Your Kidneys—A Filter for Life
BY AWAKE! CORRESPONDENT IN IRELAND
THE earth and the human body have something in common: To sustain life, both require a filter. The earth needs protection from the constant bombardment of harmful rays from the sun. The ozone layer of our atmosphere filters these out, allowing life-sustaining light to pass through to the earth. And your body? Many of the chemical processes in your body release toxic substances and waste into your bloodstream. If allowed to remain, these would cause serious problems for you, even death. They have to be continuously filtered out and removed.
This filtering is one of the principal functions of your kidneys. But how can these little organs identify, isolate, and remove harmful substances, yet at the same time make sure that vital elements remain to feed and nourish your body? And how can you help your kidneys keep you healthy?
What’s Inside Your Kidneys?
Humans normally have two kidneys—one situated on either side of the spine in the lower part of the back. Each is about four inches [10 cm] long, two inches [5 cm] wide, and one inch [2.5 cm] thick and weighs from four to six ounces [110-170 g]. Bisecting the kidney from top to bottom reveals several well-defined features, seen in the accompanying diagrams.
To visualize how the kidney works, imagine a stadium with thousands of spectators coming in for an event. First, the crowd must divide into numerous small lines. Then, the people in each line pass one by one through security gates, where individuals without tickets are turned aside. The spectators with tickets pass through to their assigned seats.
Similarly, all the many elements making up your blood need to circulate throughout your entire body. As they do so, however, they must repeatedly pass through your kidneys by means of large blood vessels, the renal arteries, one for each kidney. (See the illustration on page 24.) After entering the kidney, the renal artery fans out into smaller vessels in the kidney’s inner and outer layers. The various elements in your blood are thus channeled into smaller and more manageable “lines.”
Finally, the blood arrives at tiny clusters, each consisting of about 40 tightly looped, minute blood vessels. Each cluster, called a glomerulus, is surrounded by a two-layer membrane known as Bowman’s capsule.a Together, the glomerulus and Bowman’s capsule make up the first part of your kidney’s ‘security gate,’ a nephron—the basic filtration unit of your kidney. There are over a million nephrons in each kidney. But they are so small that you would need a microscope to examine one!—See the diagram of a nephron, greatly enlarged, on page 25.
Two-Stage Filtration of Your Blood
The blood cells and proteins in your bloodstream are indispensable. They provide your body with vital services such as oxygen supply, defense, and damage repair. To prevent the loss of blood cells and proteins, the first stage of filtration separates them from all other elements. This specialized task is accomplished by Bowman’s capsules. But how?
Blood vessels entering the glomerulus split up into tiny capillaries with very thin walls. Thus, blood pressure can force some water and other small molecules through their fine membranes, out of your bloodstream, and into Bowman’s capsule and the coiled tube connected to it. This tube is called the convoluted tubule. The larger protein molecules and all the blood cells remain in the bloodstream and continue to flow through the capillaries.
Now filtration becomes more selective. Your kidneys must make absolutely sure that nothing of value to your body escapes! The fluid flowing through the tubule at this point is a watery mixture, consisting of dissolved useful molecules along with wastes and unwanted substances. Specialized cells along the tubule’s inner wall recognize useful molecules, such as water, salts, sugars, minerals, vitamins, hormones, and amino acids. These are efficiently plucked out by being reabsorbed into the tubule wall and passed back into the surrounding network of capillaries to reenter your bloodstream. The capillaries join up again as little veins that then combine to become the blood vessel called the renal vein. By it your blood, now filtered and cleansed, leaves the kidney and goes on to sustain life in your body.
Expelling Waste Products
But what about the fluid that remains in the tubule? Obviously this contains substances that your body does not want. As the fluid continues to flow along the tubule toward the larger collecting tubule, or collecting duct, other cells in the tubule wall release additional secretions into it, including ammonia, potassium, urea, uric acid, and excess water. The final product is urine.
