Art & science life sciences: 18
The urinary system McLafferty E et al (2014) The urinary system. Nursing Standard. 28, 27, 42-49. Date of submission: October 16 2012; date of acceptance: September 2 2013.
Role of the kidneys
Abstract The urinary system plays an important role in regulating fluid and electrolyte balance and maintaining homeostatic balance within the body. Assessment and management of fluid and electrolyte balance is a vital part of the nurses’ role, therefore it is important that nurses understand the functions of the urinary system. This article explores the anatomy and physiology of the urinary system, with reference to the production and excretion of urine. It also provides an overview of urinary tract infection.
Authors Ella McLafferty Retired. Was senior lecturer, School of Nursing and Midwifery, University of Dundee. Carolyn Johnstone Lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Charles Hendry Retired. Was senior lecturer, School of Nursing and Midwifery, University of Dundee. Alistair Farley Retired. Was lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
Keywords Kidneys, nephrons, micturition, ureters, urethra, urinary system and disorders, urinary tract infection, urine production
Review All articles are subject to external double-blind peer review and checked for plagiarism using automated software.
The kidneys are the major functional units of the urinary system. They are responsible for the production of urine as well as several other functions, including balance of electrolytes such as sodium, potassium, chloride and phosphate levels. The kidneys also have a role in maintaining blood pH because of their ability to excrete hydrogen and conserve bicarbonate. They also regulate blood volume by conserving or eliminating water in the urine, which has a direct effect on blood pressure. In addition, they produce the enzyme renin, which allows control of blood pressure (Tortora and Derrickson 2011). The kidneys produce erythropoietin, which helps to stimulate the development of mature red blood cells in the bone marrow (Porth 2011). Calcitriol (activated vitamin D) is produced in the liver and kidneys and is vital for calcium absorption and thus calcium levels in the bones (Porth 2011). The kidneys are also involved in the regulation of blood glucose levels through the process of gluconeogenesis. Gluconeogenesis involves the synthesis of glucose from the amino acid glutamine, which supports normal blood glucose levels (Tortora and Derrickson 2011). In addition, the kidneys eliminate waste products such as urea (Tortora and Derrickson 2011) and some drugs such as penicillin and morphine (Porth 2011). The article focuses on the role of the kidneys in the production of urine, maintenance of fluid and electrolyte balance, regulation of blood pressure and elimination of waste products.
Anatomy of the kidneys
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THE URINARY SYSTEM is the main excretory system, consisting of two kidneys, two ureters, the urinary bladder and the urethra. The kidneys produce urine that contains metabolic waste products. Urine is transported via the ureters to the bladder for storage and is excreted by the micturition process via the urethra.
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The kidneys have been described as two bean-shaped organs lying retroperitoneally on the posterior abdominal wall, with one on each side of the vertebral column (Deshmukh and Wong 2009). They extend from the 12th thoracic vertebra to the third lumbar vertebra (Brooker and Nicol 2011), with the 11th and 12th pairs of ribs giving them a degree of protection. Damage to these ribs can lead to injury of the kidneys (Tortora and Derrickson 2011) as can a blow to the right side of the back below the rib cage. If caring for a patient with injury to the chest involving the 11th and 12th pairs of ribs, march 5 :: vol 28 no 27 :: 2014 43
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Art & science life sciences: 18
FIGURE 1
damage to the kidneys should be considered. The kidneys are outside the peritoneal cavity towards the back of the upper part of the abdomen, and because of the position of and space occupied by the liver, the right kidney is normally lower than the left (Porth 2011). This position outside the peritoneal cavity is of clinical significance because renal inflammation or infection does not, therefore, carry the same risk of peritoneal involvement as that associated with organs inside the peritoneum (Porth 2011). The kidneys are anchored in place by their outer lining, which is composed of three layers. The renal fascia is a superficial thin layer of dense connective tissue that protects the kidneys from trauma and helps to maintain their shape (Tortora and Derrickson 2011). The middle layer consists of adipose tissue, which protects the kidneys from damage and helps to keep them in place. The innermost or deepest layer is the renal capsule, which comprises dense irregular connective tissue that provides shape to the kidneys and further protection. This layer is continuous with the outer coat of the ureter (Tortora and Derrickson 2011). Because of their position close to the diaphragm, the kidneys are pushed downwards during inspiration (O’Callaghan 2009). The hilum is the concave medial border of the kidney through which the ureter emerges along with blood vessels, lymphatic vessels and nerves (Brooker and Nicol 2011). The area known as the renal pelvis is
A longitudinal section of the right kidney Cortex Renal pyramid in renal medulla
Minor calyces
Renal artery
Renal vein
Renal papilla Ureter
Capsule
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PETER LAMB
Major calyx
continuous with the ureters and consists of calyces that are cup-like structures that drain urine from the upper and lower parts of the kidneys. The kidneys are relatively small organs, around 10-12cm long, 5-7cm wide and 3cm thick, and weigh 135-150g (Tortora and Derrickson 2011). Each kidney receives about 625mL of blood per minute and together they receive in excess of 20% of the cardiac output (Munger et al 2012). The kidneys receive blood from the renal arteries, which branch from the abdominal aorta. Immediately before the renal artery enters the kidney at the hilum, it divides into five branches known as segmental arteries (Porth 2011). These segmented arteries further divide into lobular and eventually interlobular arteries as the blood supply penetrates into the kidney. The interlobular arteries enter the renal cortex, subdividing to form the afferent arterioles, with each afferent arteriole supplying one nephron (Tortora and Derrickson 2011). The afferent arteriole enters the glomerular capsule where it divides to become a small tangled network of capillaries known as the glomerulus. The capillaries of the glomerulus reunite to form the efferent arteriole (Brooker and Nicol 2011). Branches of the efferent arteriole known as the peritubular capillaries supply the renal tubules while a specialised network called the vasa recta supply the loop of Henle. The peritubular capillaries and the vasa recta are designed to allow reabsorption (Porth 2011). The peritubular capillaries join up to form the peritubular venules, interlobular veins and eventually the renal vein, which leaves the kidney at the hilum, carrying venous blood to the inferior vena cava (Tortora and Derrickson 2011). The majority of renal nerves originate from the sympathetic division of the autonomic nervous system, with most being vasomotor in nature and resulting in dilation or constriction of arterioles (Tortora and Derrickson 2011), thus controlling renal blood flow (Field et al 2010). Sympathetic stimulation of the kidneys results in a reduction in renal blood flow because of vasoconstriction of the renal arterioles (Field et al 2010) and subsequent reduction in urine production. Each kidney has two distinct regions: the renal cortex and the renal medulla (Figure 1). The renal medulla is made up of a number of cone-shaped areas known as the renal pyramids, and the apex of each pyramid (the renal papilla) points towards the hilum (Tortora and Derrickson 2011). The renal cortex is the site of the glomeruli, convoluted tubules and blood vessels (Tortora and Derrickson 2011). Together, these form the functional unit of the kidney, the nephron, with each kidney
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Production of urine Filtration
Once fluid, and a range of dissolved substances, has moved through the filtration barrier, it enters the glomerular capsule and is known as filtrate. Filtrate is similar to blood plasma, but it contains almost no proteins because proteins do not cross the filtration barrier in normal circumstances. The filtration barrier only allows certain substances to move through, and filtration is dependent on three main principles. Inside the glomerulus, a hydrostatic pressure of 55mmHg in the plasma promotes the movement of water and dissolved substances through the filtration barrier. This pressure is opposed by the filtrate hydrostatic pressure inside the glomerular capsule, which is 15mmHg. The pressure is also opposed by the osmotic pressure of the blood. Proteins exert an osmotic pressure of around 30mmHg, drawing fluid into the circulation. Overall, there is a greater pressure to push fluid out of the circulation and into the filtrate (Tortora and Derrickson 2011). The rate of filtration is known as the glomerular filtration rate, which is a measure of all the filtrate formed in one minute in both
FIGURE 2 A nephron and associated blood vessels Glomerular capsule
Efferent arteriole
Glomerulus
Proximal convoluted tubule
Afferent arteriole
Peritubular capillary network
Interlobular artery
Interlobular vein
Distal convoluted tubule
PETER LAMB
consisting of in excess of one million nephrons (Porth 2011). The nephron consists of the glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting ducts, and associated blood supply and capillary networks (Figure 2). Although all nephrons have their glomeruli in the renal cortex, about 15% of nephrons arise in the deepest part of the cortex closer to the medulla, while the remaining 85% of nephrons arise in the outer area of the cortex (Field et al 2010). The glomerulus sits inside the glomerular capsule, which together are known as the renal corpuscle (Tortora and Derrickson 2013). The afferent arteriole enters the glomerulus and divides to form the capillary network and the efferent arteriole leaves the glomerulus; this capillary network is unique because it sits between two arterioles (Tortora and Derrickson 2011). The diameter of the afferent arteriole is larger than that of the efferent arteriole, creating a difference in pressure that forces fluid out of the capillary network and into the glomerular capsule (Deshmukh and Wong 2009). The movement of fluid from the capillary network to the glomerular capsule occurs through the three-layer glomerular filtration barrier. The first layer of this barrier is the thin endothelial or lining layer of the capillary network, which has fenestrations or perforations that allow larger molecules to pass through it (O’Callaghan 2009, Porth 2011). Next is the glomerular basement membrane, a collagenbased layer with pores that help to determine the permeability of the filtration barrier in terms of the size of the molecules that can move through it (Porth 2011). The final layer is the epithelial layer of the glomerular capsule, which is a specialised layer that has pores that allow the selective filtering of substances, for example larger molecules such as the protein albumin cannot usually cross this barrier (O’Callaghan 2009). Specialised epithelial cells known as podocytes have projections that wrap around the endothelial lining of the capillaries, providing an extensive filtration membrane (Tortora and Derrickson 2011). The capillary basement membrane is susceptible to damage in people with diabetes mellitus. This damage is a result of insulin deficiency and glucose intolerance, which when combined with changes to the micro-circulation leads to thickening of the basement membrane and leakage of protein into the filtrate (Deshmukh and Wong 2009). This is known as diabetic nephropathy and is a well-recognised potential complication of diabetes mellitus.
