Biological basis of child health 15: understanding the renal system and common renal conditions in children
Intended for healthcare professionals
CPD    

Biological basis of child health 15: understanding the renal system and common renal conditions in children

Kate Davies Associate professor of paediatric prescribing and endocrinology, London South Bank University, and honorary research fellow in paediatric endocrinology, Queen Mary University of London, London, England
Suzanne Bradley Renal clinical nurse specialist, Great Ormond Street Hospital NHS Trust, Great Ormond Street Hospital For Children NHS Foundation Trust, London, England
Doreen Crawford Consultant editor of Nursing Children and Young People and nurse adviser, Crawford-McKenzie, Colsterworth, Lincolnshire, England

Why you should read this article:
  • To enhance your knowledge of the structure and functions of the renal system

  • To recognise some of the renal conditions that may be seen in children and to understand how these are managed

  • To count towards revalidation as part of your 35 hours of CPD, or you may wish to write a reflective account (UK readers)

  • To contribute towards your professional development and local registration renewal requirements (non-UK readers)

This article, the 15th and last in a series on the biological basis of child health, focuses on the renal system, in particular the kidneys. It provides an overview of their role, function, anatomy and physiology, and embryological development. The renal system has a crucial role in homeostasis, so renal function impairment can have wide-ranging and potentially serious consequences for a child’s overall health. The article describes some of the common renal conditions seen in children and how these are managed. It explains how to interpret the results of renal function tests and urine sampling conducted to assess renal function and to investigate acute and chronic disease.

Nursing Children and Young People. doi: 10.7748/ncyp.2022.e1392

Peer review

This article has been subject to open peer review and checked for plagiarism using automated software

@sausmash

Correspondence

kate.davies@lsbu.ac.uk

Conflict of interest

None declared

Davies K, Bradley S, Crawford D (2022) Biological basis of child health 15: understanding the renal system and common renal conditions in children. Nursing Children and Young People. doi: 10.7748/ncyp.2022.e1392

Published online: 09 May 2022

Aims and intended learning outcomes

The aim of this article is to provide an overview of the renal system, the tests used to assess renal function, and the manifestations and management of common renal conditions in children. After reading this article and completing the time out activities you should be able to:

  • Describe the role and functioning of the kidneys.

  • Understand the embryological development of the kidneys.

  • Explain the importance of electrolytes and how to detect and interpret abnormal electrolyte values.

  • Describe the abnormal colours of urine and what they may indicate.

  • Outline some common renal conditions in children and their management.

Key points

  • The main function of the renal or urinary system is to remove metabolic waste from the human body through the production and excretion of urine

  • The kidneys are the principal organs of the renal system and they produce urine through sequential processes of filtration, reabsorption and secretion

  • Blood tests to measure the levels of urea, creatinine and electrolytes are commonly undertaken to check kidney function and homeostasis, and the results will often guide the clinical management and care of children

  • Renal conditions that may be seen in children include urinary tract infection, nephrotic syndrome, haemolytic uraemic syndrome and chronic kidney disease

Introduction

The main function of the renal or urinary system is to remove metabolic waste from the human body through the production and excretion of urine. The kidneys are the principal organs of the renal system and their role is to filter metabolic waste from the blood and transform the filtrate into urine. The renal system is further composed of the ureters, bladder and urethra, which facilitate the passage, storing and voiding of urine. The renal system is intertwined with other body systems, notably the cardiovascular, endocrine and reproductive systems, and has a crucial role in homeostasis, so impaired renal function can have a range of negative consequences on a child’s overall health.

TIME OUT 1

Anatomy of a nephron

Examine the structure of the nephron shown in Figure 1, and use it to explain the renal system’s functions of filtration, reabsorption, secretion and excretion in relation to blood homeostasis

Anatomy and physiology

The kidneys are bean-shaped structures – with a concave side and a convex side – enclosed in a tough protective fibrous capsule. They are positioned between the T12 and L3 vertebrae behind the abdominal or peritoneal cavity (retroperitoneal space) (Chalmers 2014). At birth, each kidney is approximately 55mm long. At maturity (once the child has stopped growing), the kidneys have doubled in length to reach 110mm (Obrycki et al 2021). The kidneys receive a rich blood supply from the renal artery, which branches off the abdominal aorta. The renal vein, ureter, nerves and lymphatic vessels enter the kidneys via their concave side, known as the hilum (Odya and Norris 2017).

