The bladder
The bladder is a pear shaped muscular sac that is made up of three layers: the mucosa, transitional epithelium and lamina propria and is used as a reservoir for urine (Tortora, J.G et al. 2003). It is situated in the pelvic cavity posterior to the symphysis pubis. In males, the bladder lies in front of the rectum, but in the female is separated from the rectum by the uterus and vagina (Linn-Watson, T. 1996). The differences are shown in the diagram below.
The female bladder The male bladder
There are three openings into the bladder, one at each ureter and one at the urethra. These are arranged in the form of triangle, called the trigone (Solomon, E. et al. 1990). The bladder can contain up to 700-800ml of urine without being over distended and is normally emptied when holding 180 to 300ml of urine (Ross, J. et al. 1990).
The urethra
The urethra is the passageway of urine from the bladder to the exterior and has three layers of tissue:
- The muscular layer that is continuous with the bladder
- The thin, spongy coat
- Lining of mucous membrane continuous with that in the bladder and then changing to a layer of stratified squamous epithelium continuous with the skin near the orifice.
Tortora, J.G. et al. (2003)
The female urethra is very short tube, only about 2.5- 3.8cm in length, it leaves the base of the bladder at the apex of the trigone and reaches the exterior between the labia minora anteriorly to the opening of the vagina (Linn-Watson, T. 1996). When it leaves the bladder the female urethra is surrounded by a sphincter muscle.
The male urethra extends approximately 20cm from the bladder to its external orifice, the meatus, at the extremity of the penis. When it leaves the bladder male urethra is surrounded by sphincter muscle and prostate gland (Seeley, R. et al. 1992).
Urinary system physiology
The main function of the kidneys is to help in maintaining the homeostasis of the blood by the excretion of urine. Urine production takes place in the nephron, which contains the Bowman’s capsule, the glomerulus, and tubules. There are three mechanisms involved in production of urine:
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
(Bartholomew, M. 1997)
Carola, R., et al. 1992, p. 859
Glomerular filtration
This is a physical process that takes place in the glomeruli. Water, glucose, urea, proteins, uric acid, sodium, potassium, chloride ions and other substances are filtered from the blood into proximal convoluted tubules by a net filtration pressure (Solomon, E. et al 1990). The amount of filtrate that is produced by the kidneys is known as the glomerular filtration rate (GFR). It is important that the kidneys maintain a relatively constant GFR. If the rate is too fast then there is no time for the needed substances to be reabsorbed, and if the rate is too slow then too much of the filtrate is reabsorbed and not enough waste products are excreted.
Tubular reabsorption
Tubular reabsorption occurs in the renal tubules and collecting ducts. Most of the filtered water, salts and other useful solutes, which were filtered from the glomerular filtration phase, are reabsorbed into the blood stream by the kidney cells. Only a very small amount of the filtrate, about 1-2 litres per day, is filtered from the body each circuit (Solomon, E. et al. 1992).
Tubular secretion
This is a chemical process that takes place in the convoluted tubules. The cells lining the tubules select abnormal substances from the blood, or normal substances whose concentration is too high and pass them into the lumen of the tubules.
The product resulting from the above three processes is called urine and it passes through the collecting tubules into collecting ducts and into the renal pelvis, down the ureter by peristaltic action of its walls (Bartholomew, M. 1997). The kidneys produce urine to enable the following tasks to be carried out:
- Excretion of water and the end products of protein metabolism
- Excretion of salts or electrolytes and regulation of the blood pH
- Excretion of chemical substances that may be harmful
- Secretion and reabsorption of hormones
(Vander, A. 1995)
Excretion of water and the end products of protein metabolism
Water acts as a dynamic transport system, which helps to maintain the normal composition of the blood, tissue fluids and is also an end product of metabolism of proteins, carbohydrates and fats. Urea is one of the most important end products of protein metabolism and represents half of the total solid constituents of the urine. The normal level is 2.5 to 6.4mmol/l (Smith, T. 1998).
Another waste product is creatinine, which is formed by the breakdown of phosphates in the muscles. The normal level is 70 to 120μmol/l (Smith, T. 1998). Raised blood urea and creatinine can indicate failure of the kidneys.
Excretion of salts or electrolytes and regulation of the blood pH
One of the most important salts circulating in the blood is sodium bicarbonate, an alkali, which neutralises acids. If the alkali reserve is reduced, the blood becomes acidic and this condition is known as acidosis. The normal level in blood is 135 to 147mmol/l.
By excreting acid salts and other acidic substances, the kidneys spare the use of the alkali reserve, which would otherwise have been used to neutralise them (Smith, T. 1998)
Excretion of chemical substances that may be harmful
This process allows drugs to be taken for long period of time without accumulation in the blood and the subsequent toxic effects.
Secretion and reabsorption of hormones
Hormones (in addition to nervous impulses) are the bodies communication system for the purpose of homeostasis. The main hormones involved in the homeostasis process are:
- Sodium and water reabsorption are under the influence of antidiuretic hormone (ADH).
- Aldosterone – controls potassium content of the blood, which will affect muscle and nerve function.
