Loading...
Please wait while the page loadsChildren's Nursing · Paediatric Assessment
How the kidneys make urine, control fluid and electrolytes, what to check at the bedside, and which signs mean a child needs help early.
Save for later
Student note
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.
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.
What does the renal system actually do, and why does it matter in children?
Filtration system
Why children matter more
Urinary tract order
Extra kidney jobs
Red flags
Clinical pearl
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.
How is the kidney organised, and what does the nephron do?
Kidney layout
Nephron basics
Nephron route
Fine-tuning and drainage
Clinical pearl
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.
What are the steps from blood to urine?
Filtration
Reabsorption
Secretion
Excretion
Clinical pearl
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 are the kidneys balancing all day, and which hormones help?
Sodium and water
Potassium
ADH
Aldosterone and RAAS
Red flags
Clinical pearl
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.
What should you check at the bedside, and how do you measure it properly?
Start from the end of the bed
Urine output
Fluid balance and weight
Blood pressure, oedema, and bloods
Red flags
Clinical pearl
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.
Which conditions should you recognise, and why do renal problems affect everything?
Dehydration and UTI
AKI types
Fluid overload and electrolyte imbalance
System connections
Red flags
Clinical pearl
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.
| 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. |
Low-volume picture
Fluid-overload picture
| 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. |
Urinary tract
Urine formation
Hormonal control
Assessment rule
Sources & References
Waugh A & Grant A (2018) — Ross and Wilson Anatomy and Physiology in Health and Illness (13th edn)NICE (2019) — Acute kidney injury: prevention, detection and management (NG148)KDIGO (2012) — Clinical Practice Guideline for Acute Kidney InjuryNICE (2014) — Chronic kidney disease in adults: assessment and management (NG203)Also practice with