Urinary System

Chapter 6

Urinary System

Physiology of the urinary system

The urinary system consists of the kidneys, ureters, and bladder (Figure 6-1). The functional unit of the kidney is the nephron. Each kidney contains more than a million nephrons, which filter waste products from the blood, reabsorb water and nutrients (e.g., glucose and amino acids) from the tubular fluid, and secrete excess substances in the form of urine. In an average person, the nephron filters about 190 L of water out of glomerular blood each day. This enormous amount is many times the total volume of blood in the body. However, only a small proportion of this water (1 to 2 L) is excreted in the urine. Therefore, more than 99% of water is reabsorbed into tubular blood.

The formation of urine begins in the glomerulus, a tuft of capillaries with very thin walls and a large surface area. The blood pressure within the glomerulus is higher than that in Bowman’s capsule, which surrounds it. This difference causes the filtration of fluid into Bowman’s capsule that is equivalent to plasma containing neither protein nor red blood cells (if the nephron is healthy). The initial urine proceeds into the proximal convoluted tubule,

Radiographer Notes

Ultrasound and CT are being used with increasing frequency, and plain radiography with contrast material introduced intravenously or by means of a catheter has become a less frequently used technique for imaging the urinary system. In intravenous urography, the radiographer is responsible for preparing sterile injections of contrast material, and for operating the equipment properly and positioning the patient. In some instances, the radiographer may have to perform these functions in an operating room using sterile technique.

All radiographic studies begin with a “scout” image that is obtained before the injection of any contrast material. The radiographer should evaluate this image for proper technique and positioning so that any required alterations can be made on subsequent radiographs during the procedure. An image with correct density and contrast should demonstrate the kidney and psoas major muscle shadows, and the lumbar vertebrae and their transverse processes. A correctly positioned image should demonstrate from the superior portion of both kidneys and include the superior portion of the pubic bones (to ensure that the entire bladder is included). The radiograph must be in a true anteroposterior position with the pelvis appearing symmetric and the spinous processes of the lumbar vertebrae projected over the central portions of the vertebral bodies. The radiologist evaluates the scout radiograph to make certain that the technique and positioning are appropriate for the clinical history and to confirm proper patient preparation. The radiographic image is also checked for any radiopaque calculi or other abnormality that might be obscured after the injection of contrast material.

The radiographer must be alert to the possibility of any reaction, specifically an allergic reaction, whenever contrast agents are used. It is essential that the radiographer be aware of the proper procedures to follow in the event of any reaction and be able to initiate and maintain basic life support until advanced life-support personnel have arrived. Depending on departmental policy, it is usually the radiographer’s responsibility to assist during resuscitation procedures. Therefore, it is essential that the radiographer be familiar with the contents of the emergency cart and take responsibility for ensuring that the cart is completely stocked with appropriate medications.

All radiographs of the urinary system must be made with the patient in full exhalation so that the diaphragm assumes its highest position and does not compress the abdominal contents. Depending on the specific area being evaluated, the radiographer may have to perform oblique or erect projections, coned-down views of the kidneys or bladder, tomograms, or images made with abdominal compression. In certain pathologic conditions, radiographs must be obtained at precisely timed intervals, and delayed images may be necessary.

where a large amount of water and virtually all nutrients are reabsorbed into the blood capillaries surrounding the tubules. The amount of sodium and chloride reabsorbed is determined by the concentration of these substances in the body, and it occurs at a variable rate designed to keep the osmotic pressure of the body constant. This process is greatly influenced by two hormones: antidiuretic hormone (ADH), secreted by the posterior pituitary gland, and aldosterone, secreted by the adrenal glands.

After passing through the proximal tubule, the fluid flows through the loop of Henle, a complex structure consisting of a descending limb, a loop, and an ascending limb. Following the reabsorption of salt and water in the loop of Henle, the distal convoluted tubules permit the excretion of concentrated urine by actively secreting substances such as potassium ions (K+), hydrogen ions (H+), and some drugs. In this way the kidney plays an essential role in maintaining salt or electrolyte balance and acid-base balance of blood and body fluids.

To maintain a healthy metabolism, the pH must be kept in the very limited range of 7.35 to 7.45. If the pH of blood is lower than this (i.e., too acidic), the kidney excretes an acid urine to remove H+; if the pH of blood is higher than 7.5 (i.e., too alkaline), the kidney preserves H+ and secretes an alkaline urine.