The collecting ducts from various nephrons join together and release urine through openings in the tips of the pyramids. The urine passes into the renal pelvis and then leaves by way of the ureter, the tube connecting the kidney and the bladder. Urine is stored in your bladder before being expelled from your body.
Despite their microscopic size, the more than two million nephrons in your kidneys do a very impressive job. The New Encyclopædia Britannica states: “Nephrons . . . filter the entire five-quart water content of the blood every 45 minutes.” By the time various substances have been reabsorbed and the many processes completed, a normal, healthy body can expel about two quarts [2 L.] of waste in the form of urine every 24 hours. What a hardworking and thorough filtration system!
Take Care of Your Kidneys!
Your kidneys are self-cleaning and self-maintaining, capable of operating for a long time. However, you have a part to play in helping them do their work. A great deal of water must pass through your kidneys for your body to remain healthy. Indeed, adequate water intake is considered a primary means of preventing kidney infections and the formation of kidney stones.b Drinking water also aids your digestive and cardiovascular systems, points out Dr. C. Godec, chairman of the Urology Department of Long Island College Hospital, New York.
How much water? Dr. Godec and many other doctors recommend that in addition to taking in other foods and drinks, each person should drink at least two quarts [2 L.] of water every day. “Most people are dehydrated,” Dr. Godec told Awake! He noted that as long as your kidneys or your heart are not diseased, water is good for them. “But you have to drink enough,” Dr. Godec said. “Most people don’t.”
Some find water more palatable with a little flavor added, such as lemon. Others prefer the taste of springwater or water that has been filtered through activated charcoal. In any case, plain or very lightly flavored water is better for your kidneys than any other beverage. In fact, the sugar in fruit juices and sweetened drinks increases the body’s need for water. Drinks containing alcohol or caffeine cause the body to lose water.
Getting into the habit of drinking two quarts [2 L.] of water a day can surely be a challenge. For one thing, many people find it inconvenient or embarrassing to have to relieve themselves more often than usual. But your body will thank you for making the extra effort. Besides helping to preserve your health, drinking sufficient water can even improve your appearance. Doctors point out that good nutrition and a high fluid intake are more effective at keeping your skin good-looking than any external skin preparation.
Unfortunately, our thirst mechanism is imperfect, and it becomes even less sensitive as we get older. Thus, we cannot rely on thirst alone to tell us how much water we need. How can you be sure to take in enough? Some start their day by drinking two glasses of water, and then at regular intervals they drink another glass. Others keep a transparent container of water in view and within reach—a reminder to take a sip periodically throughout the day. Whatever method you use, drinking plenty of pure, clean water is a good way to show appreciation for your kidneys—the marvelous filter that keeps you alive.
a In the early 1840’s, English surgeon and histologist William Bowman described this small capsule and its function. It came to be named after him.
[Box on page 25]
The Nephron—The Basic Filtration Unit
THERE are over a million nephrons in each kidney. The tubular system found within each individual nephron averages about 1.2 inches [3 cm] in length and a mere .002 inch [0.05 mm] in width. Yet, if it were possible to unravel all the tubules of a single kidney, they would extend for about 19 miles [30 km]!
Bowman’s capsule is actually the indented end of the convoluted tubule of the nephron. This tubule is surrounded by a network of very small blood vessels called capillaries. The tubule runs into a larger collecting duct, which carries away the waste and toxic substances filtered out by the nephron.
[Diagram on page 24, 25]
The renal vein takes newly filtered blood out into the body
The renal artery takes unfiltered blood to the kidney
The renal pyramids are conical structures that deliver urine to the renal pelvis
The cortex contains the glomerulus of each nephron
The renal pelvis is a funnel that collects urine and channels it to the ureter
The ureter delivers urine from the kidney to the bladder
[Diagram on page 25]
The nephrons, about two million microscopic tubular filters, clean the blood
Urine is collected by convoluted tubule, then goes to bladder