The kidneys produce urine through sequential processes of filtration, reabsorption and secretion. These processes take place in the nephrons, which are the functional units of the kidneys. At maturity, there are approximately one million nephrons in each kidney. Figure 1 shows the anatomy of a nephron.

The nephron can be divided into two regions, the cortex (outermost region) and the medulla (innermost region). Located in the cortex, the Bowman’s capsule is a cup-shaped structure that encloses the glomerulus, a capillary knot branching off an afferent arteriole (Odya and Norris 2017). Blood in the glomerulus is under pressure and that pressure pushes waste-containing water out of the glomerulus into the Bowman’s capsule across the filtration membrane.

From the Bowman’s capsule, the filtrate travels through the proximal convoluted tubule, which reabsorbs useful materials such as electrolytes into the adjacent capillaries and secretes acids and bases (Odya and Norris 2017). The proximal convoluted tubule is continued by the loop of Henle, located in the medulla (Chalmers 2014). The loop of Henle is a U-shaped structure whose main function is to reabsorb water. As water is reabsorbed, the remaining filtrate becomes increasingly concentrated in waste products (Odya and Norris 2017). The loop of Henle leads to the distal convoluted tubule, which completes the processes of reabsorption of electrolytes. The distal convoluted tubule connects with the collecting duct for further reabsorption and urine concentration.

The urine produced by each nephron travels via the collecting duct to the renal pyramids in each kidney, then to the renal calyces and from there to the renal pelvis, which joins with the ureter (Odya and Norris 2017). The accumulation of urine causes the ureters to distend and the urine is propelled by peristaltic contractions into the bladder (Pocock et al 2018). The ureters enter the bladder at an oblique angle to prevent backflow (Pocock et al 2018). The area where the ureters enter the bladder is called the trigone and it is highly sensitive to pressure. Increasing pressure in the bladder activates receptors in the bladder wall called stretch receptors. Once expanded to a certain degree, the bladder signals to the brain that it needs to void (Malykhina 2017, Elsayed and Davies 2021).

In newborns and infants, a certain volume of urine in the bladder triggers the voiding reflex. Newborns and infants cannot control the voiding reflex until they are approximately two years old. Bladder capacity varies according to age and can be calculated by using the following formula (Guerra et al 2018):

Bladder capacity in mL = (child’s age in years +2) x 30

Embryological development

The embryological development of the kidneys occurs in three stages (Table 1).

Table 1.

Embryological development of the kidneys

Stage Time Normal development Potential consequences of abnormal development
Stage 1 – pronephroi (primordial kidneys)From day 21 of gestation
  • The pronephroi are non-functioning structures which degenerate by day 25 of gestation, but their ducts are retained and used by mesonephroi

Stage 2 – mesonephroi (interim kidneys)From day 28 of gestation
  • The mesonephroi are functional structures which receive fetal blood and produce urine

  • They contain primitive tubules and glomeruli

  • They degenerate by week 10 of gestation but some of their tubules and ducts are retained and used by the metanephroi

  • Urorectal septum malformations

  • Persistent urachus (umbilical disorder)

  • Abnormal kidney structure

Stage 3 – metanephroi (final kidneys)Between week 5 and week 32 of gestation
  • The metanephroi originate from two embryonic sources:

    • Metanephric blastema forms the nephrons

    • Metanephric diverticulum forms the ureters, renal pelvis, renal calyces and collecting tubules

  • The metanephroi initially appear in the pelvis and then migrate towards the diaphragm. Between week 6 and week 9 of gestation, they ascend into the retroperitoneal space, to each side of the spine between lumbar vertebra L3 and thoracic vertebra T12

  • As each nephron grows, its tubule continues to elongate, forming the proximal convoluted tubule, loop of Henle and distal convoluted tubule

  • By week 15, the cortex and medulla of each nephron are visible as two distinct regions

