Transition to extra-uterine life
Prior to birth, the placenta is the conduit that provides oxygen and nutrition to the fetus.
In the fetal circulation, pulmonary vascular resistance is high and right ventricular output is shunted across the ductus arteriosus into the systemic circulation, bypassing the lungs.
At birth, dramatic changes occur in the fetal circulation that allow transition to a lung-based gas exchange.
Pulmonary vascular resistance falls and the ductus arteriosus closes. Right ventricular output now circulates through the lungs before entering the systemic circulation via the left atrium.
Changes also occur in other organ systems...
Availability of oxygen from the lungs is better than from the placenta. The RBC count and HCT falls.
Enzyme systems are activated in the liver. For example, glucorynl transferase activation is essential for conjugating the bilirubin produced from the disintegrating RBCs.
The kidneys assume responsibility for fluid and electrolyte homeostasis.
The immune system has to mature (point to fetal immunity...see below).
More Info: Fetal Circulation