The body composition of the fetus changes dramatically during gestation. Total body water, as a proportion of body weight, progressively decreases with advancing gestation (Fig. 8.2). Total body water represents 85 % of body weight in premature infants, 75 % in full-term neonates and 60 % in older children [1]. After birth, excess total body water is mobilized and excreted. Premature infants ( Fig. 8.2) mature through several distinct phases before achieving fluid homeostasis. The pre-diuretic phase, which occurs in the first 24-48 postnatal hours, is marked by urine outputs in the range of 0.5-1.5 ml/kg/h. During the subsequent diuretic phase, urine output increases to 3-5 ml/kg/h with a decrease in sodium excretion. As a result, there is 10-15 % weight loss during the first 5-7 postnatal days (with a weight loss of up to 20 % in infants <750 g). The precise mechanisms underlying the contraction of body fluids in the first few postnatal days are not clear but have been attributed, in part, to an atrial natriuretic peptide (ANP) diuresis secondary to increased pulmonary blood flow and stretch of left atrial receptors. Another contributing factor to this diuresis and weight loss during the first postnatal week (largely extracellular fluid loss) is tubular insensitivity to aldosterone [2, 3].
Postnatal Changes in Hypothalamic, Adrenal, and Renal Physiology
Atrial Natriuretic PeptidesThe natriuretic peptides are involved in attenuating the reninangiotensin-aldosterone axis and sympathetic nervous system; suppression of vasopressin release; vasodilatation of the systemic, pulmonary, coronary, and renal circulations; and promotion of natriuresis and diuresis. When compared with older infants and children, neonates in the first few postnatal days have significantly greater circulating levels of natriuretic hormone, perhaps related to the acute increase in ventricular afterload that occurs after birth [3,4]. Plasma B-type natriuretic peptide (BNP) levels in neonates with respiratory disease, with persistent pulmonary hypertension of the newborn, are significantly greater than in those with normal right ventricular pressure. In children with known congenital heart disease, natriuretic hormone levels vary with the type and severity of the heart defect [5].
Hypothalamic-Pituitary-Adrenal AxisRemodelling of the adrenal glands after birth occurs via apoptosis of the fetal zone, by remodelling this zone and the development of other zones [6]. At birth, the concentration of free cortisol is only one-third that of maternal levels, with an inverse relationship between cortisol levels and gestational age. The size of the adrenal glands decreases by 25 % during the first 4 postnatal days. In full-term and late preterm neonates, low cord concentrations of ACTH, cortisol and free triiodothyronine are associated with lung fluid retention and transient tachypnea of the newborn (TTN) [7]. Transient insufficiency of the adrenal cortex has been reported in approximately 27 % of ELBW infants and critically ill neonates (defined as an inab...
