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Please wait while the page loadsChildren's Nursing · Free Resource
How the kidneys make urine, control fluid and electrolytes, what to check at the bedside, and which signs mean a child needs help early.
Quick framework
Urinary tract order
Kidneys → Ureters → Bladder → Urethra
Filter it, carry it, store it, release it.
Nephron basics
Glomerulus → Bowman's capsule → Proximal tubule → Loop of Henle → Distal tubule → Collecting duct
That is the route from first filtrate to final fine-tuning.
Urine output matters
Less urine often means less perfusion, less circulating volume, or less filtration.
It is one of the simplest early warning signs you have.
Golden rule
Falling urine output in a child who looks unwell is never "just a number".
Always read it alongside hydration, weight, blood pressure, and the whole child.
Student translation
Creatinine is a practical blood marker used to judge filtration. Urea is another waste product that can rise when the kidneys are not clearing well or when the child is dry. Perfusion means blood flow reaching the kidneys. Filtration is the first step where water and small solutes leave the blood at the glomerulus. Oliguria means too little urine. Fluid balance means what has gone in, what has come out, and whether the child is retaining or losing fluid overall.
A filtration and homeostasis system
Why it matters so much in children
| Structure | What it does | Why nurses care |
|---|---|---|
| Kidneys | Two highly perfused organs that filter blood, regulate water and electrolytes, help control blood pressure, and remove waste. | If kidney perfusion or function falls, urine output, blood chemistry, and fluid status can all worsen quickly. |
| Ureters | Muscular tubes that move urine from each kidney down into the bladder. | Obstruction anywhere along the route can cause pain, poor drainage, and renal injury if it is not recognised. |
| Bladder | A storage organ that holds urine until the child voids. | Retention, frequency, discomfort, or reduced output can all change the picture you are documenting. |
| Urethra | The final passage through which urine leaves the body. | Voiding symptoms, catheter issues, and infection risk often show up here at the bedside first. |
| Extra kidney job | What it means | Why it matters clinically |
|---|---|---|
| Acid-base balance | The kidneys help regulate hydrogen ions and bicarbonate. | This is why renal dysfunction can make a child acidotic and more tachypnoeic. |
| Blood pressure control | Renin links the kidneys to RAAS and circulating volume control. | Renal problems and blood pressure problems often travel together. |
| Erythropoietin | The kidneys support red blood cell production over time. | Longer-term renal disease can affect haemoglobin and energy levels. |
| Vitamin D activation | The kidneys help activate vitamin D. | So kidney disease can have wider effects than just urine and electrolytes. |
Exam tip
The kidneys are not just plumbing. They are perfusion-sensitive organs. If circulating volume falls, or the child's cardiac output drops, filtration can fall before the blood pressure looks dramatically abnormal.
Kidney layout
Nephron basics
| Structure | What it is | Why it matters |
|---|---|---|
| Cortex | The outer part of the kidney containing glomeruli and convoluted tubules | This is where filtration begins. |
| Medulla | The inner region containing loops of Henle and collecting ducts | Helps concentrate or dilute urine. |
| Renal pyramids | Triangular sections within the medulla | Channel urine towards the papillae. |
| Calyces | Cup-like spaces that collect urine from the papillae | They pass urine onwards towards the renal pelvis. |
| Renal pelvis | The funnel-like collecting area before the ureter | This is the final drainage point inside the kidney. |
| Glomerulus | A knot of capillaries under pressure | Filters water and small solutes out of the blood. |
| Bowman's capsule | The capsule wrapped around the glomerulus | Catches the filtrate that has just been formed. |
| Proximal tubule | The first long tubule after filtration | Reabsorbs most water, sodium, glucose, and other useful substances. |
| Loop of Henle | The hairpin loop dipping into the medulla | Builds the concentration gradient that helps the kidney handle water. |
| Distal tubule | The later fine-tuning segment | Adjusts electrolytes and acid-base balance further. |
| Collecting duct | The final drainage duct for multiple nephrons | ADH acts here to help control how much water is kept. |
Why children dehydrate faster
Children generally have a higher water turnover and less reserve than adults. In infants and younger children, illness can also bring rapid losses through vomiting, diarrhoea, fever, or tachypnoea. Their kidneys can conserve water, but they cannot always protect the child fast enough if the deficit is building quickly.