Vasa recta Loop of Henle
Collecting duct
march 5 :: vol 28 no 27 :: 2014 45
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Art & science life sciences: 18 kidneys. Although the glomerular filtration rate can vary from 200mL per minute to as little as a few mL per minute, the average is 125mL per minute (Porth 2011). Under normal circumstances, the glomerular filtration rate is maintained at a relatively constant level via a process known as renal autoregulation. Autoregulation is a means of maintaining blood flow via negative feedback, which can cause constriction or relaxation of the afferent arteriole, thus altering the pressure in the glomerulus and in turn influencing the filtration rate (Tortora and Derrickson 2011). The kidneys maintain efficient blood flow through autoregulation (Munger et al 2012), meaning that they are protected against fluctuations in blood pressure. The glomerular filtration rate will remain relatively constant when mean arterial blood pressure is between 80mmHg and 180mmHg, but will be affected by levels outside this; for example when arterial blood pressure falls to below 80mmHg, pressure in the renal arterioles will fall, lowering the glomerular filtration rate and slowing urine production. A healthy balance of body fluids depends upon the constant glomerular filtration rate because a rate that is too fast will not allow adequate time for reabsorption, while a rate that is too slow may lead to too much reabsorption and inadequate excretion of waste products (Tortora and Derrickson 2011). Hormonal control plays an important part in the maintenance of the glomerular filtration rate. Atrial natriuretic peptide (ANP) is produced by the atria of the heart in response to stretching of the arterial wall, which occurs when there is an increase in circulating blood volume. One of the effects of an increase in the level of ANP is an increase in the glomerular filtration rate and an increase in the amount of filtrate produced (Tortora and Derrickson 2011). The filtrate consists of fluid and dissolved substances that have passed through the filtration membrane, with only substances of 3-7nm in size crossing the filtration barrier (Brooker and Nicol 2011). In normal circumstances, large molecules such as proteins and blood cells do not cross the filtration barrier (Brooker and Nicol 2011) and will, therefore, not be found in urine when tested. The presence of these substances in urine is an indication of dysfunction of some kind. Filtrate normally contains water, sodium, chloride, bicarbonate, potassium, calcium, magnesium, glucose, amino acids, water soluble vitamins, uric acid and creatinine (Porth 2011, Tortora and Derrickson 2011). On average, 105-125mL of filtrate is produced per hour, with 99% of the filtrate being reabsorbed (McDonough and 46 march 5 :: vol 28 no 27 :: 2014
Thomson 2012). The process of reabsorption ensures that substances required by the body are reclaimed, while waste products are eliminated.
Reabsorption
With an average of 7,500mL of filtrate produced per hour, reabsorption is vital to ensure the maintenance of fluid balance. Reabsorption is a delicate balance of returning essential nutrients to the circulation, return of controlled amounts of electrolytes to the circulation and reabsorption of the majority of the fluid. Reabsorption involves the normal processes of solute transport across cell membranes, for example water and urea are passively reabsorbed while other substances such as glucose, amino acids, sodium and potassium are reabsorbed via active transport mechanisms (Porth 2011). According to Porth (2011), only 1mL of the filtrate forms urine, meaning that 124mL per minute are reabsorbed. Overall, this leads to a normal production level for urine of around 60mL per hour. In practice, it is common to use the average of 30mL per hour when measuring hourly urine volumes in patients with acute conditions. Nurses should be aware that volumes of 30mL per hour are half the average normal urine production and that levels below this may indicate diminishing renal function. Reabsorption begins in the proximal convoluted tubule, where the majority of solute reabsorption takes place (O’Callaghan 2009). Around 65% of all reabsorption processes take place in the proximal tubule and, crucially, nearly all the nutrients, for example glucose, amino acids and water-soluble vitamins, are reabsorbed from this tubule (Porth 2011). In normal circumstances, glucose is reabsorbed completely and, therefore, should not be detected in urine. Only when the level of glucose in the blood rises above normal levels does glucose appear in urine, and this is because glucose transport to the peritubular capillaries is not able to match the volume of glucose being filtered, thus some glucose remains in the filtrate and is eliminated in urine (Tortora and Derrickson 2013). The proximal convoluted tubule is highly permeable to water so there is rapid osmotic movement of water and reabsorption of 65-80% of electrolytes such as sodium, potassium, chloride and bicarbonate (Porth 2011). A more balanced approach to reabsorption is found in the loop of Henle. The loop of Henle helps control the concentration of urine (Porth 2011), with reabsorption varying along its length because of differences in the properties of its lining. In the thin descending loop, the walls are relatively impermeable to solutes but permeable to water, and the water moves by osmosis. The
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fluid in the tubule now contains a higher level of solutes and is hypertonic. The thick ascending loop is impermeable to water and is permeable to sodium and chloride, and these diffuse down the concentration gradient. The filtrate becomes hypotonic. Because water and solute movement are not always linked in the loop of Henle, the volume of body fluids can be controlled separately from the osmolality. The overall function of the loop of Henle is the reabsorption of sodium and chloride and other electrolytes, and the concentration of urine (Tortora and Derrickson 2011). The distal convoluted tubule is generally impermeable to water, therefore only solutes, especially sodium and chloride, are reabsorbed. Calcium can also be reabsorbed although this depends on stimulation by parathyroid hormone (Field et al 2010). The overall level of reabsorption in the distal convoluted tubule is influenced directly by antidiuretic hormone (ADH). If ADH is present, the tubule becomes permeable to water, causing it to be reabsorbed making the urine more concentrated. When ADH is not present, the walls remain impermeable to water and more dilute urine is formed. The role of ADH is to conserve body fluids by decreasing urine output. The production of ADH is stimulated by dehydration, loss of blood, pain or stress, and is inhibited by alcohol, which is one of the reasons for dehydration following excessive alcohol consumption (Tortora and Derrickson 2011). Sodium plays an important role in the maintenance of fluid balance. It is the most abundant electrolyte in extracellular fluid with levels controlled by the action of aldosterone, ADH and ANP. Levels of sodium are monitored constantly in the hypothalamus, with homeostatic levels of sodium maintained by an interplay of hormonal control influencing levels of reabsorption, thus the level of sodium in the blood has a direct influence on urine output (Tortora and Derrickson 2011). Several distal tubules join together to form collecting ducts, which are generally impermeable to water. These ducts are also affected by ADH in much the same way as the distal tubules, therefore sodium is reabsorbed as is some chloride, but by the time filtrate reaches the collecting ducts up to 95% of the solutes and water have been returned to the blood (Tortora and Derrickson 2011). Potassium levels in the blood are regulated by cells in the wall of the tubules because they secrete potassium back into the tubule. In addition, hydrogen and bicarbonate levels are regulated, and secretion of hydrogen is vital to the overall maintenance of pH levels in the blood. The role of the kidney in pH balance is to reabsorb the filtered bicarbonate and secrete hydrogen buffered by phosphate and ammonia. Ammonia is
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manufactured by cells of the renal tubule from the amino acid glutamine (Porth 2011).
