LIKE THE REST of your organs, I am unappealing in appearance - reddish-brown, shaped like a bean, about the size of your fist. I am your right kidney; my partner is on the other side of your lower spine. You have a very low opinion of me. You think of me simply as the producer of an unglamorous fluid - urine - and as a kind of secondary garbage-disposal unit. Brother!
Actually, I'm the master chemist of your body. And your intestinal tract is not your main waste-disposal system - I am. Blood passes through me continuously, and I clean and filter it, ridding it of wastes that are potentially deadly. I help prod production of red blood cells, watch over potassium, sodium chloride and other substances in your blood - a whisper too much or too little of any of them can be lethal. I control vital water balance - too much and your cells would drown, too little and you would simply dry up. I see to it that your blood is neither too acid nor too alkaline. As a matter of fact, I do so many things for you that the doctors still don't have a complete catalogue of my activities.
Look at my anatomy. Although I weigh only five ounces, I contain more than a million little filtering units - nephrons. Under a high-power microscope one of these looks something like a bigheaded worm, with a twisted tail called a tubule. Untangle and stretch out my tubules and there would be 70 miles of them!
Each hour my partner and I filter twice the total blood in your body. And it's mighty tricky filtering, I might add, I don't allow red blood cells or large particles of essential blood proteins to pass through my fine filters. Otherwise they might be lost in urine - with rapid, calamitous results. In my tubules, 99 percent of the fluid is reabsorbed. Essential vitamins, amino acids, glucose, hormones and so on are also returned to the bloodstream, but excess of any of them is discarded in urine.
Thus, if you have eaten two big slabs of custard pie, your urine may show enough sugar to fool a doctor into thinking you have diabetes. Let's eat a big portion of kippered herring or any other particularly salty food, and you might be in real danger if I didn't extract the salt. Salt holds water. If it were permitted to remain in the blood, excess fluid would start accumulating in the blood and intercellular spaces. Your face, feet and abdomen would puff up; and eventually your heart, pumping against the growing load of gallons of retained fluid, would simply falter and stop.
Potassium - mainly from meat and fruit juices - requires my, equally vigilant attention. Too little and muscles begin to fail, particularly breathing muscles. A pinch too much acts as a brake on the heart and can bring it to a full halt. I simply discard the excess. Or, if your diet isn't providing enough essential potassium; I hoard the existing supply like a miser.
The biggest waste I have to deal with is urea, the end product of protein digestion. Like everything else, this must be kept in precise balance. Too little means there has been damage to my upstairs neighbor, the liver. Too much and there sets in one of the ugliest diseases any doctor is apt to see - uremic poisoning. The name simply means urine in the blood. Unchecked, it can lead to shock, coma, death. As it piles up in the blood, the body makes a heroic effort to rid itself of this killer. Whitish crystals of urea "frost" may even appear on the skin as the sweat glands try to help rid the body of the stuff. Here again you need have no worries. You can eat all the steaks you want for instance, and I'll handle any resulting excess of urea.
Doing my job, I produce urine continuously - about a half a liter a day each for my partner and me. Microscopic droplets of this waste-laden fluid pass out of each of my million tubules and feed into a tiny reservoir at my center. This connects with the bladder and the bladder with the outside. Wavelike muscular action occurs every 10 to 30 seconds, pushing the fluid along the exit tubes. At night, I slow down activity to about a third of daytime levels; otherwise, you would be up every hour or so.
Like everyone else, you have noticed that certain things step up my activity. When you are chilled, for example, the blood supply to your skin is reduced - to preserve internal heat. This means an increased flow of blood to internal organs, including me. With more blood, I make more urine.
Anger produces much the same result. Your blood pressure rises, and I get an increased supply of blood for processing. Result: increased urine output.
Alcohol produces the same result via another, quite complex route. One of my main bosses is the pituitary gland on the underside of your brain. It produces an antidiuretic hormone. Left to my own devices, I might produce too much urine and you would become dangerously dehydrated. The hormone prevents this. The alcohol in your beer or martinis has no direct effect on me. But it does retard the pituitary's production of the braking hormone, so I produce urine more rapidly. If you have too many drinks, you become mildly dehydrated. That's why you crave water the next morning.
Caffeine in coffee has a similar action. The nicotine in cigarettes has the opposite effect - it steps up production of the hormone. When you smoke heavily, you need to urinate far less frequently.
When you are around 47 years old then I am beginning to show my age. I am a candidate for a lot of ills - floating kidneys, for example. You needn't worry here, mainly because you watch your weight. Normally, kidneys rest in a bed of fat. When the very obese reduce, much of this bed disappears, anchoring tissues stretch and the kidneys begin to drift.