Paul Harnetty, Class of 1999
The excesses and waste products of your body make up the urine, which flows through "ureters" (one per kidney) into the bladder, which holds the urine 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. When you aren't urinating, 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 and sugar content of urine make it a great medium for bacteria to grow in. Usually several things keep bacteria out of the urine. These include:
•The urethral sphincter: when the urethra is squeezed shut, bacteria
cannot climb up the urethra from the "meatus" (the outside opening) into
the bladder.
•The length of the urethra: it's a long way up to the bladder for a
bacterium. (Since a woman's urethra is shorter than a man's, women are
much more likely than men to get UTI's.)
•Frequent washing: any bacteria that make it into the urethra are flushed
out the next time you urinate, and since your bladder empties almost completely
when you do urinate any bacteria that get that far will be flushed out
too.
Furthermore, there are valves at the points where the ureters enter the bladder to prevent urine from "refluxing" from the bladder to the kidneys, so even if the bladder and its urine is infected the bacteria shouldn't travel up to the kidneys.
Infection from above is most often seen in newborns 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 happens than baby girls, since girls' urethrae are so 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 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.
•Urethritis is infection/inflammation of the urethra. This can be due to other things besides the organisms usually involved in UTI's; in particular, many sexually-transmitted diseases (STD's) appear initially as urethritis. However, stool-related bacteria (the most common bacteria on the skin near the meatus) will also often cause urethritis.
•Cystitis is an infection of the bladder. "-itis" means inflammation, and there are non-bacterial reasons for bladder inflammation, but they are much less common than bacterial infection.) This is the most common form of UTI; it can be aggravated if the bladder does not empty completely when you urinate.
•Ureteritis is infection of a ureter. This can occur if the bacteria entered the urinary tract from above, or if the ureter-to-bladder valves don't work properly and allow urine to "reflux" from the bladder into the ureters.
•Pyelonephritis is an infection of the kidney itself. This can happen with infection from above, or if reflux into the ureters is so bad that infected urine refluxes all the way to the kidney.
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.
Since the kidneys are located in your back, just below the 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.
Although the kidney's filter system is pretty big, it is not infinite. A sufficiently large amount of damage can reduce filter function to the point where waste products can't be removed properly. This constitutes 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 of
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 if the infection is very severe. More often, the damage done by the initial infection, even if it is not compounded by future infections, progresses over many months or years. In particular renal failure may not be complete until long after the first UTI.
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. For the initial diagnosis in small children, a sterile catheter can be inserted into the bladder through the urethra after precleaning of the meatal area.
Sometimes, it is necessary to make sure that the infection did not get beyond the bladder, and check 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. The amount of radiation involved in nuclear kidney scans is much less than even standard X-rays would give. We can also check for urine reflux from the bladder to the ureters and kidneys: we do this with a "VCUG", which 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. If there is reflux, the contrast will go up into the ureters, and perhaps the kidneys it the reflux is severe, and this will be visible on the X-rays. If reflux is bad enough, surgery can improve the function of the valves and reduce reflux in some patients.