Secretion
Secretion is the movement of substances from the blood into the tubule of the nephron and is an important renal process that regulates the level of several substances. It occurs mainly in the distal convoluted tubule and the collecting ducts (Chalmers 2008). Although creatinine is freely filtered, it is also secreted increasing the total amount in filtrate by about 20%. Other substances such as hydrogen, potassium and ammonia, and some drugs are also secreted, for example morphine, aspirin and penicillin (Porth 2011). The collecting ducts unite to form larger ducts, ultimately leading the filtrate to the pelvis of the kidney (Porth 2011) where the filtrate exits the kidneys via the ureters, and is known as urine. Normal urine consists of 95% water (with an average adult producing around 1-2L of urine per day), urea, creatinine, potassium, ammonia, uric acid, sodium, chloride, magnesium, sulphate, phosphate and calcium (Tortora and Derrickson 2013). It is important that nurses are aware of the normal levels of these elements, as well substances that do not normally appear in urine because these are important indicators of renal function and wider abnormalities in the body. For example, albumin is a plasma protein that should not appear in urine because it is too large to cross the filtration barrier and, therefore, its presence can indicate renal damage. Glucose is normally completely reabsorbed and its presence indicates that the renal threshold for reabsorption has been exceeded as occurs in diabetes mellitus. White blood cells are part of the immune response and their presence in urine indicates infection. The presence of ketones in urine has several potential causes such as starvation, a low carbohydrate diet or diabetes mellitus (Tortora and Derrickson 2013). Normal urine should be clear, although it may vary in colour from pale yellow to dark amber according to the concentration. Colour can also be influenced by food or drugs. There should be no unpleasant odour initially, but urine left standing can develop a smell of ammonia. The normal pH of urine is 4-8 with an average of 6, but this can vary considerably with diet, for example a high protein diet will increase the acidity of urine (Chalmers 2008, Tortora and Derrickson 2013).
Role and anatomy of the ureters The end result of the renal process is urine, which exits the pelvis of the kidneys via the ureters. The ureters are long tubes about 25-30cm in length march 5 :: vol 28 no 27 :: 2014 47
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Art & science life sciences: 18 that pass behind the peritoneum and connect the kidneys to the bladder (Brooker and Nicol 2011). The ureters pass under the bladder and enter obliquely, eventually opening into the bladder at the posterior inner surface (Brooker and Nicol 2011, Tortora and Derrickson 2013). This means of entering the bladder is important because it prevents back flow of urine to the ureters by creating a valve-like effect in which the ureters are compressed and the openings occluded when urine accumulates and pressure in the bladder increases. Preventing back flow of urine is important to prevent the upwards movement of microorganisms from the bladder to the kidneys, reducing the risk of transfer of infection.
Role and anatomy of the bladder and urethra The position of the bladder differs slightly in men and women. In females, the bladder is situated in front of and below the uterus and rests against the vagina, while in men it is situated directly in front of the rectum (Tortora and Derrickson 2013). The bladder is a hollow muscular sac made up of four layers: an outer fibrous coating, a longitudinal and circular layer of smooth muscle known as the detrusor muscle, a layer of connective tissue and an inner layer of transitional epithelium (Brooker and Nicol 2011). Emptying of the bladder involves both voluntary and involuntary nervous control. Activation via the parasympathetic nervous system brings about contraction of the detrusor muscle and relaxation of the internal sphincter in the bladder neck during micturition. Voluntary control is via the sympathetic nervous system, which aids bladder filling by promoting relaxation of the smooth muscle and contraction of the internal sphincter (Porth 2011). The urethra varies in length between men and women. In men, it measures about 20-25cm and passes through the prostate gland. In men, the urethra also has a sexual function in carrying semen. Semen enters the urethra via a duct known as the vas deferens. The vas deferens links with the epididymis that exits the testes (Brooker and Nicol 2011). The external sphincter is a ring of voluntary muscle and in men, it is situated where the urethra exits the prostate gland. In women, the urethra is shorter, about 3-5cm in length with its orifice between the clitoris and the vagina. The external sphincter is near the urethral orifice and is under voluntary control. The urethra is made largely of an outer layer of smooth muscle continuous with the bladder, a middle layer that has nerve, lymph and blood supply, and an inner mucous lining, which is continuous with the bladder (Brooker and Nicol 2011). 48 march 5 :: vol 28 no 27 :: 2014
Micturition
The bladder acts essentially as a store for urine until it is time to expel it from the body in the process of micturition. In normal circumstances, urine will remain in the bladder until the detrusor muscle contracts, bringing about opening of the bladder neck alongside relaxation of pelvic floor muscles and external urinary sphincter muscles (Brooker and Nicol 2011). Tortora and Derrickson (2011) stated that the pressure inside the bladder increases during filling and that when a volume of 200-400mL is reached, stretch receptors send impulses to the spinal cord triggering the micturition reflex and stimulating contraction of the detrusor muscle and relaxation of the internal sphincter. Urination or voiding is stimulated at a conscious level before initiation of the micturition reflex, and it is this conscious control that children learn via control of the external sphincter and some pelvic floor muscles (Tortora and Derrickson 2011). Conscious control is normally achieved by the age of three, but cannot be achieved until the necessary nerve pathways have matured (Brooker and Nicol 2011).
Urinary tract infection Several conditions may be associated with the urinary system such as chronic kidney disease, acute kidney injury and urinary tract infection (UTI). In England, chronic kidney disease occurs in 14% of males and 13% of females, although many individuals will be at an early stage of the condition and may be unaware that they have it (Roderick et al 2011). This long-term condition is associated with deterioration, reduced quality of life, and may necessitate dialysis. Acute kidney injury occurs in 13-18% of individuals admitted to hospital in developed countries and develops often in individuals with other conditions being cared for in non-renal specialist areas (National Institute for Health and Care Excellence 2013). Because acute kidney injury tends to develop in patients in general wards, it is essential that nurses have an understanding of basic renal function and deviations from normal plasma and urine levels that may indicate renal dysfunction. UTI is discussed here because of its prevalence and tendency to reoccur. Almost half of all women will report at least one UTI at some point in their lives and 20-30% will experience a recurrence (Patient.co.uk 2013). Although rare in men under aged 50, UTI occurs in about 3% of men in their 60s and 10% of men in their 80s (Patient.co.uk 2013). According to the Scottish Intercollegiate Guidelines Network (SIGN) (2012), UTI is the second most common reason for the use of antimicrobial treatment in both primary and
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secondary care settings. A UTI is defined as ‘the presence of bacteria or other microorganisms in the urine or genito-urinary tissues’, although the term infection is usually used when the concentration of organisms meets set quantitative criteria (Nicolle 2012). The presence of bacteria in urine does not indicate an infection that requires intervention necessarily, with an average of 16% of women and 6% of men between the ages of 65 and 74 in Scotland having asymptomatic bacteriuria (SIGN 2012). There are several risks that increase the likelihood of developing bacteriuria, including female gender, age, comorbid diabetes mellitus, institutionalisation, sexual activity and presence of an indwelling urinary catheter (SIGN 2012). Age is an important factor because post-menopausal women experience more non-specific symptoms such as low abdominal and back pain, and a delay in diagnosis (Arinzon et al 2012). Pre-menopausal women are more likely to report local signs such as pain on passing urine (Brooker and Nicol 2011, Arinzon et al 2012). Typical signs of UTI are dysuria, increased frequency of urination, suprapubic tenderness, urgency, polyuria and haematuria (SIGN 2012). A lower UTI affects the urethra and bladder, for example cystitis, and an upper UTI affects the ureters and kidneys, for example pyelonephritis (Porth 2011). Uncomplicated lower UTIs are more common than upper UTIs and generally respond well to treatment involving a short course of antibiotics. Pyelonephritis is more likely to occur in children and those with obstructions or other conditions such as those with neurogenic bladder dysfunction secondary to diabetes (Porth 2011, SIGN 2012). Pyelonephritis is potentially more serious than lower UTI, creating more widespread symptoms such as pain, nausea and pyrexia, and warranting antibiotic treatment (Brooker and Nicol 2011). However, Chalmers (2008) stated that although antibiotics are useful in treating the initial acute infection, there is little evidence to support prophylactic use to slow the development of renal disease.