  • The collecting duct and loop of Henle extend into the medulla

  • Horseshoe kidney

  • Ectopic kidney

  • Renal dysplasia

  • Renal hypoplasia

  • Polycystic kidney

The ureters, bladder and urethra start to develop in utero between week 4 and week 7 of gestation and are fully developed by week 20. The early sinus which forms the cloaca is divided into two parts by the urorectal septum, the upper portion of which forms the bladder. The pelvic part of the urogenital sinus develops from the early urethra and some of the reproductive tract. Abnormal development of the ureters, bladder and/or urethra can result in duplication of the ureters, congenital megalourethra, bladder exstrophy, urethral valves and some hypospadias (Webster and de Wreede 2016).

Functions of the kidneys and renal system

The kidneys have a central role in the removal of metabolic waste from the human body through the production and excretion of urine. They also support the regulation of pH and maintenance of the acid-base balance in the body and contribute to the production of red blood cells and to bone growth. Examples of how the kidneys carry out these functions include (Gormley-Fleming 2015, Odya and Norris 2017):

  • Excreting acids via the urine to maintain the homeostatic range of blood pH between 7.3 and 7.4.

  • Producing the growth factor erythropoietin, which stimulates the bone marrow to produce red blood cells.

  • Regulating calcium and phosphate homeostasis and producing calcitriol, the active form of vitamin D.

Furthermore, the renal system has a central role in regulating blood pressure through its involvement in the renin-angiotensin-aldosterone system (Figure 2).

Figure 2.

Renin-angiotensin-aldosterone system

ncyp.2022.e1392_0002.jpg

In the renin-angiotensin-aldosterone system, complex interactions involving the kidneys and organs from other body systems take place with the aim of adjusting the diameter of blood vessels and the blood and extracellular fluid volume. In an area of the nephron called the macula densa, specialised cells lining the walls of the distal convoluted tubule monitor the level of sodium in the filtrate. In parallel, the juxtaglomerular cells located in the walls of the afferent arterioles monitor blood pressure. The level of sodium gives the macula densa an indication of filtrate osmolarity. If filtrate osmolarity falls too low, the macula densa causes the afferent arteriole to dilate, therefore increasing the pressure at the glomerulus and increasing the glomerular filtration rate (GFR). The decrease in blood pressure in the afferent arteriole prompts the juxtaglomerular cells to secrete a protein and enzyme called renin (Samuel et al 2018).

Renin converts angiotensinogen – a protein and hormone precursor produced by the liver – into angiotensin 1. Angiotensin-1 is converted into angiotensin-2 by an angiotensin-converting enzyme, found in the endothelium of the lungs and kidneys. Angiotensin-2 is a potent vasoactive peptide which causes blood vessels to constrict, resulting in increased blood pressure. It also stimulates the adrenal cortex to secrete the hormone aldosterone (Samuel et al 2018). Aldosterone stimulates the proximal convoluted tubules of the nephrons to increase the reabsorption of sodium and water into the blood. This increase in sodium increases blood osmolarity, shifting fluid into the blood and extracellular fluid volume. This subsequently increases blood vessel tone and therefore blood pressure (Samuel et al 2018).

TIME OUT 2

Watch this video explaining glomerular filtration: youtu.be/t0zxeStWWGc How would you explain glomerular filtration to a parent who is concerned about their child’s glomerular filtration rate?

Interpreting urea, creatinine and electrolytes levels

Urine is 95% water and 5% particles such as urea, creatinine, ammonia, uric acid and electrolytes (Gormley-Fleming 2015). These particles are excreted in the urine and/or reabsorbed in the kidneys. Measuring the levels of urea, creatinine and electrolytes in the blood is a commonly requested laboratory test to check kidney function and homeostasis (Blann 2014). The results will often guide the clinical management and care of children, for example in hypovolaemia due to gastroenteritis, dehydration or sepsis (Lissauer and Carroll 2017).

