What is urology?

Science and Profession

The urinary system is a complex series of structures that includes the kidneys, ureters, urinary bladder, and urethra. Since in males the urinary tract is closely associated with the genital tract, urology properly deals with disorders of the male genitourinary tract and the female urinary tract. Urologists may also study disorders of the adrenal glands, which are closely associated with the kidneys.

Urine production begins in the kidneys, a pair of bean-shaped organs found within the abdomen. Urine is produced through a complex system of units called nephrons; approximately one million nephrons are found within each kidney. Each nephron consists of a ball-shaped capillary network called the glomerulus, which is surrounded by a capsule (Bowman’s capsule) through which the actual filtration of blood takes place. Blood enters the glomerulus under high pressure, forcing the liquid and dissolved material through the basement membrane into the renal tubules that extend from the capsule.

The long, convoluted tubule that extends from each capsule follows a circuitous route through the kidney. As it emerges from the capsule, the proximal convoluted tubule is found within the outer region, or cortex, of the kidney. The tubule then passes through the inner portion, or medulla, of the kidney, forming an extended loop called the loop of Henle. The tubule winds its way back to the cortical region as the distal convoluted tubule. Blood circulates completely through the kidneys about twenty times each hour. Approximately 20 percent of the plasma (liquid portion of the blood) is filtered through the Bowman’s capsules during this time, the equivalent of some 180 liters of fluid per day. Much of the plasma and nearly all the nutrient material found within the liquid that passes through the tubules are reabsorbed into the capillary network surrounding the tubules. Approximately 80 percent is absorbed within the proximal convoluted tubule, with the remainder being absorbed as it flows through the tubule system. The rest of the fluid, approximately one liter per day for the average person, contains nitrogenous material such as urea, salts, and other metabolic wastes, which are voided.

The distal tubules emerge from the cortex of the kidney and again pass into the medulla, where they now merge into increasingly larger collecting ducts. The collecting ducts form clearly visible pyramids, or papillae, within the medulla. The merging of the largest ducts within the renal pelvis, the lowest portion of the kidney, results in the formation of a single tube, the ureter. One ureter emerges from each kidney to empty the urine into the bladder.

The ureters are thick-walled tubes that extend through the pelvic region. They enter the bladder in a slanted manner, which helps prevent backup of the urine from the bladder when it is full. Urine is pumped through the ureters by means of peristaltic, or rhythmic, contraction of the smooth muscle that lines the ureters.

The urinary bladder is a membranous organ in the pelvis that serves to store and discharge urine. The average individual's bladder is capable of holding approximately one-third to one-half of a liter of liquid. When full, it can cause the lower abdomen to bulge visibly. Since the structure is adjacent to the uterus in women, conditions such as pregnancy may significantly lower the carrying capacity of the bladder.

The musculature in the lower portion of the bladder is thickened, forming the bladder neck, and serves to retain the liquid within the organ. The muscle, in turn, is continuous with that of the urethra, the tubular structure that drains the urine from the bladder.

In women, the urethra is three to four centimeters in length and emerges just in front of the vagina. In men, the tube is approximately twenty centimeters long. Emerging from the bladder in the male, it passes through the prostate gland and into the penis, where it serves as a passage both for the elimination of urine and for semen during ejaculation.

Since urine formation begins in the kidney, urology may overlap with nephrology at times. Strictly speaking, however, nephrology deals with the kidney as a regulatory organ for fluid and salt levels in the body, in addition to its role as an endocrine gland; urology deals with disorders of the urinary tract, in addition to problems associated with the genitourinary tract in males, since the two systems are so closely associated.