Diagnosis
UTIs in men should be confirmed with a urine sample for culture. Because there is a strong correlation between UTI in men and an enlarged prostate, this must be considered during investigation and diagnosis (SIGN 2012). Most bacteria enter the urinary tract via the urethra and travel to the bladder, with Escherichia coli accounting for the majority of infections in the community and half of all hospital-acquired infections (Brooker and Nicol 2011). A UTI diagnosis is based on the presenting symptoms
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and urine sample testing. In otherwise healthy women, it is recommended that diagnosis is made and treatment with antibiotics is considered when three or more of the previously mentioned typical symptoms are present (SIGN 2012). The likelihood of the presence of a UTI is further confirmed when urine is cloudy with suspended particles – the presence of white blood cells (pus) creates this cloudiness and is known as pyuria. A dipstick test may be useful when fewer symptoms are present.
Treatment
Treatment of UTI varies with gender, and type and severity of infection. Antibiotic therapy with a three-day course of trimethoprim or nitrofurantoin should be considered in non-pregnant women under 65 when there are three or more typical symptoms (SIGN 2012). Pregnant women should have a urine sample taken for culture before antibiotic treatment, be treated with an antibiotic chosen from the local formulary and usually for seven days, and have a further urine sample taken for culture one week after completion of antibiotic therapy to ensure the infection has resolved (SIGN 2012). Recurrent UTIs in older women often lead to the repeated use of antibiotic therapy. McMurdo et al (2009) found that in older women with recurrent UTI, there is little difference between the effectiveness of cranberry extract and trimethoprim in terms of prevention of infection, suggesting that these women may wish to consider this option, in discussion with their clinician, as a means of reducing the risk of antimicrobial resistance. Infection in men should be treated with quinolone when there is a suggestion of prostatitis, and referral to a urology specialist should be considered when there is failure to respond to antibiotic therapy, recurrent UTI or upper UTI (SIGN 2012). The link between UTI and catheterisation is well-recognised, with increasing duration of catheterisation increasing the risk of infection (SIGN 2012). While catheterisation is an essential part of nursing management in many situations, it is vital that the catheter is removed as soon as it is no longer clinically indicated to reduce the risk of UTI and that catheterisation is not viewed as a solution to urinary incontinence (Hooton et al 2010). Catheter-associated urinary tract infections may present with worsening fever, rigors, altered mental state, flank pain, acute haematuria or pelvic discomfort. Dipstick testing will be unreliable as pyuria is common in patients who have been catheterised and is not predictive of infection (SIGN 2012). Hooton et al (2010) stated that prevention measures such as a closed drainage system are march 5 :: vol 28 no 27 :: 2014 49
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Art & science life sciences: 18 important in reducing the risk of infection. SIGN (2012) states that hospital admission is required when systemic symptoms such as fever, rigors, chills or vomiting are present. In terms of antibiotic therapy, treatment should be guided by symptoms and follow local antibiotic policy. Long-term catheters should be changed before commencement of antibiotic therapy, and therapy should then be guided by local policy. It should be noted that because of the prevalence of bacteriuria in people who are catheterised, screening would be unhelpful and there is no need to treat those with asymptomatic bacteriuria (SIGN 2012).
The kidneys are the primary active units in the urinary system filtering more than 600mL of blood per minute; however, this can create vulnerability because of the close contact the nephrons have with blood and any potential toxins (Porth 2011). Assessment and monitoring of renal function is a major part of the nurses’ role and is inherently linked to fluid balance. Nurses need to have an understanding of renal physiology and fluid management. In addition, they need to be aware of how to identify and treat UTI because of the prevalence of this condition and the potential for associated complications NS
Conclusion Although the primary function of the urinary system is the production of urine, it has a range of other roles that link it closely to other systems in the body. For example, the kidneys have a vital role in the maintenance of blood pressure while blood pH is maintained by the interplay of actions of the urinary and respiratory systems. Producing urine is a major means of eliminating waste products and toxic substances from the body, thus maintaining fluid balance in the body.
POINTS FOR PRACTICE List some of the signs and symptoms of a lower urinary tract infection (UTI) and explain why these might occur. Refer to local protocols and outline a nursing care plan for a patient presenting with a lower UTI. Review local guidelines in relation to catheterisation and catheter management with a view to reducing or preventing the incidence of UTI.
References Arinzon Z, Shabat S, Peisakh A, Berner Y (2012) Clinical presentation or urinary tract infection (UTI) differs with aging in women. Archives of Gerontology and Geriatrics. 55, 1, 145-147. Brooker C, Nicol M (2011) Alexander’s Nursing Practice. Fourth edition. Churchill Livingstone Elsevier, Edinburgh. Chalmers CA (2008) Applied anatomy and physiology and the renal disease process. In Thomas N (Ed) Renal Nursing. Third edition. Bailliere Tindall Elsevier, Edinburgh, 27-72. Deshmukh SR, Wong NWK (2009) The Renal System Explained. An Illustrated Core Text. Nottingham University Press, Nottingham. Field M, Pollock C, Harris D (2010) The Renal System: Basic Science And Clinical Conditions. Second edition. Churchill Livingstone Elsevier, Edinburgh. Hooton TM, Bradley SF, Cardenas DD et al (2010)
Diagnosis, prevention and treatment of catheter-associated urinary tract infections in adults: 2009 International Clinical Practice Guidelines from the Infectious Diseases Society of America. Clinical Infectious Diseases. 50, 5, 625-663. McDonough AA, Thomson SC (2012) Metabolic basis of solute transport. In Taal MW, Chertow GM, Marsden PA, Skorecki K, Yu ASL, Brenner BM (Eds) Brenner & Rector’s The Kidney. Ninth edition. Elsevier Saunders, Philadelphia PA, 138-157. McMurdo MET, Argo I, Phillips G, Daly F, Davey P (2009) Cranberry or trimethoprim for the prevention of recurrent urinary tract infections? A randomized controlled trial in older women. Journal of Antimicrobial Chemotherapy. 63, 2, 389-395. Munger KA, Kost Jr. CK, Brenner BM, Maddox DA (2012) The renal circulations and glomerular ultrafiltration. In Taal MW, Chertow GM,
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Marsden PA, Skorecki K, Yu ASL, Brenner BM (Eds) Brenner & Rector’s The Kidney. Ninth edition. Elsevier Saunders, Philadelphia PA, 94-137. National Institute for Health and Care Excellence (2013) Acute Kidney Injury: Prevention, Detection and Management Up to the Point of Renal Replacement Therapy. Clinical guideline No. 169. NICE, London. Nicolle LE (2012) Urinary tract infection in adults. In Taal MW, Chertow GM, Marsden PA, Skorecki K, Yu ASL, Brenner BM (Eds) Brenner & Rector’s The Kidney. Ninth edition. Elsevier Saunders, Philadelphia PA, 1356-1382.
