Clinical meaning
The transition from intrauterine to extrauterine life is the most complex physiological adaptation a human being undergoes. Within seconds to minutes of birth, the neonate must transition from dependence on the placenta for gas exchange to independent pulmonary respiration, and the cardiovascular system must convert from a parallel fetal circulation to a series adult circulation. Understanding these transition mechanisms is essential to appreciating what the APGAR score measures and why rapid assessment is critical. In utero, the fetal lungs are filled with approximately 30 mL/kg of lung fluid produced by the pulmonary epithelium. The fetal pulmonary vascular resistance is very high (because the fluid-filled alveoli compress the capillaries), so only about 10% of cardiac output flows through the fetal lungs. The majority of oxygenated blood from the placenta bypasses the lungs through two shunts: the foramen ovale (an opening between the right and left atria) and the ductus arteriosus (a vessel connecting the pulmonary artery to the aorta). At birth, the first breath generates a negative intrathoracic pressure of -40 to -100 cmH2O, which draws air into the alveoli, displaces...