Nephron in one pass
Filter
The glomerulus makes the initial filtrate from plasma.
Fine-tune
The tubules reclaim what matters and add extra waste to the fluid.
Drain
The collecting duct sends final urine towards the calyces, pelvis, ureter, bladder, and urethra.
Simple sequence
Filter it. Keep what the body still needs. Add extra waste. Then excrete what is left.
Filtration and reabsorption
Secretion and excretion
| Step | What happens | What the body keeps or loses |
|---|---|---|
| Filtration | Pressure pushes water and small solutes out of the glomerular capillaries into Bowman's capsule. | Cells and most large proteins should stay in the blood. |
| Reabsorption | The tubules take back what the body still needs. | Water, sodium, glucose, bicarbonate, and other useful substances are reclaimed. |
| Secretion | The tubules actively move extra substances from the blood into the tubular fluid. | This helps get rid of acids, potassium, some drugs, and additional waste. |
| Excretion | What is left becomes urine and leaves through the collecting ducts, calyces, pelvis, ureter, bladder, and urethra. | This is what the body has decided not to keep. |
Bedside meaning
Dark concentrated urine often means the body is trying to conserve water, but one nappy or one void does not tell the whole story. Always match urine appearance with intake, measured output, perfusion, weight, and how the child actually looks.
What the kidneys are balancing all day
Water
The kidneys decide whether water is being conserved or excreted.
Electrolytes
Sodium and potassium are constantly being adjusted to keep cells working safely.
Acid-base and pressure
Kidneys help regulate pH and play a major role in blood pressure control.
ADH in plain English
ADH tells the kidney to save more water. If a child is dehydrated, the body will often increase ADH so urine becomes more concentrated and output falls.
Aldosterone in plain English
Aldosterone helps the body retain sodium, and water tends to follow sodium. It also increases potassium excretion, which is why potassium must always be interpreted carefully.
| Factor | Why it matters | Bedside meaning |
|---|---|---|
| Sodium | The main extracellular electrolyte; water tends to follow it. | Too much or too little changes fluid distribution and can affect neurological status. |
| Potassium | Mostly inside cells, but the kidneys help keep the blood level in a safe range. | Abnormal potassium can cause weakness and dangerous heart rhythm problems. |
| Water | The kidneys decide how much water to conserve or excrete. | This is why dehydration and fluid overload both show up so clearly in renal assessment. |
| ADH | Antidiuretic hormone helps the collecting ducts reabsorb more water. | When ADH is high, urine usually becomes more concentrated and output can fall. |
| Aldosterone | Promotes sodium reabsorption and potassium excretion in the distal nephron. | This helps defend circulating volume, but it can also worsen potassium loss. |
| Bicarbonate / acid-base | The kidneys help maintain pH by handling hydrogen ions and bicarbonate. | Renal dysfunction can contribute to metabolic acidosis and compensatory fast breathing. |
Important note
Children can move from compensated dehydration to significant clinical compromise faster than adults. Sodium and water problems are therefore never just "lab issues" if the child is also dry, tachycardic, drowsy, or not passing urine.
Perfusion pressure matters
RAAS link in plain English
Renal-cardiovascular connection
The relationship goes both ways. The kidneys need the circulation to work well, but injured kidneys can then worsen the cardiovascular picture by retaining fluid, disturbing electrolytes, and contributing to hypertension.
RAAS in three steps
Renin
Released when the kidney senses reduced perfusion or reduced sodium delivery.
Angiotensin II
Raises vascular tone and helps defend blood pressure.