Eventually, urine passes from the collecting tubules, whose openings are in the papillae, into the calyces, and on to the funnel-shaped renal pelvis and tubular ureters (Figure 6-2). Peristaltic waves (about 1 to 5 per minute) force the urine down the ureters and into the bladder. The ureters enter the bladder through an oblique tunnel that functions as a valve to prevent backflow of urine into the ureters (vesicoureteral reflux) during bladder contraction.

The bladder acts as a reservoir for the urine before it leaves the body (Figure 6-3). The openings of the two ureters lie at the posterior corners of the triangle-shaped floor (the trigone), and the urethral opening is situated at the anterior lower corner. Filling of the bladder (about 250 mL in the average person) stimulates autonomic nerve endings in the wall that are perceived as a distended sensation and the desire to void (micturate). A complicated sequence of bladder contractions and relaxation of the sphincter muscles permits the bladder to expel urine from the body through the urethra. Voluntary contraction of the external sphincter to prevent or terminate micturition is learned and is possible only if the motor system is intact. Nervous system injury (cerebral hemorrhage, spinal cord injury) results in involuntary emptying of the bladder at intervals (incontinence).

The kidney is also important in the production of red blood cells and in the control of blood pressure. Erythropoietin, a substance produced by the kidney, stimulates the rate of production of red blood cells. Therefore, renal failure is often associated with a severe anemia. Juxtaglomerular apparatus refers to specialized cells within renal arterioles that secrete renin, an enzyme that acts with one of the plasma proteins to produce angiotensin. Decreased blood flow through these arterioles increases the secretion of renin and thus the blood level of angiotensin, which constricts peripheral arterioles throughout the body and elevates the blood pressure.

Congenital/hereditary diseases

Anomalies of Number and Size

Unilateral renal agenesis (solitary kidney) is a rare anomaly that may be associated with a variety of other congenital malformations (Figure 6-4). Before the diagnosis can be made, it is essential to exclude a nonfunctioning, diseased kidney and prior nephrectomy. Unilateral renal agenesis results from a failure of the embryonic renal bud or renal vascular system to form. In true renal agenesis, the ureter and corresponding half of the trigone are missing also. Ultrasound (ultrasonography) or computed tomography (CT) can demonstrate the absence of renal tissue. A solitary kidney tends to be larger than expected, reflecting compensatory hypertrophy.

A supernumerary kidney is also a rare anomaly. The third kidney is usually small and rudimentary and possesses a separate pelvis, ureter, and blood supply. Although supernumerary kidneys function normally, they tend to cause secondary infections that eventually may require their removal.

A small, hypoplastic kidney often appears as a miniature replica of a normal kidney, with good function and a normal relationship between the amount of parenchyma and the size of the collecting system (Figure 6-5). Renal hypoplasia must be differentiated from an acquired atrophic kidney, which is small and contracted because of vascular or inflammatory disease that has reduced the volume of renal parenchyma.

Compensatory hypertrophy is an acquired condition that develops when one kidney is forced to perform the function normally carried out by two kidneys (see Figure 6-5). This phenomenon may follow unilateral renal agenesis, hypoplasia, atrophy, or nephrectomy. The ability of the kidney to undergo compensatory hypertrophy is greatest in children and diminishes in adulthood. Ultrasound demonstrates the size of the renal parenchyma, calyces, and pelvis without the use of a contrast agent or ionizing radiation to provide a diagnosis.

Anomalies of Rotation, Position, and Fusion

Malrotation of one or both kidneys may produce a bizarre appearance of the renal parenchyma, calyces, and pelvis that suggests a pathologic condition when in reality the kidney is otherwise entirely normal (Figure 6-6). Abnormally positioned kidneys (ectopic kidney) may be found in various locations, from the true pelvis (pelvic kidney) (Figure 6-7) to above the diaphragm (intrathoracic kidney) (Figure 6-8). Pelvic kidneys occur much more frequently than intrathoracic kidneys. Whenever only one kidney is seen on intravenous urography, a full view of the abdomen is essential to search for an ectopic kidney. Although the ectopic kidney usually functions, the nephrogram and the pelvicalyceal system may be obscured by overlying bone and fecal contents. Patient history can distinguish a true pelvic kidney from a kidney transplant, which typically is located in the right pelvis. Crossed ectopia refers to a situation in which an ectopic kidney lies on the same side as the normal kidney and is very commonly fused with it.

