Clinical overview
Birth is the most abrupt physiological transition a human ever undergoes. In the space of a few minutes the fetus must convert from an organism whose lungs are fluid-filled and bypassed, whose oxygenation and clearance are entirely placental, and whose circulation runs largely in parallel, into a neonate that ventilates air, oxygenates across its own alveoli, and runs its circulation in series. Most babies accomplish this themselves: roughly 85% breathe spontaneously within 10–30 seconds of birth and a further 10% respond to drying and stimulation, so that only about 5% require any active resuscitation and around 1% need extensive intervention (standard ILCOR/NRP teaching). The registrar's task is to understand the physiology well enough to recognise the small but important minority in whom transition fails, and to support — not interrupt — the majority in whom it succeeds.
The "first few days" of the objective spans three overlapping phases: the immediate cardiorespiratory transition (minutes to hours), consolidation of an air-breathing circulation and metabolic independence over the first 24 hours, and the slower closure of fetal shunts, establishment of feeding, thermal and glycaemic stability, and the early detection of disease over days one to three. Failure at any phase has obstetric relevance because the commonest precipitants — intrapartum hypoxia, prematurity, meconium, maternal sedation, infection, and cold stress — are largely things the labour ward and theatre can either cause or prevent. This chapter sits alongside Initiation of respiration and Neonatal resuscitation; the emphasis here is the physiology of adaptation and what to watch for as it unfolds.
Core knowledge
The fetal starting point
In utero the lungs are not collapsed but distended with fetal lung fluid actively secreted by the pulmonary epithelium via chloride-driven transport. Pulmonary vascular resistance (PVR) is high — pulmonary arterioles are thick-walled and constricted in the low-oxygen fetal environment — so only a small fraction of combined ventricular output perfuses the lungs. The rest is diverted through two right-to-left shunts: the foramen ovale (right atrium to left atrium) and the ductus arteriosus (pulmonary artery to descending aorta). Oxygenated blood returns from the placenta via the umbilical vein, partly bypassing the liver through the ductus venosus. The result is a parallel circulation tuned to deliver the best-oxygenated blood to the heart and brain.
The respiratory switch
Figure L3.1 — How the newborn moves from placental parallel circulation to air-breathing series circulation: lung liquid clears, PVR falls, SVR rises and fetal shunts functionally close while remaining reactive.
Three events must coincide. First, lung fluid must clear. Labour itself drives a catecholamine and steroid surge that switches the alveolar epithelium from chloride/fluid secretion to sodium absorption through epithelial sodium channels, beginning reabsorption before delivery; the remainder is cleared by the mechanical and transpulmonary pressure changes of the first breaths and by lymphatic and capillary uptake. This is the physiological reason elective caesarean before labour, particularly before 39 weeks, carries a higher rate of transient tachypnoea of the newborn (TTN) — the lung-fluid clearance program has not been triggered.
Second, the first breaths must aerate the lung. The initial inflations generate large negative (and, with crying, positive) transpulmonary pressures — classically of the order of tens of cmH₂O — to overcome surface tension and the viscosity of residual fluid, and to establish a functional residual capacity. Surfactant, produced by type II pneumocytes and present in mature quantity from roughly 34–35 weeks, lowers alveolar surface tension and prevents end-expiratory collapse; its deficiency is the basis of respiratory distress syndrome (RDS) of prematurity and the rationale for antenatal corticosteroids (see Preterm birth and pprom).
Third, oxygenation must rise. As the alveoli fill with air, alveolar oxygen tension climbs.
The circulatory switch
Lung aeration is the master switch for the circulation. Alveolar distension and the rise in oxygen tension cause a sharp fall in pulmonary vascular resistance, so pulmonary blood flow increases several-fold. This raises pulmonary venous return and left atrial pressure. Simultaneously, clamping the cord (or the natural cessation of umbilical flow) removes the low-resistance placental bed, raising systemic vascular resistance. Left atrial pressure now exceeds right atrial pressure, functionally closing the foramen ovale. The ductus arteriosus constricts in response to the rising arterial oxygen tension and the fall in circulating placentally-derived prostaglandins; functional closure usually occurs within the first day or two, with anatomical closure over the following weeks. The ductus venosus closes as umbilical flow ceases. The circulation has gone from parallel to series.