Aldosterone
Promotes sodium and water retention, with potassium excretion.
| Test | Plain-English meaning | Why trends matter |
|---|---|---|
| Creatinine | A waste product used as a practical marker of filtration. | A rising creatinine usually means filtration is worsening. A 'normal' value can still be worrying if it has risen from that child's baseline. |
| Urea | A waste product from protein breakdown. | It can rise with dehydration as well as renal impairment, so interpret it with the overall clinical picture. |
| Sodium | A clue to water balance rather than just 'salt level'. | Look at symptoms and trends. Sodium problems can affect behaviour, consciousness, and seizure risk. |
| Potassium | One of the most urgent electrolytes to notice when abnormal. | If potassium is significantly high or low, escalate because the heart may be at risk. |
| Bicarbonate | A clue to acid-base balance. | A low bicarbonate can fit with dehydration, poor perfusion, sepsis, or renal dysfunction. |
| Urinalysis | A bedside test that can show blood, protein, nitrites, leucocytes, glucose, or ketones. | It does not replace blood tests, but it often gives the first clue about what direction the problem is taking. |
Trends beat snapshots
A single result matters less than the direction of travel. A creatinine that is still inside the reference range can still be clinically important if it has risen quickly, especially alongside oliguria, dehydration, or new oedema.
How to start your assessment
Start from the end of the bed
Before you chase numbers, look at alertness, colour, breathing pattern, puffiness, and whether the child looks dry or overloaded.
Measure output properly
Use catheter totals or accurate nappy weights, and match them to time. "Not much urine" is not good enough documentation.
Join the clues up
Urine output, daily weight, hydration, blood pressure, oedema, and bloods make more sense together than they do on their own.
| Urine output pattern | Rough bedside guide | What to think about |
|---|---|---|
| Usual bedside guide | Children should usually pass around at least 1 ml/kg/hour; neonates and young infants may pass more. | Always use local guidance and the wider clinical picture, but children generally need more urine per kg than adults. |
| Oliguria | Persistently reduced urine output. | At the bedside, a child drifting below roughly 1 ml/kg/hour should make you pause and review perfusion, hydration, and renal function. |
| Severe oliguria / anuria | Very little or no urine over hours. | This is urgent, especially if the child is also tachycardic, oedematous, hypertensive, or becoming drowsy. |
| How to measure it properly | Use catheter totals if present, or carefully weigh nappies and document time periods clearly. | A guessed output is not the same as a measured output. Trends over time matter more than one short interval. |
| Assessment | What it tells you | Key points |
|---|---|---|
| Urine output | A practical bedside marker of renal perfusion and filtration. | Track it accurately and look at the trend. Falling output is often an early warning sign. |
| Colour, clarity, and frequency | Can hint at hydration, infection, blood, or concentration. | Dark, cloudy, red, or unusually frothy urine should not be ignored. |
| Fluid balance chart | The running picture of intake versus output. | Useful, but only if it is complete. Missed drinks, vomits, flushes, or nappies make the chart misleading. |
| Daily weights | One of the best ways to spot fluid gain or fluid loss over time. | A 1 kg change is roughly 1 litre of fluid, so daily weights are often more reliable than memory-based fluid estimates. |
| Hydration status | How full the circulation and tissues appear to be. | Check mucous membranes, tears, capillary refill, skin turgor, sunken eyes or fontanelle, and overall alertness. |
| Blood pressure | A key renal and cardiovascular observation. | The kidneys both affect blood pressure and depend on it. Hypertension and hypotension can both matter. |
| Oedema | A clue that fluid is moving into tissues or that the body is retaining salt and water. | Look at eyelids, sacrum, ankles, hands, and whether clothes or nappies are tighter than expected. |
| Bloods and medication review | Shows whether the picture matches dehydration, AKI, electrolyte imbalance, or treatment effects. | NSAIDs, nephrotoxins, diuretics, and recent IV fluids all matter when you interpret the renal picture. |
Fluid management principles
Replace what is missing
If the child is dehydrated, the aim is to restore circulating volume safely and reassess whether urine output and perfusion improve.