Horseshoe kidney is the most common type of fusion anomaly. In this condition, both kidneys are malrotated and their lower poles are joined by a band of normal renal parenchyma (isthmus) or connective tissue (Figure 6-9). The ureters arise from the kidneys anteriorly instead of medially, and the lower pole calyces point medially rather than laterally. The pelves are often large and flabby and may simulate obstruction. Obstruction at the ureteropelvic junction may occur because of the anterior position of the ureters. Complete fusion of the kidneys is a rare anomaly that produces a single irregular mass that has no resemblance to a renal structure. The resulting bizarre appearance has been given such varied names as disk, cake, lump, and doughnut kidney.

Anomalies of Renal Pelvis and Ureter

Duplication (duplex kidney) is a common anomaly that may vary from a simple bifid pelvis (Figure 6-10) to a completely double pelvis ureter (Figure 6-11) and ureterovesical orifice. The ureter draining the upper renal segment enters the bladder below the ureter draining the lower renal segment. Complete duplication can be complicated by obstruction or by vesicoureteral reflux with infection. Vesicoureteral reflux and infection more commonly involve the ureter draining the lower renal segment; obstruction more frequently affects the upper pole, where it can cause a hydronephrotic mass that displaces and compresses the lower calyces.

In cases of renal congenital anomalies, to minimize radiation exposure to the patient, ultrasound and plain radiography are the first imaging modalities performed, rather than CT.


A ureterocele is a cystic dilatation of the distal ureter near its insertion into the bladder. In the simple (adult) type, the opening in the ureter is situated at or near the normal position in the bladder, usually with stenosis of the ureteral orifice and with varying degrees of dilatation of the proximal ureter. The stenosis leads to prolapse of the distal ureter into the bladder and dilatation of the lumen of the prolapsed segment. Ectopic ureteroceles are found almost exclusively in infants and children; most are associated with ureteral duplication.

Radiographic Appearance

The appearance of a ureterocele on intravenous urography depends on whether opaque medium fills it. If it is filled, the lesion appears as a round or oval density surrounded by a thin radiolucent halo representing the wall of the prolapsed ureter and the mucosa of the bladder (cobra head sign) (Figure 6-12A). When the ureterocele is not filled with contrast material, it appears as a radiolucent mass within the opacified bladder in the region of the ureteral orifice (Figure 6-12B). Ultrasound is the modality of choice to evaluate infants and children. Children with ureteral duplication have an 80% incidence of an associated ureterocele. On ultrasound images, a ureterocele appears as a round cystlike structure within the bladder (Figure 6-12C).

On intravenous urography, an ectopic ureterocele typically appears as a large, eccentric filling defect impressing the floor of the bladder (Figure 6-13). The ureterocele arises from the ureter draining the upper segment of the duplicated collecting system. A mass effect, representing hydronephrosis, often involves the upper pole of the kidney and causes downward and lateral displacement of the lower portion of the collecting system.

Posterior Urethral Valves

Posterior urethral valves are thin, transverse membranes, found almost exclusively in males, that cause bladder outlet obstruction and may lead to severe hydronephrosis, hydroureter, and renal damage. The thin, transverse membranes work as a reverse valve, meaning that catheterization is normal but the valve prevents antegrade flow. They are best demonstrated on a voiding cystourethrogram (Figure 6-14). The proximal urethra is dilated and the thin, lucent, transverse membrane of the valve can be identified.

Inflammatory disorders


Glomerulonephritis is a nonsuppurative inflammatory process involving the tufts of capillaries (glomeruli) that filter the blood within the kidney. It represents an antigen-antibody reaction that most commonly occurs several weeks after an acute upper respiratory or middle ear infection with certain strains of hemolytic streptococci. In recent years, western countries have seen a decrease in acute glomerulonephritis caused by streptococci. More frequently, the inflammatory process is due to a chronic autoimmune disorder. The inflammatory process makes

the glomeruli extremely permeable, allowing albumin and red blood cells to leak into the urine (resulting in proteinuria or hematuria). A decreased glomerular filtration rate results in oliguria, a smaller-than-normal amount of urine.

