The "urinary tract" consists of the various organs of the body that produce, store, and get rid of urine. These include the kidneys, the ureters, the bladder, and the urethra.
Our kidneys are chemical filters for our blood. About one-quarter of the blood pumped by the heart goes through the kidneys. The kidneys filter this blood, and the "filtrate" is processed to separate out waste products and excess amounts of minerals, sugar, and other chemicals. Since it sees so much of the body's blood flow, the kidneys also contain pressure-sensitive tissue which helps the body control blood pressure, and some of the minerals and water are saved or discarded partly to keep your blood pressure in the proper range.
The waste products and "extras" make up the urine, which flows through "ureters" (one per kidney) into the bladder, where it is held until you are ready to get rid of it. When you urinate, muscles in the bladder wall help push urine out of the bladder, through the urethra, and out. (In men, the urethra passes through the penis; in women, the urethra opens just in front of the vagina.) When you aren't urinating (which is most of the time) a muscle called the "sphincter" squeezes the urethra shut to keep urine in; the sphincter relaxes when you urinate so that urine can flow out easily.
Urine is normally sterile -- that is, it does not normally contain bacteria. This is a good thing, since the mineral content of urine make it a great medium for bacteria to grow in. (If you have sugar in your urine, it's an even better culture medium, but that shouldn't happen unless you are diabetic, or are one of the rare people -- like me -- who are not diabetic but still have sugar in their urine.) Usually several things keep bacteria out of the urine. These include:
The urinary tract can be infected from above (by bacteria entering the kidneys from the bloodstream and travelling downward) or from below (by bacteria entering the urethra and travelling upward).
Infections from above are often seen in newborns seen with generalized infection or sepsis. If there are many bacteria in the bloodstream, some are likely to get through the filters of the kidney to the urine. This is especially likely if the filters are immature, or if there are a lot of bacteria.
In older children and adults infection most often starts from below. In small children still using diapers, stool (which is largely bacteria) can sit for some time right at the meatus; the longer it sits there, the more likely it is that bacteria may enter the urethra. Baby boys are less likely to have this happen than baby girls, because girls' urethrae are much shorter and the head of the penis isn't as likely to sit in stool. (Note, though, that bacteria can hang out in any moist, warm area, and that UTI's in boys under 1 year old seem to happens more often in uncircumcised boys than in circumcised boys since bacteria can accumulate beneath the foreskin.) Older girls may become prone to UTI's through wiping back-to-front when they are first toilet-trained, which pulls stool into the vaginal/meatal area. Sexually active teenage and adult women are more prone to UTI's because of friction at the meatus, which tends to push bacteria into the urethra (urinating after intercourse helps avoid UTI's); the same mechanism may cause UTI's in teenage boys and adult men, although they are again less prone to UTI's than women of the same age.
In general, the farther the organ in the urinary tract from the place where the bacteria enter, the less likely the organ is to be infected.
The symptoms you have with a UTI depend on how old you are and on where in the urinary tract the infection is located.
Urethritis usually appears as burning on urination. Often this burning occurs mainly when you start urinating, since the bacteria and infected urine in the urethra cause the inflammation but are flushed out when "fresh" urine flows through the urethra on its way out of the bladder.
Cystitis may show up as burning on urination, often in the "middle" of urination. However, it may have no symptoms other than fever, lower abdominal (way down -- just above the pubic bone) pain, or even just a funny smell or colour or appearance (cloudy, dark, even blood-tinged) to your urine.
Blood in the urine can be a sign -- sometimes the only sign at first -- of a urinary tract infection. It can result from microscopic bleeding within the kidneys, or from an abscess if the infection is far advanced. Blood can also appear in urine from a bleed anywhere between the kidneys and the urinary meatus (the end of the urethra, from where the urine emerges); in particular, cystitis can result in bleeding inside the bladder, which will certainly leave blood in the urine -- whether as blood-tinging, blood clots in the urine, or something in between. When we ask patients what part of the urine stream the blood appears in, we are trying to figure out where the blood is entering the urine: for example, blood that appears just as you start to urinate and clears up as the flow continues indicates that the bleeding is in the urethra, where it accumulates until you urinate and is then flushed out by the flowing urine. On the other hand, blood that is uniformly mixed with the urine is likely coming from the kidneys, the ureters, or the bladder.
Since your kidneys are located in your back, just below your bottom ribs, pyelonephritis may appear as pain in your back or flank(s), or in the abdomen. Fever usually (but not always) comes along with the pain. If the kidneys are severely affected, you may also start seeing some of the complications due to kidney malfunction.
Urinary tract infections can make you pretty miserable. They can do other things, too.
The biggest problem with a UTI is if it progresses to pyelonephritis. This can cause scarring and damage to the kidney tissue. The kidney's filter system is pretty big, but it's not infinite. If there is enough damage to the filter system, waste products can't be removed properly. This is kidney failure, and if it is bad enough and long-lasting enough the only solutions are dialysis (filtering your blood through an "artificial kidney" which isn't nearly as good as the real one and requires you to sit hooked up to a lot of plumbing three times a week) or a kidney transplant (which also poses many risks and problems).
