Infections of the urinary tract are the second most common type of infection in the body. The urinary system consists of the kidneys, ureters, bladder, and urethra. The key elements in the system are the kidneys, a pair of purplish-brown organs located below the ribs toward the middle of the back.

The kidneys remove excess liquid and wastes 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.

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.

How the infection occurs

Normally, urine is sterile. It is usually free of bacteria, viruses, and fungi but does contain fluids, salts, and waste products. An infection occurs when tiny organisms, usually bacteria from the digestive tract, cling to the opening of the urethra and begin to multiply.

The urethra is the tube that carries urine from the bladder to outside the body. Most infections arise from one type of bacteria known as Escherichia coli, which normally lives in the colon.

In many cases, bacteria first travel to the urethra. When bacteria multiply, an infection can occur. An infection limited to the urethra is called urethritis.

If bacteria move to the bladder and multiply, abladder infection called cystitis occurs. If the infection is not treated promptly, bacteria may then travel further up the ureters to multiply and infect the kidneys. A kidney infection is called pyeloneprhitis. 

Other micro-organisms known as Chlamydia and mycoplasma may also cause urinary tract infections 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 such infections require treatment of both partners.

The urinary system is 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 secretions that slow bacterial growth. In both sexes, immune defenses also prevent infection. But despite these safeguards, infections still occur.

Risks

Some people are more prone to getting a urinary tract infection than others.  Any abnormality of the urinary tract that obstructs the flow of urine such as a kidney stone sets the stage for an infection.

An enlarged prostate gland also can slow the flow of urine, thus raising the risk of infection.

A common source of infection is catheters, or tubes, placed in the urethra and bladder. A person who cannot void or who is unconscious or critically ill often needs a catheter that stays in place for a long time.

Some people, especially the elderly or those with nervous system disorders who lose bladder control, may need a catheter for life. Bacteria on the catheter can infect the bladder, so hospital staff takes special care to keep the catheter clean and remove it as soon as possible.

People with diabetes have a higher risk of this kind of infection because of changes in the immune system. Any other disorder that suppresses the immune system raises the risk of a urinary infection.

The urinary tract infection may occur in infants, both boys and girls, who are born with abnormalities of the urinary tract, which sometimes need to be corrected with surgery. However, these infections are rarer in boys and young men.
In adult women, the rate of this kind of infection gradually increases with age. One factor may be that a woman’s urethra is short, allowing bacteria quick access to the bladder.

Also, a woman’s urethral opening is near sources of bacteria from the anus and vagina. For many women, sexual intercourse seems to trigger an infection, although the reasons for this linkage are unclear.

Research also shows that women who use a diaphragm are more likely to develop urinary tract infection women who use other forms of birth control. Recently, researchers found that women whose partners use a condom with spermicidal foam also tend to have growth of E. coli bacteria in the vagina.

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