Porth CM (2011) Essentials of Pathophysiology. Third edition, Wolter Kluwer Health/Lippincott Williams & Wilkins, Philadelphia PA. Roderick P, Roth M, Mindell J (2011) Prevalence of chronic kidney disease in England: Findings from the 2009 Health Survey for England. Journal of Epidemiology & Community Health. 65, Suppl 2, A12. Scottish Intercollegiate Guidelines Network (2012) Management of Suspected Bacterial Urinary Tract Infection in Adults. A national clinical guideline. SIGN, Edinburgh.
O’Callaghan C (2009) The Renal System at A Glance. Third edition. Blackwell Publishing, Oxford.
Tortora GJ, Derrickson BH (2011) Principles of Anatomy and Physiology. Volume 2 – Maintenance and Continuity of the Human Body. 13th edition. John Wiley and Sons, Asia.
Patient.co.uk (2013) Urinary Tract Infections in Adults. www. patient.co.uk/doctor/urinary-tractinfection-in-adults (Last accessed: February 14 2014.)
Tortora GJ, Derrickson BH (2013) Essentials of Anatomy and Physiology. Ninth edition International Student Version. John Wiley and Sons, Singapore.
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The urinary system McLafferty E et al (2014) The urinary system. Nursing Standard. 28, 27, 42-49. Date of submission: October 16 2012; date of acceptance: September 2 2013.
Role of the kidneys
Abstract The urinary system plays an important role in regulating fluid and electrolyte balance and maintaining homeostatic balance within the body. Assessment and management of fluid and electrolyte balance is a vital part of the nurses’ role, therefore it is important that nurses understand the functions of the urinary system. This article explores the anatomy and physiology of the urinary system, with reference to the production and excretion of urine. It also provides an overview of urinary tract infection.
Authors Ella McLafferty Retired. Was senior lecturer, School of Nursing and Midwifery, University of Dundee. Carolyn Johnstone Lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Charles Hendry Retired. Was senior lecturer, School of Nursing and Midwifery, University of Dundee. Alistair Farley Retired. Was lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
Keywords Kidneys, nephrons, micturition, ureters, urethra, urinary system and disorders, urinary tract infection, urine production
Review All articles are subject to external double-blind peer review and checked for plagiarism using automated software.
The kidneys are the major functional units of the urinary system. They are responsible for the production of urine as well as several other functions, including balance of electrolytes such as sodium, potassium, chloride and phosphate levels. The kidneys also have a role in maintaining blood pH because of their ability to excrete hydrogen and conserve bicarbonate. They also regulate blood volume by conserving or eliminating water in the urine, which has a direct effect on blood pressure. In addition, they produce the enzyme renin, which allows control of blood pressure (Tortora and Derrickson 2011). The kidneys produce erythropoietin, which helps to stimulate the development of mature red blood cells in the bone marrow (Porth 2011). Calcitriol (activated vitamin D) is produced in the liver and kidneys and is vital for calcium absorption and thus calcium levels in the bones (Porth 2011). The kidneys are also involved in the regulation of blood glucose levels through the process of gluconeogenesis. Gluconeogenesis involves the synthesis of glucose from the amino acid glutamine, which supports normal blood glucose levels (Tortora and Derrickson 2011). In addition, the kidneys eliminate waste products such as urea (Tortora and Derrickson 2011) and some drugs such as penicillin and morphine (Porth 2011). The article focuses on the role of the kidneys in the production of urine, maintenance of fluid and electrolyte balance, regulation of blood pressure and elimination of waste products.
Anatomy of the kidneys
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THE URINARY SYSTEM is the main excretory system, consisting of two kidneys, two ureters, the urinary bladder and the urethra. The kidneys produce urine that contains metabolic waste products. Urine is transported via the ureters to the bladder for storage and is excreted by the micturition process via the urethra.
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The kidneys have been described as two bean-shaped organs lying retroperitoneally on the posterior abdominal wall, with one on each side of the vertebral column (Deshmukh and Wong 2009). They extend from the 12th thoracic vertebra to the third lumbar vertebra (Brooker and Nicol 2011), with the 11th and 12th pairs of ribs giving them a degree of protection. Damage to these ribs can lead to injury of the kidneys (Tortora and Derrickson 2011) as can a blow to the right side of the back below the rib cage. If caring for a patient with injury to the chest involving the 11th and 12th pairs of ribs, march 5 :: vol 28 no 27 :: 2014 43
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Art & science life sciences: 18
FIGURE 1
damage to the kidneys should be considered. The kidneys are outside the peritoneal cavity towards the back of the upper part of the abdomen, and because of the position of and space occupied by the liver, the right kidney is normally lower than the left (Porth 2011). This position outside the peritoneal cavity is of clinical significance because renal inflammation or infection does not, therefore, carry the same risk of peritoneal involvement as that associated with organs inside the peritoneum (Porth 2011). The kidneys are anchored in place by their outer lining, which is composed of three layers. The renal fascia is a superficial thin layer of dense connective tissue that protects the kidneys from trauma and helps to maintain their shape (Tortora and Derrickson 2011). The middle layer consists of adipose tissue, which protects the kidneys from damage and helps to keep them in place. The innermost or deepest layer is the renal capsule, which comprises dense irregular connective tissue that provides shape to the kidneys and further protection. This layer is continuous with the outer coat of the ureter (Tortora and Derrickson 2011). Because of their position close to the diaphragm, the kidneys are pushed downwards during inspiration (O’Callaghan 2009). The hilum is the concave medial border of the kidney through which the ureter emerges along with blood vessels, lymphatic vessels and nerves (Brooker and Nicol 2011). The area known as the renal pelvis is
A longitudinal section of the right kidney Cortex Renal pyramid in renal medulla
Minor calyces
Renal artery
Renal vein
Renal papilla Ureter
Capsule
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PETER LAMB
Major calyx
continuous with the ureters and consists of calyces that are cup-like structures that drain urine from the upper and lower parts of the kidneys. The kidneys are relatively small organs, around 10-12cm long, 5-7cm wide and 3cm thick, and weigh 135-150g (Tortora and Derrickson 2011). Each kidney receives about 625mL of blood per minute and together they receive in excess of 20% of the cardiac output (Munger et al 2012). The kidneys receive blood from the renal arteries, which branch from the abdominal aorta. Immediately before the renal artery enters the kidney at the hilum, it divides into five branches known as segmental arteries (Porth 2011). These segmented arteries further divide into lobular and eventually interlobular arteries as the blood supply penetrates into the kidney. The interlobular arteries enter the renal cortex, subdividing to form the afferent arterioles, with each afferent arteriole supplying one nephron (Tortora and Derrickson 2011). The afferent arteriole enters the glomerular capsule where it divides to become a small tangled network of capillaries known as the glomerulus. The capillaries of the glomerulus reunite to form the efferent arteriole (Brooker and Nicol 2011). Branches of the efferent arteriole known as the peritubular capillaries supply the renal tubules while a specialised network called the vasa recta supply the loop of Henle. The peritubular capillaries and the