Creatinine and urea levels reflect the GFR, which represents the flow of plasma from the glomerulus across the filtration membrane into the Bowman’s capsule. The GFR is an important indicator of kidney function and is typically recorded in millilitres per minute per body surface (mL per min per 1.73m2). At birth, GFR is approximately 20mL per min per 1.73m2. It reaches its maximum (adult) level of approximately 90mL per min per 1.73m2 when the child is about two years old (Mian and Schwartz 2017). Table 2 describes the main components of urine.

Table 2.

Main components of urine

ncyp.2022.e1392_0002_tb1.jpg

TIME OUT 3

What methods do you and your colleagues use to collect a urine sample from infants and young children? What are the advantages and disadvantages of each of these methods? In your experience, what circumstances or factors mandate an invasive procedure to collect a urine sample?

Urine sampling

Collecting a urine sample

A urine sample is usually collected to investigate possible infection. The sample will be sent to the laboratory for urinalysis, which will potentially detect the presence of disease markers such as leukocytes and nitrites. For example, it may show the presence of leucocytes which would indicate an infection (Kaufman et al 2019).

The ‘clean catch’ method, whereby urine is collected by holding a sterile specimen bottle in the urine stream, is the recommended method for collecting a urine sample in children (Morris 2018). In older and cooperative children, the parent or nurse can attempt to use the clean catch method when the child goes to the toilet.

In infants, the clean catch method can be combined with the voiding stimulation or ‘quick wee’ method. Voiding is stimulated by cleaning the infant’s suprapubic area (below the umbilicus) with cold water. This prompts the infant to urinate in minutes, enabling the nurse to use the clean catch method to collect a urine sample (Morris 2018). However, the clean catch method is often impractical and may not be possible in younger, preverbal and/or frightened children who are unwell.

If it is not possible to collect a urine sample using the clean catch method, another non-invasive method is to use a urine collection pad. When using urine collection pads, it is important to follow the manufacturer’s instructions. Cotton wool balls, gauze and sanitary towels should not be used routinely to collect urine in infants and children, since these are not sterile (Rogers and Saunders 2008).

Alternatively, a urine collection bag can be secured onto the skin and placed inside the infant’s nappy. The nappy could also be slit and the bag placed outside the nappy so it can be inspected. The optimal time to apply a urine collection bag is when the infant or young child is sleepy and positioned on their back. Such bags are convenient but can easily leak, detach or become contaminated with faeces. Infants may find the bag uncomfortable and their skin may become irritated, so care needs to be taken when removing the bag (Kaufman et al 2020).

When it is not practical or possible to collect urine by any of the non-invasive methods described, catheter sampling or suprapubic aspiration should be used. Before attempting suprapubic aspiration, an ultrasound should be performed to check that urine is present in the bladder (National Institute for Health and Care Excellence (NICE) 2018).

Inspecting a urine sample

The odour of urine is usually due to the presence of ammonia (Odya and Norris 2017) but is also influenced by the presence of certain bacteria, the consumption of foods such as asparagus and/or the intake of vitamin supplements. The normal urinary pH range is 5-6 (Chalmers 2014). Acidic urine (a lower pH) could indicate decreased nutritional intake, a high-protein diet or a urinary tract infection (UTI), whereas more alkaline urine (a higher pH) may be seen after meals or in infants with distal renal tubular acidosis (van Biljon 2012).

The colour of urine is due to urobilin resulting from haem degradation. The normal colour of a child’s urine ranges from light yellow to golden and fluctuates over the course of a day depending on their level of hydration – the more urine is concentrated, the darker it appears. A visual inspection of the child’s urine sample may reveal an abnormal colour. Table 3 details abnormal urine colours and potential causes.

Table 3.