Approximately 20 percent of adult visits to a physician involve problems associated with the genitourinary tract. Urinalysis—the physical, chemical, and microscopic evaluation of collected urine—thus becomes an important diagnostic tool. The process begins with proper collection of urine in a sterile specimen container. The sample initially undergoes a macroscopic examination in which color and appearance are evaluated. Since recent ingestion of food may result in the discoloration of urine or alteration in its pH, it is best to obtain the sample several hours after the patient has eaten. Generally, the odor is unimportant; for example, by-products of asparagus ingestion may produce a rather characteristic odor in urine that is of no medical significance. Nevertheless, a pungent aroma may signify an infection. Metabolic diseases may also lead to by-products that have characteristic smells.

Macroscopic examination of urine also involves a determination of the specific gravity, or density, of the solution and its pH. Densities outside the normal density range for urine may be indicative of diabetes mellitus or renal dysfunction. The pH is a measurement of hydrogen ion concentration in the fluid. A pH of 7.0 is neutral. Normal levels in urine vary considerably, from an acid level of 4.6 to an alkaline pH of 8.0. Generally, urine samples obtained soon after a meal will be slightly alkaline, but a consistently alkaline level may be indicative of a urinary tract infection. Other macromolecules that may be observed in urine as a result of various pathologies include elevated levels of protein or sugar and the presence of blood (hematuria).

Microscopic analysis of urine is a necessary part of a thorough urinalysis. The urine sample is centrifuged, or spun at high speed, to concentrate material in a smaller volume. The pellet from the
centrifugation is then stained and observed for bacteria or blood cells. Normally, the number of bacteria and white blood cells in urine is low, and some bacterial contamination of the specimen during collection is common. Large numbers of either may be indicative of an infection. The presence of red blood cells in urine is always considered abnormal and may signify inflammation or bleeding within the urinary system.

Diagnostic and Treatment Techniques

A thorough urinary system assessment involving the examination of the kidneys, ureters, bladder, and urethra may be necessary for an accurate diagnosis of certain pathologies. In addition to the normal urinalysis, including the use of a catheter for obtaining a urine sample, the study includes the patient’s medical history and vital signs. The diagnosis of urinary problems may include procedures for obtaining images of the urinary tract, such as x-rays of the kidneys or urinary tract, as well as excretory or intravenous urography. The latter involves the injection of a radiopaque solution into the system, followed by x-ray analysis as the solution passes through the tract. Direct observation through cystoscopy may also be carried out. Other methodologies developed during the 1980s include computed tomography (CT) scanning and magnetic resonance imaging (MRI).

Depending on the problem, treatment may be as simple as prescribing antibiotics. Urologic surgery becomes necessary if diagnostic procedures reveal a tumor or obstruction. Such circumstances may require surgical removal, reconstruction, or relocation. For example, damage to the urinary system as a result of neurologic or neoplastic (cancerous) conditions may require the diversion of urine through an opening in the abdomen, a ureteroileostomy, instead of through normal channels.

Pathologic conditions of the urinary tract may take a variety of forms, such as an obstruction that interferes with urinary flow or an infection by any of a wide array of bacteria. Either condition may lead to inflammation and subsequent urinary problems. Damage may also result from external forces, such as injuries caused by falling or blunt force.

Urinary tract obstructions are generally classified on the basis of several characteristics: the etiology or source of the obstruction, the length of time over which the obstruction takes place (acute or chronic), and the site of the obstruction. The source of the obstruction may be congenital, often resulting from a stenosis, or narrowing, of the meatus (opening or tunnel) within the urethra. An additional congenital abnormality may result from the inability of the ureterovesical junction, the site at which the ureters enter the bladder, to prevent urine reflux, or backflow, into the ureter. The result of any such obstruction is frequently pyelonephritis, an infection within the urinary system. Since any infection may ascend to the kidney, damage can occur at any site in the urinary tract.

Obstructions may result from injury to the urinary tract, from benign or malignant tumors, or from the formation of stones. In women, extension of the uterus during pregnancy may impinge on the ureters, interfering with normal flow. The obstructions may develop anywhere along the urinary tract. An obstruction in the lower urinary tract (the region along the urethra) may cause ballooning or dilation of the urethra; in men, this dilation may extend into the prostate gland. The weakening of the urethral wall may result in the formation of diverticula, pouchlike herniations in the muscle wall. If the region becomes infected, a likely possibility, the increased hydrostatic pressure coupled with the weakening of the wall may cause the urethra to rupture.

