIs
Men are less likely than women to have a first UTI. But once a man has a UTI, he is likely to have another because bacteria can hide deep inside prostate tissue.