Most urinary tract infections (UTIs) involve only the bladder and urethra (the lower urinary system). Pyelonephritis results when a UTI progresses to involve the upper urinary system (the kidneys and ureters).
The kidneys filter the blood to produce urine. Two tubes called the ureters carry urine from the kidneys down to the bladder. Urine travels from the bladder out of the body through the urethra.
Urinary tract infections (UTIs) are bacterial infections in the body's urinary system -- the bladder, kidneys, ureters, and urethra. In the kidneys, the infection is called pyelonephritis; in the bladder, it is called cystitis.
Urinary tract infections are common in women. Children with UTIs may show different symptoms than adults.