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Please wait while the page loadsAnatomy & Physiology · Children's & Adult Nursing
How the body keeps its internal balance, where fluid sits, what electrolytes actually do, and how dehydration and overload show up differently in children and adults.
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Student note
A cation is a positively charged ion (sodium and potassium are the two you will use most). Osmosis is water moving towards a higher solute concentration. ORS stands for oral rehydration solution. U&Es refers to the urea and electrolytes blood test used to check kidney function and electrolyte balance.
Core loop
Receptor detects change → control centre decides → effector responds. Most homeostasis topics use this pattern.
Age rule
More body water, more of it extracellular, higher metabolic rate – infants dehydrate faster than adults.
Potassium rule
Both high and low potassium can cause dangerous arrhythmias – treat abnormal results with urgency.
Trend rule
A single fluid balance number matters less than the trend over the last 24–48 hours.
How does the body keep its internal environment stable?
What homeostasis means
Negative feedback
Positive feedback
Key players
Clinical pearl
Almost every system you learn about – temperature, blood glucose, blood pressure, fluid balance – comes back to this same loop: receptor detects change, control centre decides, effector responds. Learning that one pattern well makes many other topics easier to understand.
Where is water held in the body, and how does this change with age?
Fluid compartments
Water movement basics
Why infants differ
Why this matters clinically
Clinical pearl
Infants are not just "small adults" when it comes to fluid balance. A higher proportion of body water, more of it in the easily-lost extracellular compartment, and a higher metabolic rate all mean the same percentage fluid loss can become clinically significant much faster than in an adult.
What do the main electrolytes actually do, and what happens when they go out of range?
Sodium (Na⁺)
Potassium (K⁺)
Calcium (Ca²⁺)
Magnesium (Mg²⁺)
Red flags
Clinical pearl
Potassium is the electrolyte most likely to kill quickly if badly deranged, because both very high and very low levels can cause life-threatening cardiac arrhythmias. Any significantly abnormal potassium result deserves prompt attention, not a "we’ll check again later" approach.
How do you assess and grade dehydration in a child?
Why children are higher risk
Clinical signs to check
Grading severity
Management approach
Red flags
Clinical pearl
A child can look reasonably well and then deteriorate quickly once dehydration becomes severe, because children compensate well until they suddenly do not. Reassessing regularly matters more than relying on a single snapshot assessment.
What changes in adult fluid assessment, especially in older adults and long-term conditions?
Dehydration in adults
Fluid overload
Assessment in adults
Special situations
Red flags
Clinical pearl
Skin turgor is a less reliable sign of dehydration in older adults because skin naturally loses elasticity with age. New or worsening confusion is often a more useful early clue in this group and should not automatically be written off as "just their normal."
How do you monitor and document fluid balance accurately at the bedside?
Fluid balance chart
Bedside checks
Escalation triggers
Communication
Clinical pearl
A single fluid balance figure tells you much less than a trend. A patient who is "balanced" today after being significantly negative yesterday still needs watching – the direction of travel often matters as much as the number itself.
| Compartment | Location | Note |
|---|---|---|
| Intracellular fluid (ICF) | Inside cells | The largest fluid compartment by volume; potassium is the dominant cation here. |
| Interstitial fluid | Between cells, outside blood vessels | Part of the extracellular fluid; bathes tissues and exchanges with plasma and cells. |
| Plasma | Inside blood vessels | The fluid portion of blood; sodium is the dominant cation in the extracellular fluid overall. |
ECF splits into interstitial fluid (around the cells) and plasma (inside vessels) — proportions are illustrative
| Electrolyte | Typical adult range | Main role |
|---|---|---|
| Sodium (Na⁺) | Roughly 135–145 mmol/L (adult) | Fluid balance, nerve conduction, acid-base balance |
| Potassium (K⁺) | Roughly 3.5–5.0 mmol/L (adult) | Cardiac rhythm, nerve and muscle function |
| Calcium (Ca²⁺) | Roughly 2.2–2.6 mmol/L (adult, total) | Bone strength, muscle contraction, blood clotting |
| Magnesium (Mg²⁺) | Roughly 0.7–1.0 mmol/L (adult) | Enzyme function, muscle and nerve activity |
Student note
These ranges are typical adult reference ranges used widely in UK nursing education. Paediatric ranges can differ slightly by age, and every lab publishes its own local reference range on the result – always check the range printed alongside the result rather than relying on memory alone.
| Direction | Signs | Common causes |
|---|---|---|
| Hyponatraemia (low) | Headache, nausea, confusion, lethargy; severe cases can cause seizures. | Excess water relative to sodium, some diuretics, SIADH, vomiting with water replacement only. |
| Hypernatraemia (high) | Thirst, dry mucous membranes, confusion, irritability, lethargy. | Insufficient water intake, excessive water loss, or excess sodium intake relative to water. |
| Direction | Signs | Common causes |
|---|---|---|
| Hypokalaemia (low) | Muscle weakness, cramps, fatigue, cardiac arrhythmias. | Diuretics, vomiting, diarrhoea, inadequate intake. |
| Hyperkalaemia (high) | Muscle weakness, tingling, potentially dangerous cardiac arrhythmias. | Renal impairment, certain medications, tissue injury, excess intake. |
| Grade | Typical signs | Usual approach |
|---|---|---|
| Mild (no clinical signs) | Child looks well, alert, normal mucous membranes and skin turgor. | Encourage normal fluids; ORS often not needed unless losses continue. |
| Moderate | Slightly reduced activity, dry mucous membranes, reduced urine output. | Oral rehydration solution (ORS) given in small, frequent amounts is usually first line. |
| Severe | Lethargy, sunken eyes/fontanelle, prolonged capillary refill, features of shock. | Needs urgent assessment; IV fluids are often required alongside close monitoring. |
| Entry type | What it includes |
|---|---|
| Input | Oral fluids, IV fluids, enteral feed, any fluid given with medications. |
| Output | Urine, vomit, diarrhoea, drain losses, stoma output, significant wound exudate. |
| Running total | Input minus output, usually calculated over a 24-hour period, reviewed at handover. |
Clinical pearl
If you are worried about fluid status in either a child or an adult: check trends not single numbers, look at the whole clinical picture (alertness, colour, perfusion, urine output), escalate early, and always cross-reference against U&E results where available rather than fluid balance alone.
Feedback types
Negative reverses change (common); positive amplifies change (rare, e.g. labour)
Compartments
ICF (inside cells) is largest; ECF splits into interstitial fluid and plasma
Big two ions
Sodium: mainly extracellular. Potassium: mainly intracellular. Both critical to monitor
Escalate on
Confusion, breathlessness, reduced urine output, or abnormal potassium result
Sources & References
Waugh A & Grant A (2018) — Ross and Wilson Anatomy and Physiology in Health and Illness (13th edn)NICE (2015, updated 2021) — Intravenous fluid therapy in children and young people in hospital (NG29)NICE (2013, updated 2017) — Intravenous fluid therapy in adults in hospital (CG174)NICE (2009, updated 2019) — Diarrhoea and vomiting caused by gastroenteritis in under 5s (CG84)Also practice with