How the Urinary Tract Works

Dr. Patrick Walsh's Guide to Surviving Prostate Cancer
by Janet Farrar Worthington, Patrick C. Walsh, M.D.

(Page 4 of 4)

The kidneys are the body's main filters. With each heartbeat, they cleanse the blood of toxic wastes, excess water and salts, and (among many other chores) help maintain the body's balance of fluids and minerals. With more than a million tiny, wadded-up filters called nephrons, the kidneys sift through an incredible volume of fluid - about 45 gallons a day for a 150-pound man. (See Fig. 1.5.) Every sip of water we drink is refined, reabsorbed, and then processed again. (If the water and minerals weren't reabsorbed, our bodies would become seriously dehydrated within hours.) Not all of this material returns to the body, however; much of it passes out as urine. Every day, the average man excretes about two quarts of urine (a concentrated mixture of water, sodium, chloride, bicarbonate, potassium, and urea, the breakdown product of proteins).

Urine exits each kidney through a pipeline called the ureter. The ureters work like toothpaste tubes, squeezing or “milking” urine from the kidneys. Each ureter is about a foot long, and narrow - less than a half-inch wide at its broadest point. Ureters are one-way streets: Urine always flows the same way through them - straight toward the bladder.

The bladder is a big bag. Stretched to its fullest, this muscular tank can hold about a pint of urine. Unlike the kidneys and ureters, the bladder-in normal circumstances-allows us some voluntary control; it generally obeys our decision to eliminate or hold urine. (The inability to control urination is called incontinence.) With intricately woven layers of muscle and connective tissue, the bladder can collapse or expand, depending on the amount of fluid it's asked to hold at a given time. A sophisticated backup system protects the bladder from extreme distention and the risk of rupture: When the bladder is very full, it signals the kidneys to slow down the production of urine. At the neck of the bladder is a gate called the trigone. The purpose of the trigone is to make sure urine flows only one way-downward, away from the ureters and kidneys. The trigone's valve makes a tight seal that prevents urine from backing up into the kidneys, even when the bladder is distended.

The next stop of urine's downward passage is the urethra, another muscular tube, about 8 inches long. This one begins at the neck of the bladder, then tunnels through the prostate at a 35-degree angle and continues into the penis. The urethra is divided into three segments-prostatic (the part that runs through the prostate), membranous (in between the prostate and penis-this is where the external sphincter is located), and penile. Like the prostate, it plays a role in both the urinary and reproductive systems; it serves as a conduit not only for urine, but for sexual fluids. The prostatic urethra has its own gate to prevent fluid backup-a ring of smooth muscle that works with the bladder neck as a clamp during ejaculation. This keeps semen from flowing the wrong way-up into the bladder-and directs its course downward, out the urethra.

That's it for the anatomy crash course. Over the course of this book, as we describe diagnostic procedures, treatments, and complications, you may need to return to this chapter. That's what it's for - to give you a working familiarity with the territory we'll be covering in the next chapters. If it helps, think of these pages as your Michelin Guide to male anatomy. Now that we've discussed the context of the prostate - as a significant gland in both the urinary and reproductive systems - it's time to explain why this tiny gland is so important, and what can go wrong.

Copyright © 2001 by Patrick C. Walsh and Janet Farrar Worthington

About the Author

Janet Farrar Worthington is a science writer and commentator on American Public Radio.

Patrick C. Walsh, M.D., urologist-in-chief at Johns Hopkins Hospital and director of the Department of Urology at Johns Hopkins University School of Medicine is the surgeon who created the Nerve-Sparing Technique (also known as the Walsh procedure), which has made it possible to preserve potency in men who lose their prostate.