The urinary tract can be infected from above by bacteria from another part of the body that enter the kidney through the bloodstream, or a UTI can occur from bacteria entering the urethra. Infection from above is most often seen in newborn babies with a systemic infection known as sepsis. Infection from below is much more common in both children and adults. 
Urinary Tract 
The urinary tract is made up of the kidneys, two ureters, the bladder, and urethra. The major components are the kidneys, a pair of bean-shaped organs located below the ribs near the middle of one's back. The kidneys comprise a complex filtration system made up of individual nephrons that work together to remove waste products from the blood, which are eliminated from the body in the form of urine. The kidneys also function to maintain a stable balance of salts and other substances in the blood, as well as to produce a hormone erythropoietin, which triggers the production of red blood cells in the bone marrow. 

The ureters are tube-like structures that transport the urine from the kidneys to the bladder where the urine is stored. Muscles called sphincters tighten around the urethra to prevent urine from leaking out. There are two sphincters: the internal is not controlled consciously, while the external sphincter is under voluntary control. The bladder is elastic and expands as it fills with urine. When the bladder reaches a certain capacity, which differs for each individual, the brain sends impulses to the internal sphincter to relax and other impulses to a muscle called the detrusor to contract and expel the urine out the urethra. This process is under the voluntary control of the individual, who can hold the urine until social circumstances allow for urination. (Loss of this control is urinary incontinence.) 

Urine is normally "sterile," meaning that it usually contains no bacteria. The body accomplishes this through several methods. First, the two sphincter muscles that prevent urine leaking from the bladder to the urethra, also prevent the bacteria that normally colonize the skin from ascending through the meatus (the opening in the urethra) into the bladder. Second, the length of the urethra makes it difficult for bacteria to get to the bladder. The fact that women have a much shorter urethra than men accounts for the fivefold increase of UTIs among women compared to men. Finally, if bacteria do make it to the bladder, the body is equipped with valves where the ureters empty into the bladder, a region known as the trigone. These valves prevent urine and any bacteria from backing up into the kidneys. Further, the bladder almost completely empties when urination occurs, so that any bacteria present should be excreted as well. Despite these defense mechanisms, infections sometimes occur.