Radiographic Appearance

The intravenous urographic findings in glomerulonephritis depend on the duration and severity of the disease process and on the level of renal function. In patients with acute glomerulonephritis, the kidneys may be normal or diffusely increased in size with smooth contours and normal calyces. A loss of renal substance in chronic glomerulonephritis produces bilateral small kidneys (Figure 6-15). The renal outline remains smooth and the collecting system is normal, unlike the irregular contours and blunted calyces seen in chronic pyelonephritis. Ultrasound may demonstrate these findings more efficiently without contrast enhancement or radiation exposure.


Pyelonephritis is a suppurative inflammation of the kidney and renal pelvis caused by pyogenic (pus-forming) bacteria. Unlike glomerulonephritis, which primarily involves the parenchyma (glomeruli and tubules) of the kidney, the inflammatory process of pyelonephritis affects the interstitial tissue between the tubules. The infection is patchy in distribution, often involves only one kidney, and is asymmetric if both kidneys are involved. Although it may spread from the bloodstream or lymphatics, the infection usually originates in the bladder and ascends by means of the ureter to involve the kidneys. Pyelonephritis often occurs in women and children. The disease frequently develops in patients with obstruction of the urinary tract (due to enlarged prostate gland, kidney stone, congenital defect), which causes stagnation of the urine and provides a breeding ground for infection. Instrumentation or catheterization of the ureter is also an important contributing factor to the development of pyelonephritis.

Patients with pyelonephritis have high fever, chills, and sudden back pain that spreads over the abdomen. Painful urination (dysuria) usually occurs. Large amounts of pus may be detected in the urine (pyuria), and bacteria can be cultured from the urine or observed in the urinary sediment.

Radiographic Appearance

In most patients with acute pyelonephritis, the intravenous urogram findings are normal. Occasional abnormalities include generalized enlargement of the kidney on the symptomatic side, delayed calyceal opacification, and decreased density of the contrast material. A characteristic finding is linear striation in the renal pelvis, which probably represents mucosal edema.

The urographic hallmark of chronic pyelonephritis is patchy calyceal clubbing with overlying parenchymal scarring (Figure 6-16). Initially, there is blunting of the calyces, which then become rounded or clubbed. Fibrotic scarring causes a cortical depression overlying the dilated calyx. Progressive cortical atrophy and thinning may be so extensive that the tip of the blunted calyx appears to lie directly beneath the renal capsule. The urographic findings may be unilateral or bilateral and are often most pronounced at the poles. If calyceal changes are minimal, the overlying cortical depressions may simulate lobar infarctions or normal kidney lobulations. However, in chronic pyelonephritis, the cortical depression lies directly over a calyx rather than between calyces as in lobar infarctions or congenital lobulation. Chronic pyelonephritis may progress to end-stage renal disease with small, usually irregular, poorly functioning kidneys.

CT has become the imaging modality of choice for this disease. Contrast-enhanced CT demonstrates the cortical changes and may show an associated abscess. Ultrasound also can demonstrate an abscess but is less sensitive. Both CT and ultrasound can detect hydronephrosis, which would indicate that urinary tract obstruction is a predisposing factor. In chronic nephritis, ultrasound shows loss of renal parenchyma as an increase in echogenicity in the area of the scar and extension of the central renal sinus echoes to the periphery of the affected area. Dimercaptosuccinic acid scintigraphy (nuclear medicine study) can demonstrate inflammation and scarring of the kidney and determine the distribution of function in the renal cortex. Some clinicians prefer this study to CT because of the lower radiation dose.

Emphysematous Pyelonephritis

Emphysematous pyelonephritis is a severe form of acute parenchymal and perirenal infection with gas-forming bacteria that occurs virtually only in diabetic patients and causes an acute necrosis of the entire kidney. The presence of radiolucent gas shadows within and around the kidney is pathognomonic of emphysematous pyelonephritis (Figure 6-17). CT is the preferred modality for localizing the gas patterns within and around (perinephric) the kidney. The affected region appears as mottled areas of low attenuation.

Apr 10, 2017 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Urinary System
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