vasa recta are designed to allow reabsorption (Porth 2011). The peritubular capillaries join up to form the peritubular venules, interlobular veins and eventually the renal vein, which leaves the kidney at the hilum, carrying venous blood to the inferior vena cava (Tortora and Derrickson 2011). The majority of renal nerves originate from the sympathetic division of the autonomic nervous system, with most being vasomotor in nature and resulting in dilation or constriction of arterioles (Tortora and Derrickson 2011), thus controlling renal blood flow (Field et al 2010). Sympathetic stimulation of the kidneys results in a reduction in renal blood flow because of vasoconstriction of the renal arterioles (Field et al 2010) and subsequent reduction in urine production. Each kidney has two distinct regions: the renal cortex and the renal medulla (Figure 1). The renal medulla is made up of a number of cone-shaped areas known as the renal pyramids, and the apex of each pyramid (the renal papilla) points towards the hilum (Tortora and Derrickson 2011). The renal cortex is the site of the glomeruli, convoluted tubules and blood vessels (Tortora and Derrickson 2011). Together, these form the functional unit of the kidney, the nephron, with each kidney
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Production of urine Filtration
Once fluid, and a range of dissolved substances, has moved through the filtration barrier, it enters the glomerular capsule and is known as filtrate. Filtrate is similar to blood plasma, but it contains almost no proteins because proteins do not cross the filtration barrier in normal circumstances. The filtration barrier only allows certain substances to move through, and filtration is dependent on three main principles. Inside the glomerulus, a hydrostatic pressure of 55mmHg in the plasma promotes the movement of water and dissolved substances through the filtration barrier. This pressure is opposed by the filtrate hydrostatic pressure inside the glomerular capsule, which is 15mmHg. The pressure is also opposed by the osmotic pressure of the blood. Proteins exert an osmotic pressure of around 30mmHg, drawing fluid into the circulation. Overall, there is a greater pressure to push fluid out of the circulation and into the filtrate (Tortora and Derrickson 2011). The rate of filtration is known as the glomerular filtration rate, which is a measure of all the filtrate formed in one minute in both
FIGURE 2 A nephron and associated blood vessels Glomerular capsule
Efferent arteriole
Glomerulus
Proximal convoluted tubule
Afferent arteriole
Peritubular capillary network
Interlobular artery
Interlobular vein
Distal convoluted tubule
PETER LAMB
consisting of in excess of one million nephrons (Porth 2011). The nephron consists of the glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting ducts, and associated blood supply and capillary networks (Figure 2). Although all nephrons have their glomeruli in the renal cortex, about 15% of nephrons arise in the deepest part of the cortex closer to the medulla, while the remaining 85% of nephrons arise in the outer area of the cortex (Field et al 2010). The glomerulus sits inside the glomerular capsule, which together are known as the renal corpuscle (Tortora and Derrickson 2013). The afferent arteriole enters the glomerulus and divides to form the capillary network and the efferent arteriole leaves the glomerulus; this capillary network is unique because it sits between two arterioles (Tortora and Derrickson 2011). The diameter of the afferent arteriole is larger than that of the efferent arteriole, creating a difference in pressure that forces fluid out of the capillary network and into the glomerular capsule (Deshmukh and Wong 2009). The movement of fluid from the capillary network to the glomerular capsule occurs through the three-layer glomerular filtration barrier. The first layer of this barrier is the thin endothelial or lining layer of the capillary network, which has fenestrations or perforations that allow larger molecules to pass through it (O’Callaghan 2009, Porth 2011). Next is the glomerular basement membrane, a collagenbased layer with pores that help to determine the permeability of the filtration barrier in terms of the size of the molecules that can move through it (Porth 2011). The final layer is the epithelial layer of the glomerular capsule, which is a specialised layer that has pores that allow the selective filtering of substances, for example larger molecules such as the protein albumin cannot usually cross this barrier (O’Callaghan 2009). Specialised epithelial cells known as podocytes have projections that wrap around the endothelial lining of the capillaries, providing an extensive filtration membrane (Tortora and Derrickson 2011). The capillary basement membrane is susceptible to damage in people with diabetes mellitus. This damage is a result of insulin deficiency and glucose intolerance, which when combined with changes to the micro-circulation leads to thickening of the basement membrane and leakage of protein into the filtrate (Deshmukh and Wong 2009). This is known as diabetic nephropathy and is a well-recognised potential complication of diabetes mellitus.
Vasa recta Loop of Henle
Collecting duct
march 5 :: vol 28 no 27 :: 2014 45
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Art & science life sciences: 18 kidneys. Although the glomerular filtration rate can vary from 200mL per minute to as little as a few mL per minute, the average is 125mL per minute (Porth 2011). Under normal circumstances, the glomerular filtration rate is maintained at a relatively constant level via a process known as renal autoregulation. Autoregulation is a means of maintaining blood flow via negative feedback, which can cause constriction or relaxation of the afferent arteriole, thus altering the pressure in the glomerulus and in turn influencing the filtration rate (Tortora and Derrickson 2011). The kidneys maintain efficient blood flow through autoregulation (Munger et al 2012), meaning that they are protected against fluctuations in blood pressure. The glomerular filtration rate will remain relatively constant when mean arterial blood pressure is between 80mmHg and 180mmHg, but will be affected by levels outside this; for example when arterial blood pressure falls to below 80mmHg, pressure in the renal arterioles will fall, lowering the glomerular filtration rate and slowing urine production. A healthy balance of body fluids depends upon the constant glomerular filtration rate because a rate that is too fast will not allow adequate time for reabsorption, while a rate that is too slow may lead to too much reabsorption and inadequate excretion of waste products (Tortora and Derrickson 2011). Hormonal control plays an important part in the maintenance of the glomerular filtration rate. Atrial natriuretic peptide (ANP) is produced by the atria of the heart in response to stretching of the arterial wall, which occurs when there is an increase in circulating blood volume. One of the effects of an increase in the level of ANP is an increase in the glomerular filtration rate and an increase in the amount of filtrate produced (Tortora and Derrickson 2011). The filtrate consists of fluid and dissolved substances that have passed through the filtration membrane, with only substances of 3-7nm in size crossing the filtration barrier (Brooker and Nicol 2011). In normal circumstances, large molecules such as proteins and blood cells do not cross the filtration barrier (Brooker and Nicol 2011) and will, therefore, not be found in urine when tested. The presence of these substances in urine is an indication of dysfunction of some kind. Filtrate normally contains water, sodium, chloride, bicarbonate, potassium, calcium, magnesium, glucose, amino acids, water soluble vitamins, uric acid and creatinine (Porth 2011, Tortora and Derrickson 2011). On average, 105-125mL of filtrate is produced per hour, with 99% of the filtrate being reabsorbed (McDonough and 46 march 5 :: vol 28 no 27 :: 2014
Thomson 2012). The process of reabsorption ensures that substances required by the body are reclaimed, while waste products are eliminated.