Midtract obstructions are associated with the bladder. To compensate for increased resistance to urine flow, the muscle of the bladder may initially thicken, sometimes increasing in thickness by a factor of two or three. The increased size of the musculature of the bladder may in turn actually decrease the urine flow from the ureter as a result of the downward pull on these tubes. The resulting backflow may cause damage to the ureters or kidneys.

The increased pressure within the bladder may also force the tissue, or mucosa, between bundles of musculature, resulting in pockets called cellules. Continued pressure may result in larger pockets, or diverticula, being formed within the bladder wall. Since these regions tend to retain urine, infections are common, and surgery may be necessary to remove the diverticula.

Obstructions of the upper urinary tract are associated with the ureters and kidneys. Increased pressure from backflow may cause dilation of the ureter wall, with an increase in muscle development as compensation. This stage is generally followed by one of decompensation, in which the ureters lose their ability to contract and maintain urine flow. Likewise, the kidneys may be subjected to increased pressure. Normally, the pressure on the kidneys from within the urinary tract is very low. When the pressure is increased on the kidney pelvis, the regions in which the collecting ducts form, the pelvis becomes subject to pressure, which ultimately has an impact on blood flow. The result is ischemia, or lack of oxygen to the region. The kidney itself may atrophy, followed by renal failure.

Generally, obstructions can be visualized through a variety of procedures. Tumors or calcified stones within the tract will show on x-rays. An excretory urogram, a technique in which the urinary tract is x-rayed following injection of a radiopaque substance, may reveal the precise site of the obstruction. The urogram is preferred for observation of certain forms of urinary tract stones that may not appear on conventional x-rays. It can also be used to observe sites of both dilation and stenosis.

Depending on the source of obstruction, urologic surgery may become necessary for its removal. If kidney function is significantly reduced, temporary or permanent dialysis or even transplantation may be necessary. However, temporary urinary diversion may provide relief to the system, allowing natural healing to repair dilated tubes once the obstruction has been removed. Ureteroileostomy, in which a portion of the ureter is diverted through an opening, or stoma, in the intestine, has been used under such circumstances.

Urinary stones remain the most common cause of obstructions. The formation of stones is related to a variety of factors, including the diet and metabolic state of the patient, genetics, and the anatomic features of the urinary tract. The result is increased deposition of salts such as calcium around an initial foreign body in the urine. Eventual crystallization leads to steady increases in the size of the stone and, unless it is passed naturally within the urine, eventual obstruction. Stones may form anywhere in the tract, but they tend to be less common in the urethra. In general, stones are crystals of either calcium salts or, less often, uric acid.

A variety of techniques exists for the elimination of urinary stones. Stone dissolution, including lithotripsy (the breaking up of the stone with a surgical instrument or shock waves), is preferred, since it requires minimal invasiveness and hospitalization. Hemiacidrin, a magnesium-containing solution, has been used successfully in dissolving certain stones. Ultrasonographic lithotripsy, which uses ultrasonic vibrations to dissolve the stone, has also proved successful. Some stones, however, particularly those composed of calcium, may not respond adequately to these forms of treatment. If the obstruction is significant, and particularly if an infection is present, surgical removal may become necessary.

Urinary tract infections may be primary (a direct result of contamination) or secondary (the result of other pathological conditions, such as obstructions). Infections may be confined to a single site or may spread to other organs or areas. Since the clinical signs of infection may resemble those of other conditions, recognition of the microbial cause is necessary for proper treatment. Infections that spread to the kidneys may cause significant damage or organ failure.

