A urinary tract infection (UTI) is a condition in which one or more parts of the urinary system (the kidneys, ureters, bladder, and urethra) become infected. UTIs are the most common of all bacterial infections and can occur at any time in the life of an individual. Nearly 95% of cases of UTIs are caused by bacteria that typically multiply at the opening of the urethra and travel up to the bladder. Much less often, bacteria spread to the kidney from the bloodstream.
The Urinary System. The urinary system helps maintain proper water and salt balance throughout the body and also expels urine from the body. It is made up of the following organs and structures:
- The two kidneys, located on each side below the ribs and toward the middle of the back, play the major role in this process. They filter waste products, water, and salts from the blood to form urine.
- Urine passes from each kidney to the bladder through thin tubes called ureters.
- Ureters empty the urine into the bladder, which rests on top of the pelvic floor. The pelvic floor is a muscular structure similar to a sling running between the pubic bone in front to the base of the spine.
- The bladder stores the urine. When the bladder becomes filled, the muscle in the wall of the bladder contracts, and the urine leaves the body via another tube called the urethra. In men the urethra is enclosed in the penis. In women, it leads directly out.
Defense Systems Against Bacteria. Infection does not always occur when bacteria are introduced into the bladder. A number of defense systems protect the urinary tract against infection-causing bacteria:
- Urine itself functions as an antiseptic, washing potentially harmful bacteria out of the body during normal urination. (Urine is normally sterile, that is, free of bacteria, viruses, and fungi.)
- The ureters join into the bladder in a manner designed to prevent urine from backing up into the kidney when the bladder squeezes urine out through the urethra.
- The prostate gland in men secretes infection-fighting substances.
- The immune system defenses and antibacterial substances in the mucous lining of the bladder eliminate many organisms.
- In healthy women, the vagina is colonized by lactobacilli, beneficial microorganisms that maintain a highly acidic environment (low pH) that is hostile to other bacteria. Lactobacilli produce hydrogen peroxide, which helps eliminate bacteria and reduces the ability of Escherichia coli (E. coli) to adhere to vaginal cells. (E. coli is the major bacterial culprit in urinary tract infections.)
Types of UTIs. UTIs are generally classified as:
Review Date: 06/17/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.