Do not overdo it
IV fluids are a treatment, not neutral water. Too much can worsen oedema, blood pressure, and breathing if the kidneys cannot excrete it.
Keep reassessing
Weights, fluid balance, urine output, blood pressure, bloods, and the child's general appearance tell you whether your plan is helping.
Low-volume picture
Fluid-overload picture
| Condition | What's going on | Key signs |
|---|---|---|
| Dehydration | Reduced circulating volume lowers renal perfusion, so the kidneys conserve water and urine output falls. | Dry mucous membranes, reduced tears, tachycardia, delayed capillary refill, weight loss, concentrated urine |
| UTI | Infection anywhere in the urinary tract can irritate the bladder or ascend towards the kidneys. | Dysuria, frequency, foul-smelling urine, abdominal pain, fever, vomiting, irritability, or non-specific illness in younger children |
| AKI | An abrupt drop in kidney function caused by poor perfusion, intrinsic injury, or obstruction. | Falling urine output, rising creatinine, fluid retention, worsening electrolytes, acidosis, drowsiness |
| Fluid overload | Too much fluid is being retained or given for the child's current ability to excrete it. | Puffy eyes, oedema, rapid weight gain, rising blood pressure, crackles, increased work of breathing |
| Electrolyte imbalance | Sodium or potassium handling is disrupted by illness, renal dysfunction, or treatment. | Weakness, irritability, arrhythmia risk, altered consciousness, seizures, muscle cramps |
| AKI type | What the problem is | Common causes |
|---|---|---|
| Pre-renal | The kidneys are not being perfused properly. | Dehydration, sepsis, blood loss, poor cardiac output |
| Intrinsic renal | The kidney tissue itself is injured. | Inflammation, nephrotoxins, prolonged hypoperfusion, direct renal disease |
| Post-renal | Urine cannot drain properly because of obstruction. | Blocked catheter, anatomical obstruction, severe retention |
Pre-renal problem
This means the kidneys themselves may still be structurally intact, but they are not being perfused properly. Dehydration, haemorrhage, and sepsis can all cause this. If the cause is recognised early and corrected, renal function may recover quickly.
Intrinsic renal problem
This means the kidney tissue itself is injured. In that situation, simply giving fluid may not solve the whole picture, and the child may need more senior review, blood monitoring, or specialist input.
UTI in younger children
Urinary tract infection does not always present with classic dysuria in infants and younger children. Fever, vomiting, poor feeding, abdominal discomfort, irritability, or just a generally unwell child may be the clue.
| Link | What connects them | Bedside meaning |
|---|---|---|
| Renal + cardiovascular | The kidneys need blood flow and pressure to filter. If cardiac output or circulating volume drops, urine output often falls before blood pressure becomes dramatically abnormal. | Poor perfusion can trigger RAAS activation, fluid retention, and worsening blood pressure problems. |
| Renal + endocrine | The kidneys respond to ADH and aldosterone, release renin, activate vitamin D, and help with erythropoietin production. | Renal problems can therefore affect water handling, blood pressure control, bone health, and longer-term haemoglobin. |
| Renal + neurological | The brain is very sensitive to sodium, water shifts, uraemia, and severe hypertension. | Confusion, irritability, headache, seizures, or reduced consciousness can all be renal red flags. |
| Renal + respiratory | Fluid overload and metabolic acidosis both change breathing. | A child with renal dysfunction may become tachypnoeic because they are acidotic, or breathless because they are overloaded. |
Red flags - escalate immediately
Escalation mindset
Do not wait for the renal picture to become dramatic. A child with falling urine output, a dry appearance, rising heart rate, or new oedema is giving you a pattern. Escalate the pattern early rather than hoping the next set of numbers will explain it away.
Quick recap