Neonatal
Adaptation for life: a review of neonatal physiology

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Abstract

The neonatal period (first 28 days of life or 44 weeks postconceptual age) contains the most dramatic and rapid physiological changes seen in humans. They vary from the immediate changes seen in the respiratory and cardiovascular systems to the slower progression seen in the hepatic, haematological and renal systems. These adaptations support life during the development from intrauterine physiology to adult physiology. This article describes neonatal physiological changes in a system-based approach, including slower changes that may extend beyond the neonatal period.

Section snippets

The fetal respiratory system

Lungs develop from the third week of gestation with completion of the terminal bronchioles by week 16. However, type I and II pneumocytes are distinguishable only by 20–22 weeks and surfactant is present only after 24 weeks, making this the watershed time for pulmonary gas exchange and therefore extra-uterine survival. Surfactant production can be increased after 24 weeks by giving betamethasone to the mother, thereby improving neonatal lung function if premature delivery is anticipated.

The fetal circulation

Oxygenated placental blood is preferentially delivered to the brain, myocardium and upper torso, with lower oxygen tension blood distributed to the lower body and placenta. Preferential splitting is achieved via intra- and extracardiac shunts that direct blood into two parallel circulations (Figure 1). Oxygenated blood returning from the placenta divides equally to pass either through the liver or via the ductus venosus to reach the inferior vena cava. Oxygenated blood from the ductus venosus

Haematology

Neonatal blood contains both adult (HbA) and fetal haemoglobin (HbF). HbF is made up of the four globin chains α2γ2 in contrast to HbA, which is made up of α2β2. This structural difference of HbF provides a greater affinity for oxygen and helps maintain the molecular structure and function in a more acidic environment. The increased oxygen affinity of HbF facilitates oxygen transfer across the placenta from maternal HbA. Postdelivery, the high oxygen affinity of HbF becomes detrimental as

Heat loss

Neonates and, in particular, premature neonates are at high risk of heat loss and subsequent hypothermia. Hypothermic preterm babies have a poor outcome in the intensive care setting and therefore body temperature must be aggressively regulated. Neonates have a 2.5–3.0 times higher surface area to bodyweight ratio compared with adults, increasing the relative potential surface for heat loss. This is exacerbated by the limited insulating capacity from subcutaneous fat and the inability of

Hepatic

Most enzymatic pathways are present in the neonate, but are inactive at birth and generally become fully active at 3 months postdelivery. An example of this is the conjugation pathway for bilirubin, which is inactive at birth but is fully established at 2 weeks. Unconjugated bilirubin levels rise during the first 48 hours because of the rapid breakdown of HbF and poor conjugating abilities of the immature liver. This rise can be exacerbated in the presence of haemolysis, sepsis, dehydration or

Renal

A full complement of 1 million nephrons is present by 34 weeks’ gestation. The glomeruli and nephrons are immature at birth, resulting in a reduced glomerular filtration rate (GFR) and limited concentrating ability. Lack of renal medulla osmotic gradient and absence of medullary tubules limit urinary concentrating ability. The concentrating capacity of the neonatal kidney (600 mOsm/kg) is about half that of the adult (1200–1400 mOsm/kg). GFR is gestational age related; the GFR is reduced with

Body fluid composition

At term, 75% of neonatal bodyweight is water, which is distributed predominantly in the extracellular compartment (40%). In preterm neonates the water content is even higher at 80–85% of bodyweight, split in a ratio of 2:1, extracellular to intracellular. For the first 12–24 hours of life, urine output is limited to 0.5 ml/kg/hour due to poor renal perfusion, which improves with circulatory adaptation. Following this initial oliguric phase, a period of natriuresis ensues. Isotonic fluid is lost

Nutrition

The neonate must rapidly adapt from receiving all nutrient and energy requirements via the placenta to obtaining them orally. In utero, the gastrointestinal tract is fully formed by 25 weeks and by term provides approximately 0.3 g/kg/24 hours of protein from swallowed amniotic fluid. During the last 6 weeks of normal gestation body fat deposition almost doubles to 15% of bodyweight. Glycogen stores increase during the last 9 weeks of gestation to 2–3 times that seen in adults. These late

Nervous system

The nervous system is precocious in development compared with other organ systems and accounts for 10% of total body weight at birth. This system is immature and continues to develop to achieve a full complement of cortical and brainstem cells by 1 year. The brain increases its size threefold during the first year of life, producing a high metabolic demand. This is reflected in the neonatal cerebral circulation receiving one-third of cardiac output compared with one-sixth of cardiac output in

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