Reabsorption
With an average of 7,500mL of filtrate produced per hour, reabsorption is vital to ensure the maintenance of fluid balance. Reabsorption is a delicate balance of returning essential nutrients to the circulation, return of controlled amounts of electrolytes to the circulation and reabsorption of the majority of the fluid. Reabsorption involves the normal processes of solute transport across cell membranes, for example water and urea are passively reabsorbed while other substances such as glucose, amino acids, sodium and potassium are reabsorbed via active transport mechanisms (Porth 2011). According to Porth (2011), only 1mL of the filtrate forms urine, meaning that 124mL per minute are reabsorbed. Overall, this leads to a normal production level for urine of around 60mL per hour. In practice, it is common to use the average of 30mL per hour when measuring hourly urine volumes in patients with acute conditions. Nurses should be aware that volumes of 30mL per hour are half the average normal urine production and that levels below this may indicate diminishing renal function. Reabsorption begins in the proximal convoluted tubule, where the majority of solute reabsorption takes place (O’Callaghan 2009). Around 65% of all reabsorption processes take place in the proximal tubule and, crucially, nearly all the nutrients, for example glucose, amino acids and water-soluble vitamins, are reabsorbed from this tubule (Porth 2011). In normal circumstances, glucose is reabsorbed completely and, therefore, should not be detected in urine. Only when the level of glucose in the blood rises above normal levels does glucose appear in urine, and this is because glucose transport to the peritubular capillaries is not able to match the volume of glucose being filtered, thus some glucose remains in the filtrate and is eliminated in urine (Tortora and Derrickson 2013). The proximal convoluted tubule is highly permeable to water so there is rapid osmotic movement of water and reabsorption of 65-80% of electrolytes such as sodium, potassium, chloride and bicarbonate (Porth 2011). A more balanced approach to reabsorption is found in the loop of Henle. The loop of Henle helps control the concentration of urine (Porth 2011), with reabsorption varying along its length because of differences in the properties of its lining. In the thin descending loop, the walls are relatively impermeable to solutes but permeable to water, and the water moves by osmosis. The
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fluid in the tubule now contains a higher level of solutes and is hypertonic. The thick ascending loop is impermeable to water and is permeable to sodium and chloride, and these diffuse down the concentration gradient. The filtrate becomes hypotonic. Because water and solute movement are not always linked in the loop of Henle, the volume of body fluids can be controlled separately from the osmolality. The overall function of the loop of Henle is the reabsorption of sodium and chloride and other electrolytes, and the concentration of urine (Tortora and Derrickson 2011). The distal convoluted tubule is generally impermeable to water, therefore only solutes, especially sodium and chloride, are reabsorbed. Calcium can also be reabsorbed although this depends on stimulation by parathyroid hormone (Field et al 2010). The overall level of reabsorption in the distal convoluted tubule is influenced directly by antidiuretic hormone (ADH). If ADH is present, the tubule becomes permeable to water, causing it to be reabsorbed making the urine more concentrated. When ADH is not present, the walls remain impermeable to water and more dilute urine is formed. The role of ADH is to conserve body fluids by decreasing urine output. The production of ADH is stimulated by dehydration, loss of blood, pain or stress, and is inhibited by alcohol, which is one of the reasons for dehydration following excessive alcohol consumption (Tortora and Derrickson 2011). Sodium plays an important role in the maintenance of fluid balance. It is the most abundant electrolyte in extracellular fluid with levels controlled by the action of aldosterone, ADH and ANP. Levels of sodium are monitored constantly in the hypothalamus, with homeostatic levels of sodium maintained by an interplay of hormonal control influencing levels of reabsorption, thus the level of sodium in the blood has a direct influence on urine output (Tortora and Derrickson 2011). Several distal tubules join together to form collecting ducts, which are generally impermeable to water. These ducts are also affected by ADH in much the same way as the distal tubules, therefore sodium is reabsorbed as is some chloride, but by the time filtrate reaches the collecting ducts up to 95% of the solutes and water have been returned to the blood (Tortora and Derrickson 2011). Potassium levels in the blood are regulated by cells in the wall of the tubules because they secrete potassium back into the tubule. In addition, hydrogen and bicarbonate levels are regulated, and secretion of hydrogen is vital to the overall maintenance of pH levels in the blood. The role of the kidney in pH balance is to reabsorb the filtered bicarbonate and secrete hydrogen buffered by phosphate and ammonia. Ammonia is
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manufactured by cells of the renal tubule from the amino acid glutamine (Porth 2011).
Secretion
Secretion is the movement of substances from the blood into the tubule of the nephron and is an important renal process that regulates the level of several substances. It occurs mainly in the distal convoluted tubule and the collecting ducts (Chalmers 2008). Although creatinine is freely filtered, it is also secreted increasing the total amount in filtrate by about 20%. Other substances such as hydrogen, potassium and ammonia, and some drugs are also secreted, for example morphine, aspirin and penicillin (Porth 2011). The collecting ducts unite to form larger ducts, ultimately leading the filtrate to the pelvis of the kidney (Porth 2011) where the filtrate exits the kidneys via the ureters, and is known as urine. Normal urine consists of 95% water (with an average adult producing around 1-2L of urine per day), urea, creatinine, potassium, ammonia, uric acid, sodium, chloride, magnesium, sulphate, phosphate and calcium (Tortora and Derrickson 2013). It is important that nurses are aware of the normal levels of these elements, as well substances that do not normally appear in urine because these are important indicators of renal function and wider abnormalities in the body. For example, albumin is a plasma protein that should not appear in urine because it is too large to cross the filtration barrier and, therefore, its presence can indicate renal damage. Glucose is normally completely reabsorbed and its presence indicates that the renal threshold for reabsorption has been exceeded as occurs in diabetes mellitus. White blood cells are part of the immune response and their presence in urine indicates infection. The presence of ketones in urine has several potential causes such as starvation, a low carbohydrate diet or diabetes mellitus (Tortora and Derrickson 2013). Normal urine should be clear, although it may vary in colour from pale yellow to dark amber according to the concentration. Colour can also be influenced by food or drugs. There should be no unpleasant odour initially, but urine left standing can develop a smell of ammonia. The normal pH of urine is 4-8 with an average of 6, but this can vary considerably with diet, for example a high protein diet will increase the acidity of urine (Chalmers 2008, Tortora and Derrickson 2013).
Role and anatomy of the ureters The end result of the renal process is urine, which exits the pelvis of the kidneys via the ureters. The ureters are long tubes about 25-30cm in length march 5 :: vol 28 no 27 :: 2014 47
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Art & science life sciences: 18 that pass behind the peritoneum and connect the kidneys to the bladder (Brooker and Nicol 2011). The ureters pass under the bladder and enter obliquely, eventually opening into the bladder at the posterior inner surface (Brooker and Nicol 2011, Tortora and Derrickson 2013). This means of entering the bladder is important because it prevents back flow of urine to the ureters by creating a valve-like effect in which the ureters are compressed and the openings occluded when urine accumulates and pressure in the bladder increases. Preventing back flow of urine is important to prevent the upwards movement of microorganisms from the bladder to the kidneys, reducing the risk of transfer of infection.
Role and anatomy of the bladder and urethra The position of the bladder differs slightly in men and women. In females, the bladder is situated in front of and below the uterus and rests against the vagina, while in men it is situated directly in front of the rectum (Tortora and Derrickson 2013). The bladder is a hollow muscular sac made up of four layers: an outer fibrous coating, a longitudinal and circular layer of smooth muscle known as the detrusor muscle, a layer of connective tissue and an inner layer of transitional epithelium (Brooker and Nicol 2011). Emptying of the bladder involves both voluntary and involuntary nervous control. Activation via the parasympathetic nervous system brings about contraction of the detrusor muscle and relaxation of the internal sphincter in the bladder neck during micturition. Voluntary control is via the sympathetic nervous system, which aids bladder filling by promoting relaxation of the smooth muscle and contraction of the internal sphincter (Porth 2011). The urethra varies in length between men and women. In men, it measures about 20-25cm and passes through the prostate gland. In men, the urethra also has a sexual function in carrying semen. Semen enters the urethra via a duct known as the vas deferens. The vas deferens links with the epididymis that exits the testes (Brooker and Nicol 2011). The external sphincter is a ring of voluntary muscle and in men, it is situated where the urethra exits the prostate gland. In women, the urethra is shorter, about 3-5cm in length with its orifice between the clitoris and the vagina. The external sphincter is near the urethral orifice and is under voluntary control. The urethra is made largely of an outer layer of smooth muscle continuous with the bladder, a middle layer that has nerve, lymph and blood supply, and an inner mucous lining, which is continuous with the bladder (Brooker and Nicol 2011). 48 march 5 :: vol 28 no 27 :: 2014
Micturition
The bladder acts essentially as a store for urine until it is time to expel it from the body in the process of micturition. In normal circumstances, urine will remain in the bladder until the detrusor muscle contracts, bringing about opening of the bladder neck alongside relaxation of pelvic floor muscles and external urinary sphincter muscles (Brooker and Nicol 2011). Tortora and Derrickson (2011) stated that the pressure inside the bladder increases during filling and that when a volume of 200-400mL is reached, stretch receptors send impulses to the spinal cord triggering the micturition reflex and stimulating contraction of the detrusor muscle and relaxation of the internal sphincter. Urination or voiding is stimulated at a conscious level before initiation of the micturition reflex, and it is this conscious control that children learn via control of the external sphincter and some pelvic floor muscles (Tortora and Derrickson 2011). Conscious control is normally achieved by the age of three, but cannot be achieved until the necessary nerve pathways have matured (Brooker and Nicol 2011).
