Age-related physiology

In the progression from fetus through childhood to adulthood, important physiological differences and changes are seen in thermoregulation, control of breathing, pulmonary mechanics, fluid homeostasis, bilirubin metabolism, haemoglobin and cardiovascular physiology. Babies are especially prone to losing heat, and the smaller the baby, the greater this loss. Heat production is by non-shivering thermogenesis and the ideal environmental temperature for the neonate is known as the neutral thermal environment. In the kidneys, by 34 weeks' gestation, glomerulogenesis and nephrogenesis are complete. However, even at term, the glomeruli are much smaller than in adults, and tubular function does not reach adult levels until 12 months of age. Cardiovascular physiology and anatomy also change with age. In the newborn baby there is right ventricular dominance and these anatomical changes are reflected in the neonatal ECG, which has some important differences to paediatric and adult ECGs. Interpretation is therefore age related. Fetal red cells contain a unique haemoglobin (haemoglobin F) that has a higher affinity for oxygen and the oxyhaemoglobin dissociation curve is shifted to the left of the adult curve. From birth, haemoglobin concentrations fall and this physiological anaemia of the newborn is a normal adaptation to extrauterine life. In babies there is an increased rate of bilirubin production due to an increased red blood cell mass and shortened erythrocyte half-life. Bilirubin metabolism pathways are immature and a rise in unconjugated bilirubin in the first 2 weeks of life is almost universal.