- Rennin – plays a key role in filtration rate of the kidneys and the production of angiotensin II, which is the body’s most potent vasoconstrictor.
(Tortora, J.G. et al. 2003)
Urinary system pathology
A common pathology of the urinary system is renal calculi, known as kidney stones (Figure 1).
Renal calculi
Figure 1
There are five distinct varieties of calculi that vary in size, shape, contour, colour and consistency (See Appendix B). Renal calculi usually originate in collecting tubules or in renal papillae and then pass into the renal pelvis where they may increase in size (Ross, J. et al. 1990). Some calculi become too large to pass through the ureter and may obstruct the flow of urine. Others pass to the bladder and are either excreted or increase in size and obstruct urethra. Occasionally stones may originate in bladder. These stones are either single or multiple (Figure 2) and vary in size. Bladder stones have a tendency to form from stagnant urine that is unable to be passed, due to urethral outlet obstruction (Linn-Watson, T. 1996).
Renal bladder
calculi
Figure 2
Calculi are usually asymptomatic until they begin to descend through the ureters or until they cause an obstruction (Linn-Watson, T. 1996). The exact cause of the formation of calculi is not known, but several conditions are implicated:
- Dehydration – this leads to increased reabsorption of water from the tubules, but does not change solute reabsorption, which results in a reduced volume of highly concentrated filtrate in the collecting tubules.
- pH of urine – when the normally acid filtrate becomes alkaline some substances, such as phosphate, may be precipitated. This may occur when the kidney system is defective and in some infections.
- Renal infection – necrotic material and pus provide foci upon that solutes in the filtrate may be deposited and the products of infection may alter the pH of the urine.
-
Tumours – Pressure caused by tumours may restrict the flow of urine and may cause ischemia (lack of blood flow) and necrosis (death of cells or tissues) or predispose to infection
- Obstruction – There are three most common sites for the stones to cause obstructions: The ureteropelvic junction, the pelvic brim and the ureterovesical junction (Mace, J.D. et al. 1998). The obstruction can lead to hydronephrosis (Figure 3 arrows), which is a dilation of renal pelvis and calyces caused by accumulation of urine.
Figure 3
Bilateral renal hydronephrosis (arrows)
It is important to identify stones, because renal failure can occur from an obstruction of the ureter.
To diagnose renal calculi a number of radiographic procedures can be carried out, that will be discussed in the next section.
Urinary system imaging procedures
Many different imaging procedures can be used to study the various aspects of the urinary system. It is desirable for the patient to undergo an ultrasound in the first place, since it is quick, painless, non ionising and comfortable for the patient (no preparation is needed unless full bladder is required) (Chapman, S. et al 2001). It is also good for the patients who are allergic to contrast media, have renal failure or are pregnant. Ultrasound is useful in diagnosis of kidney stones (Figure 4), calcifications, hydronephrosis, abscesses, renal masses, cysts and to asses renal atrophy (Mace, J.D., et al. 1998). The applications of ultrasound in the urinary tract can be found in the Appendix C. Ultrasound demonstrates anatomy without the necessity for adequate renal function, but gives no functional information, therefore is the ideal complement to nuclear medicine imaging (Chapman, S., et al. 2001).
Figure 4
Calculus of the renal calyx and renal pelvis (highlighted areas)
()
Radionuclide evaluation of the urinary system (Figure 5) includes the imaging of the structural abnormalities, making estimates regarding renal perfusion and function (Linn-Watson, T. 1996). Two types of nuclear medicine studies are done for the renal area:
- An architectural renal scan – uses a nuclide that localises in the tubular cells and it is done to differentiate renal masses from pseudotumours
- A dynamic renogram – shows perfusion as well as excretion. Usually, it is performed to see if a transplanted kidney is functioning poorly, because of rejection or tubular necrosis. The renogram can also evaluate renovascular hypertension.
(Linn-Watson, T. 1996)
Figure 5
Radionuclide evaluation of the urinary system
()
Computed Tomography (CT) is more useful than ultrasound in obese patients since fat attenuates sound transmission, but enhances the boundaries of kidney under examination by CT. CT is also useful when staging renal and bladder neoplasms, evaluating constricting ureteral lesions and in cases of diagnostic doubt. Intravenous contrast medium can be used to assess enhancement and outline the urinary tract structures (Figure 6). The applications of CT scanning in the urinary tract are summarised in Appendix D.
Figure 6
The most valuable examination of the urinary tract is the Intravenous Urogram (IVU), which gives excellent anatomical images of the kidneys and, to some extent, an indication of function (Grainger, R.G. et al. 1986). The examination involves the use of a positive water-soluble iodine based contrast agent that is injected into the blood stream via median cubital vein (see Appendix E for the route of contrast agent to the kidneys). The positive contrast agent will increase the atomic number of a structure or its surroundings, which will in turn increase the amount of x-ray beam that is attenuated. The increase in attenuation means that there will be areas of reduced density on the radiographs.