Infections are categorized as being either specific or nonspecific. Specific infections are those in which a disease is manifested as a result of a particular agent. For example, sexually transmitted diseases (STDs) are specific in the sense that gonorrhea is caused only by Neisseria gonorrhoeae and urinary tuberculosis by Mycobacterium tuberculosis. Nonspecific infections are diseases in which the pathology or manifestation may be similar but the symptoms may be caused by any of a variety of bacteria. For example, common causes of nonspecific urinary infection include Escherichia coli (E. coli) and members of the genera Proteus and Staphylococcus.

The most common cause of urinary tract infections is E. coli, a natural colon bacillus. Secondary problems may also result from specific agents. For example, members of the genus
Proteus produce urease, an enzyme capable of splitting urea to form ammonia. The result is a rise in pH, an alkaline condition that may cause precipitation of magnesium or calcium salts and subsequent stone formation.

The specific physical manifestation of the infection is generally related to the site within the urinary tract. Urethritis, accompanied by reduced or painful urination, often results from STDs. Infection may spread as far up as the kidney, with resulting pyelonephritis. Both E. coli and STDs are common causes, though other bacteria may also cause similar types of infections. Proper diagnosis of bacterial infections generally requires the isolation and identification of the organism, if possible, and the ruling out of other possible causes of the symptoms, such as diabetes. The agent may be isolated from pus, from urine, or through the insertion of a needle into the lesion itself. Treatment usually involves antimicrobials (antibiotics) suited to the particular etiological agent. Abscesses, particularly those in the kidney, may require surgical drainage. If the abscess is too large or does not respond to treatment, then nephrectomy (the surgical removal of a kidney) may be necessary.

Damage from external sources may also result in injury to the urinary tract. Depending on the damage, surgical repair or realignment of the urethra, bladder, or ureters may be necessary. Observations via x-rays, cystograms, or urethrograms are routinely used for such assessment.

Perspective and Prospects

The understanding of urine formation and excretion has its roots in the work of the Roman physician Galen during the second century CE. Though observations had been carried out before this period, it remained unclear whether the source of urine was the kidney or the bladder. Galen settled the issue by tying off the ureters in animals, demonstrating that no urine would be found below the stricture, and thus urine formation began in the kidney.

Urology as a branch of medicine, and indeed clinical interest in urine formation, arguably began in the early decades of the nineteenth century. In 1827, English physician Richard Bright described a form of chronic nephritis, which became known as Bright disease, in which progressive kidney failure generally resulted in the death of the individual. Bright demonstrated that as a result of kidney failure, instead of urine being secreted from the body, its constituents are retained in body fluids. It was also in 1827 that German chemist Friedrich Wöhler chemically synthesized urea, the first demonstration of the synthesis of an organic
compound from inorganic materials.

Beginning in 1844, Carl Ludwig attempted to explain urine formation on the basis of a purely physical process. He suggested the hydrostatic pressure of the blood is sufficiently high that a protein-free filtrate is forced through the kidney glomeruli, followed by passage through the tubules, and ultimately into the ureters. The first definitive work on urine secretion was Arthur Robertson Cushny’s 1917 monograph The Secretion of Urine. Cushny believed that urine secretion involves both an active and a passive process: mechanical filtration and movement through the urinary tract, and active tubular reabsorption of most nutrients before the liquid leaves the kidney. He was subsequently proved to be essentially correct in this belief, though Cushny’s mechanics of reabsorption were less than accurate and were later refined by others.

The development of noninvasive techniques for the elimination of stones and improved surgical methods for urinary diversion marked much of the progress in urology in the 1970s and 1980s. Extracorporeal shock-wave lithotripsy (ESWL), the use of ultrasonic vibration for the disintegration of stones, eliminated the need for the surgical removal of these obstructions in most cases. The use of ureterosigmoidostomy (implantation of the ureter into the intestinal tract) dates to the nineteenth century. It was replaced with alternate methods of bladder augmentation. The ureter itself could be replaced with segments of intestinal ileum, or it could be joined to the other ureter (ureteroureterostomy).

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