Pathophysiology
Clinical meaning
Children are not small adults; their organ systems are developmentally immature with higher metabolic rates, more precarious fluid-electrolyte balance, and developing immune systems. Growth follows predictable patterns: birth weight doubles by 4-6 months and triples by 12 months, posterior fontanelle closes by 2-3 months, anterior fontanelle by 12-18 months. Development proceeds cephalocaudal (head to toe) and proximodistal (center to periphery). Vital sign ranges are age-dependent, and pediatric dehydration can progress rapidly because children have higher body surface area to weight ratios and greater insensible fluid losses.