Urinary tract infection Several conditions may be associated with the urinary system such as chronic kidney disease, acute kidney injury and urinary tract infection (UTI). In England, chronic kidney disease occurs in 14% of males and 13% of females, although many individuals will be at an early stage of the condition and may be unaware that they have it (Roderick et al 2011). This long-term condition is associated with deterioration, reduced quality of life, and may necessitate dialysis. Acute kidney injury occurs in 13-18% of individuals admitted to hospital in developed countries and develops often in individuals with other conditions being cared for in non-renal specialist areas (National Institute for Health and Care Excellence 2013). Because acute kidney injury tends to develop in patients in general wards, it is essential that nurses have an understanding of basic renal function and deviations from normal plasma and urine levels that may indicate renal dysfunction. UTI is discussed here because of its prevalence and tendency to reoccur. Almost half of all women will report at least one UTI at some point in their lives and 20-30% will experience a recurrence (Patient.co.uk 2013). Although rare in men under aged 50, UTI occurs in about 3% of men in their 60s and 10% of men in their 80s (Patient.co.uk 2013). According to the Scottish Intercollegiate Guidelines Network (SIGN) (2012), UTI is the second most common reason for the use of antimicrobial treatment in both primary and
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secondary care settings. A UTI is defined as ‘the presence of bacteria or other microorganisms in the urine or genito-urinary tissues’, although the term infection is usually used when the concentration of organisms meets set quantitative criteria (Nicolle 2012). The presence of bacteria in urine does not indicate an infection that requires intervention necessarily, with an average of 16% of women and 6% of men between the ages of 65 and 74 in Scotland having asymptomatic bacteriuria (SIGN 2012). There are several risks that increase the likelihood of developing bacteriuria, including female gender, age, comorbid diabetes mellitus, institutionalisation, sexual activity and presence of an indwelling urinary catheter (SIGN 2012). Age is an important factor because post-menopausal women experience more non-specific symptoms such as low abdominal and back pain, and a delay in diagnosis (Arinzon et al 2012). Pre-menopausal women are more likely to report local signs such as pain on passing urine (Brooker and Nicol 2011, Arinzon et al 2012). Typical signs of UTI are dysuria, increased frequency of urination, suprapubic tenderness, urgency, polyuria and haematuria (SIGN 2012). A lower UTI affects the urethra and bladder, for example cystitis, and an upper UTI affects the ureters and kidneys, for example pyelonephritis (Porth 2011). Uncomplicated lower UTIs are more common than upper UTIs and generally respond well to treatment involving a short course of antibiotics. Pyelonephritis is more likely to occur in children and those with obstructions or other conditions such as those with neurogenic bladder dysfunction secondary to diabetes (Porth 2011, SIGN 2012). Pyelonephritis is potentially more serious than lower UTI, creating more widespread symptoms such as pain, nausea and pyrexia, and warranting antibiotic treatment (Brooker and Nicol 2011). However, Chalmers (2008) stated that although antibiotics are useful in treating the initial acute infection, there is little evidence to support prophylactic use to slow the development of renal disease.
Diagnosis
UTIs in men should be confirmed with a urine sample for culture. Because there is a strong correlation between UTI in men and an enlarged prostate, this must be considered during investigation and diagnosis (SIGN 2012). Most bacteria enter the urinary tract via the urethra and travel to the bladder, with Escherichia coli accounting for the majority of infections in the community and half of all hospital-acquired infections (Brooker and Nicol 2011). A UTI diagnosis is based on the presenting symptoms
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and urine sample testing. In otherwise healthy women, it is recommended that diagnosis is made and treatment with antibiotics is considered when three or more of the previously mentioned typical symptoms are present (SIGN 2012). The likelihood of the presence of a UTI is further confirmed when urine is cloudy with suspended particles – the presence of white blood cells (pus) creates this cloudiness and is known as pyuria. A dipstick test may be useful when fewer symptoms are present.
Treatment
Treatment of UTI varies with gender, and type and severity of infection. Antibiotic therapy with a three-day course of trimethoprim or nitrofurantoin should be considered in non-pregnant women under 65 when there are three or more typical symptoms (SIGN 2012). Pregnant women should have a urine sample taken for culture before antibiotic treatment, be treated with an antibiotic chosen from the local formulary and usually for seven days, and have a further urine sample taken for culture one week after completion of antibiotic therapy to ensure the infection has resolved (SIGN 2012). Recurrent UTIs in older women often lead to the repeated use of antibiotic therapy. McMurdo et al (2009) found that in older women with recurrent UTI, there is little difference between the effectiveness of cranberry extract and trimethoprim in terms of prevention of infection, suggesting that these women may wish to consider this option, in discussion with their clinician, as a means of reducing the risk of antimicrobial resistance. Infection in men should be treated with quinolone when there is a suggestion of prostatitis, and referral to a urology specialist should be considered when there is failure to respond to antibiotic therapy, recurrent UTI or upper UTI (SIGN 2012). The link between UTI and catheterisation is well-recognised, with increasing duration of catheterisation increasing the risk of infection (SIGN 2012). While catheterisation is an essential part of nursing management in many situations, it is vital that the catheter is removed as soon as it is no longer clinically indicated to reduce the risk of UTI and that catheterisation is not viewed as a solution to urinary incontinence (Hooton et al 2010). Catheter-associated urinary tract infections may present with worsening fever, rigors, altered mental state, flank pain, acute haematuria or pelvic discomfort. Dipstick testing will be unreliable as pyuria is common in patients who have been catheterised and is not predictive of infection (SIGN 2012). Hooton et al (2010) stated that prevention measures such as a closed drainage system are march 5 :: vol 28 no 27 :: 2014 49
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Art & science life sciences: 18 important in reducing the risk of infection. SIGN (2012) states that hospital admission is required when systemic symptoms such as fever, rigors, chills or vomiting are present. In terms of antibiotic therapy, treatment should be guided by symptoms and follow local antibiotic policy. Long-term catheters should be changed before commencement of antibiotic therapy, and therapy should then be guided by local policy. It should be noted that because of the prevalence of bacteriuria in people who are catheterised, screening would be unhelpful and there is no need to treat those with asymptomatic bacteriuria (SIGN 2012).
The kidneys are the primary active units in the urinary system filtering more than 600mL of blood per minute; however, this can create vulnerability because of the close contact the nephrons have with blood and any potential toxins (Porth 2011). Assessment and monitoring of renal function is a major part of the nurses’ role and is inherently linked to fluid balance. Nurses need to have an understanding of renal physiology and fluid management. In addition, they need to be aware of how to identify and treat UTI because of the prevalence of this condition and the potential for associated complications NS
Conclusion Although the primary function of the urinary system is the production of urine, it has a range of other roles that link it closely to other systems in the body. For example, the kidneys have a vital role in the maintenance of blood pressure while blood pH is maintained by the interplay of actions of the urinary and respiratory systems. Producing urine is a major means of eliminating waste products and toxic substances from the body, thus maintaining fluid balance in the body.
POINTS FOR PRACTICE List some of the signs and symptoms of a lower urinary tract infection (UTI) and explain why these might occur. Refer to local protocols and outline a nursing care plan for a patient presenting with a lower UTI. Review local guidelines in relation to catheterisation and catheter management with a view to reducing or preventing the incidence of UTI.
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Diagnosis, prevention and treatment of catheter-associated urinary tract infections in adults: 2009 International Clinical Practice Guidelines from the Infectious Diseases Society of America. Clinical Infectious Diseases. 50, 5, 625-663. McDonough AA, Thomson SC (2012) Metabolic basis of solute transport. In Taal MW, Chertow GM, Marsden PA, Skorecki K, Yu ASL, Brenner BM (Eds) Brenner & Rector’s The Kidney. Ninth edition. Elsevier Saunders, Philadelphia PA, 138-157. McMurdo MET, Argo I, Phillips G, Daly F, Davey P (2009) Cranberry or trimethoprim for the prevention of recurrent urinary tract infections? A randomized controlled trial in older women. Journal of Antimicrobial Chemotherapy. 63, 2, 389-395. Munger KA, Kost Jr. CK, Brenner BM, Maddox DA (2012) The renal circulations and glomerular ultrafiltration. In Taal MW, Chertow GM,
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