👶 Paediatric Anatomy & Physiology
How children differ from adults anatomically and physiologically — covering the airway, cardiovascular system, respiratory system, fluid balance, renal function, thermoregulation, and age-based normal values relevant to the MRCS examination.
Overview & Age Definitions
Children are not simply small adults. Anatomical proportions, physiological parameters, and the response to injury and illness differ substantially across the paediatric age spectrum. Understanding these differences is fundamental to safe perioperative and surgical care of children.
Children’s bodies differ fundamentally in composition from adults — these differences have direct relevance to drug pharmacokinetics, fluid management, and vulnerability to environmental stressors.
| Parameter | Neonate / Infant | Adult | Clinical relevance |
|---|---|---|---|
| Total body water | 75–80% body weight | 55–60% body weight | Greater dilution of water-soluble drugs; higher volume of distribution |
| Extracellular fluid (ECF) | ~45% body weight | ~20% body weight | Larger ECF → greater susceptibility to electrolyte shifts |
| Body fat | Very low (neonates ~10%) | ~15–30% | Less fat-soluble drug reservoir; reduced insulation → heat loss |
| Surface area : mass ratio | Very high | Lower | Rapid heat and water loss; higher metabolic rate per kg |
| Plasma protein | Reduced (albumin lower) | Normal | More unbound (active) drug fraction; altered pharmacodynamics |
| Glycogen stores | Minimal (especially preterm) | Adequate | Risk of hypoglycaemia during fasting or stress — dextrose-containing fluids often needed |
The Weight Estimation Formulae
Exact weight is essential for drug dosing and fluid calculation. When scales are unavailable, use:
- Age 1–5 years: Weight (kg) = 2 × (age + 4)
- Age 6–12 years: Weight (kg) = 3 × age + 7
- Broselow tape: Length-based estimation — most accurate for children under 25 kg
- APLS formula (age 1–10): Weight (kg) = 2 × (age + 4)
Children have a significantly higher basal metabolic rate per kilogram than adults. Oxygen consumption in a neonate is approximately 6–8 mL/kg/min, compared to 3–4 mL/kg/min in an adult. This means:
- Neonates desaturate far more rapidly during apnoea (smaller FRC relative to oxygen demand)
- Caloric requirements per kg are substantially higher
- Carbon dioxide production is proportionally elevated → need for higher minute ventilation per kg
- Hypoglycaemia develops more rapidly during fasting or physiological stress