The contrast agent used for the IVU examination is non-ionic and has low osmolarity. Non-ionic contrast agents do not ionise when injected into blood (they remain as one particle), which means that there are less particles to draw water from the body. Thus non-ionic contrast agents has half the osmolarity of ionic contrast agents, but will still have the same concentration of iodine (Whitley, A.S., et al 1998).
There are some contraindications for performing an IVU examination. Patients who have renal failure, myelomas or any known contrast sensitivity with the chance of severe reaction(s) should avoid having examination (Whitley, et al. 1998). Precautions should be taken when dealing with patients who suffer from asthma, have history of allergies, epileptic or are diabetic taking Metformin (See Appendix F).
Reactions to contrast agents can be classified into three areas of severity:
- Minor reactions – These reactions do not interfere with examination and do not need treatment, for example flushing, nausea, vomiting, mild rash, arm pain etc
- Moderate reactions – these reactions interfere with the examination, but do not require treatment, for example facial oedema, hypotension, bronchospasm etc.
- Severe reactions – these reactions interfere with the examination and require treatment, for example hypotensive shock, laryngeal oedema, convulsions, respiratory and cardiac arrest etc.
Appendix A
Anterior view of the kidneys showing the areas of contact with associated structures
(Ross, J. et al. 1990)
Posterior view of the kidneys showing the areas of contact with associated structures
(Ross, J. et al. 1990)
The following important structures are related to the kidneys:
Appendix B
Calcium Oxalate and phosphate stones
Calcium oxalate and phosphate stones (Figure 1) are the most common and account for 50 to 75% of all stones. They are sharp and densely opaque. They may be pure or of a mixed variety. Pure phosphate stones are usually large and may form a cast of the entire renal pelvis and calyces. These large phosphate stones are known as staghorn calculi (Laudicina, P.F. 1989).
Phosphate stone
Figure 1
Phosphate stone forming complete cast of the renal pelvis and calices (staghorn calculus)
(Sutton, D., et al. 1998, p. 1139)
Struvite stones (or Magnesium ammonium phosphate stones) (Figure 2)
These types of stone represents 20% of urinary calculi and are associated with urinary tract infection. The infection is usually a urea-splitting organism, such as proteus, which alkalinises the urine (Grainger, R.G., et al. 1986).
Figure 2
Struvite stones
Uric acid stones (Figure 3)
These form 5% of all calculi. Gout is responsible for a quarter of uric acid stones formed. Ulcerative colitis and other causes of chronic diarrhoea result in a highly concentrated acid urine and the formation of uric acid calculi (Grainger, R.G., et al. 1986).
Figure 3
Intravenous urography with multiple bilateral uric acid calculi
(Grainger, R.G., et al. 1986, p. 1062)
Cystine stones
These stones account for about 3%. Cystinuria (Figure 4), a genetic disorder, is characterised by excessive excretion of the dibasic amino acids cystine, lysine, arginine and ornithine, due to a transport defect in the renal tubules. This defect will result in recurrent cystine stones (Grainger, R.G., et al. 1986).
Figure 4
Cystinuria. Plain x-ray film showing cystine staghorn stones
()
Calcified papillae
Papillary necrosis (Figure 5) can lead to sloughed papillae remaining in the pelvicalyceal system and theses may then calcify (Grainger, R.G., et al. 1986).
Figure 5
Renal Intravenous urography (Tomogram) Papillary necrosis
()
Appendix C
Some applications of ultrasound in the urinary tract
(Grainger, R.G., et al. 1986)
Appendix D
Some applications of Computed Tomography in the urinary tract
(Grainger, R.G., et al. 1986)
Appendix E
Route of contrast agent from the median cubital vein to the kidneys
Median cubital vein
↓
Basilic and cephalic veins (superficial veins of the arm – medial and lateral respectively)
↓
Axillary vein
↓
Subclavian veins
↓
Branchiocephalic vein
↓
Superior vena cava
↓
Right atrium
↓
Right ventricle
↓
Pulmonary artery
↓
Pulmonary veins
↓
Left atrium
↓
Left ventricle
↓
Aorta – ascending, arch and descending thoracic, then abdominal
↓
Renal artery
(Tortora, J.G., et al. 2003)
Appendix F
If the patient is on Metformin the following are the Royal College of Radiologists recommendations for patients with Diabetes Mellitus, who are receiving Metformin and who are referred for a radiological investigation using intravascular contrast media:
- The referring clinician should take responsibility for assessing the patients’ renal function either by checking the serum creatinine or accepting a normal level within the past year.
- In patients with normal renal function, although there are as yet no reports of Metformin-induced lactic acidosis in the United Kingdom after intravenous contrast agents, there is a remote theoretical risk of interaction. Metformin should therefore be discontinued at the time of the investigation and withheld for the subsequent 48 hours
- For those patients with abnormal renal function, Metformin should similarly be discontinued at the time of the investigation and the subsequent 48 hours, and only reinstated when renal function has been re-evaluated and found to be normal
- As the British Association states that Metformin is contra-indicated in the presence of abnormal renal function, it is suggested that such patients, who require intravascular contrast examinations should have their drug history reviewed by the appropriate physician to ensure suitability of the drug regime.
()
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