Abnormal urine colours and potential causes

Abnormal urine colour Potential causes
Orange or red
  • Orange or red urine may indicate haematuria and urine should be tested for the presence of haem

  • Medicines including rifampicin, isoniazid, riboflavin, sulfasalazine and warfarin sodium can cause an orange or red urine colour

  • The consumption of carrots can result in orange urine

  • The consumption of beetroot, rhubarb or sweets containing red pigments can result in dark red urine

  • Poisoning with mercury or lead can cause red urine, requiring urgent medical attention

Brown
  • Antibiotics such as nitrofurantoin and metronidazole, and antimalarials such as chloroquine and primaquine can cause a brown urine colour

  • The consumption of fava beans, rhubarb and/or aloe can result in dark brown urine

  • Brown urine is common in liver failure and jaundice

  • Cloudy brown urine is rare and indicates acute tubular necrosis, requiring urgent medical attention

White
  • White urine may indicate the presence of mucus, calcium sediments or phosphate precipitates

  • Other causes of white urine include fungal infection and bacterial infection, which may also manifest as pyuria (the presence of pus in the urine)

  • White urine may indicate the presence of lymph in the urinary tract. Chyluria (urine containing lymph) is associated with an obstruction to lymph flow. Chyluria is rare and usually caused by a parasite, but it can also be seen in pregnancy, after scoliosis surgery and in the presence of enlarged abdominal lymph nodes or tumours

Blue or green
  • Blue or green urine can indicate metabolic disorders where tryptophan metabolism is altered

  • Amitriptyline hydrochloride, propofol and medicines containing methylene blue can cause blue or green urine – however, they are not commonly used in children

  • The consumption of asparagus or liquorice can cause blue or green urine

Purple or black
  • Purple urine can indicate metabolic disorders such as porphyria

  • Very dark urine can indicate the rare genetic disease alkaptonuria, also known as black urine disease

  • The use of senna or cascara can darken urine

  • Poisoning with iodine, copper or phenol can blacken urine

  • Purple or black urine may result from the application of purple or black hair dye containing the highly toxic ingredient paraphenylenediamine

(Adapted from Gill 2014, Singh et al 2014, Roth 2021)

Nurses should explain the rationale for taking a blood sample or for collecting and inspecting a urine sample to parents. It is important that parents understand that conducting these investigations does not invariably mean that there is long-term damage, that abnormal test values can be reversible, and that repeat urinalysis may be conducted to determine the extent to which treatment is proving beneficial. When communicating with parents, nurses need to avoid the overuse of abbreviations and medical terms. They also need to explain clearly the risks, benefits and short-term and long-term implications of undertaking investigations for the health and well-being of the child.

TIME OUT 4

Reflect on how parents tend to perceive childhood renal conditions. Do most of them understand the wide-ranging implications of a renal condition for other body systems and the overall health of their child? Do they tend to underestimate or overestimate the significance of abnormal renal function test results? Think about how you can explain their child’s condition, and the child’s associated short-term and long-term healthcare needs, to parents

Renal conditions in children

Urinary tract infection

According to NICE (2018), infants and children who have bacteriuria but no systemic signs or symptoms have a lower UTI (cystitis), whereas infants and children who have bacteriuria and fever of 38°C or higher – or those presenting with fever lower than 38°C with loin pain or tenderness and bacteriuria – have an upper UTI (acute pyelonephritis).

The child or young person with a UTI may be afebrile or have a low-grade pyrexia. Untreated UTIs can lead to chronic kidney disease (CKD) where there is bilateral scarring of the kidneys and hypertension (Rees et al 2019). Signs and symptoms of UTI can include fever, lethargy and abdominal pain. If a UTI is suspected, a urine sample should be obtained and analysed locally for leukocytes and nitrites. If the sample is positive, antibiotics need to be administered and a second urine sample should be sent to the laboratory for urgent microscopy and culture.

Nephrotic syndrome

Nephrotic syndrome occurs in 1 in 50,000 children usually aged 2-5 years (NHS 2019a). Primary nephrotic syndrome is the most common form and is caused by kidney disease. Secondary nephrotic syndrome is rarer and occurs as a consequence of other diseases, for example diabetes mellitus.

In nephrotic syndrome, damage to the glomerulus means the filtration membrane becomes increasingly permeable, which means albumin proteins that would normally remain the bloodstream filter through and are excreted in the urine, resulting in low serum albumin levels and characteristic oedema. A reduced volume of fluid in the circulation results in the kidney secreting increased amounts of renin, which activates salt retention, followed by water retention and further oedema. This oedema is secondary to reduced oncotic pressure. It is most often seen in the legs, feet, or ankles but it may also affect the hands and face. Children with nephrotic syndrome can also present with haematuria, fever, lethargy, abdominal pain, anorexia, diarrhoea and hypertension. Treatment may involve corticosteroids, antibiotics, diuretics, albumin infusions and a low-salt diet (Noone et al 2018).