A different complication occurs if the pressure-regulation tissues in the kidney are scarred. If this is bad enough, your blood pressure may be kept too low (and you'll faint frequently at the very least) or too high (leading to strokes, heart disease, and other nasty things).
Both of these problems may occur rapidly, but only with very severe infections. More often, the damage done by the initial infection, even if it is not compounded by future infections, happens over many months or years. In particular, renal failure may not be complete until long after the first UTI.
The first step in treating a UTI is to make sure there really is one. The only certain way to know if there is a UTI is to take a sample of urine and "culture" it: try to grow bacteria from the sample. If there are bacteria, we can then test several antibiotics to see which ones kill the bacteria most efficiently.
The problem here is in getting a good sample of urine for culture. Simply urinating into a sterile cup may not stop contamination by bacteria on the skin, especially with girls. If you can control your urine, it is possible to use a "clean-catch" sample. You get this by cleaning the meatus and the surrounding area thoroughly with antiseptics (such as iodine solution), then urinating a little into the toilet before filling the sample cup, and finishing your urination in the toilet. This flushes out bacteria that may be in the urethra or meatus.
Unfortunately, small children can't cooperate well enough to do this sort of collection, even if they are toilet-trained. We can collect urine with a bag ("puck") that is taped over the meatus and genitals. However, this almost guarantees contamination by skin bacteria. We sometimes use pucks to collect samples for follow-up culture, but such samples just don't work well for the initial diagnosis where we have to know whether or not there really is an infection. For the initial diagnosis in small children we usually use a sterile catheter inserted into the bladder through the urethra (after cleaning the meatal area with iodine or another soap that kills bacteria). This may sound barbaric, but it is the only way to be sure if a small child has a UTI or not. In newborn babies who may be septic, we may go even farther and draw urine out of the bladder with a needle inserted over the pubic bone (a suprapubic bladder tap) -- which may sound even more barbaric than the catheter, but the stakes are a lot higher in a newborn baby who doesn't have the defenses against infections that older children and adults have, and a suprapubic urine culture is postive if there are any bacteria growing in it -- no ifs, ands, or buts.
Once we have diagnosed a UTI we treat the patient with antibiotics. Typical antibiotics used for UTIs include trimethoprim-sulfamethoxamole, nitrofurantoin, ciprofloxacin, levofloxacin, or their chemical relatives, and certain penicillins such as amoxicillin. In some cases, when we are pretty sure from the symptoms that you actually have a UTI, we will start antibiotics right after we get the urine culture; if the culture result shows that we need a different antibiotic, we can always change. We often repeat the culture 3-5 days after starting antibiotics to make sure that we are actually killing all the bacteria, and again soon after the antibiotics are finished to make sure we killed everything that needed killing.
We also need to make sure that the infection did not get beyond the bladder, or, if it did, that the kidneys haven't been damaged. This is usually done with "nuclear scans" in which a tiny amount of a radioactive medicine is injected into the patient's bloodstream, where it heads for the kidneys to be excreted. The medicine can be detected with radiation detecting cameras, giving a picture of the kidneys: damaged kidney tissue will appear on the picture. (Older methods involving X-rays don't produce pictures nearly as good as the nuclear scan pictures, and expose you to much more radiation. The amount of radiation involved in nuclear kidney scans is much less than even standard X-ray machines would give.)
Ultrasound images of the kidneys, ureter, and bladder can show abscesses that may be present, as well as abnormalities in the "plumbing" (such as duplicate ureters or blocked ureters). It won't necessarily show the source of microscopic bleeding, but if the bleeding is microscopic it may stop after the infection is treated and we may never know precisely where the blood was entering the urine.
A voiding cystourethrogram, or "VCUG", is an X-ray of the kidneys and bladder taken after a "contrast medium" (a medicine which blocks X-rays) is injected into the bladder through a catheter in the urethra. We use the VCUG to look for reflux: if there is reflux, the contrast medium will go up into the ureters, and perhaps the kidneys it the reflux is severe, and this will be visible on the X-rays. As you can imagine, this isn't very comfortable for the patient, but the VCUG is the only practical way to find out if there is reflux. If reflux is bad enough, surgery can improve valve function and reduce reflux in some patients. Milder cases of reflux will often improve as a child grows; for intermediate grades of reflux we may decide to give a child low doses of antibiotics until the reflux improves (which may take several months). The antibiotics we use to treat UTIs are excreted from the body through the kidney and urine -- in fact, that's why we use those antibiotics -- so even low doses give levels of the antibiotic in the urine that are high enough to kill the few bacteria that might stray into the bladder, and resistance isn't as much of a problem as it might otherwise be.