Haemolytic uraemic syndrome

Haemolytic uraemic syndrome is the abrupt onset of renal dysfunction resulting in an imbalance of electrolytes, fluids and waste products (Ciccia and Devarajan 2017). It may occur in microangiopathic haemolytic anaemia (anaemia resulting from the destruction of red blood cells), thrombocytopenia or acute kidney injury (Walsh and Johnson 2018). Acute kidney injury can have prerenal causes (for example sepsis), intrarenal causes (due to damage in the nephrons) or postrenal causes (for example obstruction in the upper or lower urinary tract affecting outflow from the kidneys), or it can be due to previous infections that have damaged the kidney tissues (Poole 2019).

The clinical presentation of children with haemolytic uraemic syndrome is similar to that seen in gastroenteritis, with vomiting, bloody diarrhoea, abdominal pain, fever, chills and headache as the condition progresses. Children may also present with anaemia, thrombocytopenia, purpura, lethargy and pallor (Grisaru 2014). Anaemia can be managed with blood transfusions. If there is an acute deterioration in kidney function in the presence of active disease, dialysis may be required. Most children recover well from haemolytic uraemic syndrome but some experience serious consequences, such as blood clotting disorders, which can lead to bleeding, stroke, seizures, coma, heart conditions and CKD (Davies 2014).

Chronic kidney disease

In CKD, there is irreversible damage to the parenchyma of the kidney resulting in abnormal renal function (Rees et al 2019). It is usually a consequence of other conditions – such as congenital abnormalities, hereditary diseases, untreated or unmanaged infections and nephrotic syndrome – or trauma. From the age of two years and above, CKD is described in five stages according to GFR values (American Kidney Fund 2022):

  • Stage 1 – normal kidney function, GFR of 90mL per min per 1.73m2 or above.

  • Stage 2 – mild loss of kidney function, GFR 60-89mL per min per 1.73m2.

  • Stage 3 – moderate loss of kidney function, GFR 45-59mL per min per 1.73m2.

  • Stage 4 – severe loss of kidney function, GFR 15-24mL per min per 1.73m2.

  • Stage 5 – kidney failure (end-stage renal disease), GFR <15mL per min per 1.73m2.

An individual fluid restriction regimen is an important element of CKD management, alongside treatment of any infections and anaemia. In stage 5 CKD, optimal management usually entails a renal transplant. If that is not possible, or while the patient is waiting for a transplant, they require dialysis to remove metabolic waste and excess fluid (NHS 2019b). Dialysis usually takes place in specialist centres, although home dialysis is possible. Each session typically lasts 3-5 hours. A patient may need to receive dialysis several times per week (Kaspar et al 2016).

Conclusion

The main function of the renal system is to remove metabolic waste from the body through the production and excretion of urine. Renal function impairment can have serious negative consequences for a child’s overall health. It is important for children’s nurses to understand renal function so they can not only provide effective care for children, but also enhance parents’ understanding of their child’s condition and its management. In particular, children’s nurses need to have an adequate understanding of the various blood and urine tests conducted to assess renal function and to support the diagnosis and management of various conditions.

TIME OUT 5

Identify how Biological basis of child health 15: understanding the renal system and common renal conditions applies to your practice and the requirements of your regulatory body

TIME OUT 6

Now that you have completed the article, reflect on your practice in this area and consider writing a reflective account: rcni.com/reflective-account

Further resources

National Institute of Diabetes and Digestive and Kidney Disease – Kidney disease

niddk.nih.gov/health-information/kidney-disease

Khan Academy – Tubular reabsorption

khanacademy.org/test-prep/mcat/organ-systems/the-renal-system/a/tubular-reabsorption-article

National Kidney Foundation – Paediatric GFR calculator

kidney.org/professionals/Kdoqi/gfr_calculatorPed

infoKID – Information for parents and carers about kidney conditions in children

infokid.org.uk

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