Circulation after birth

• From birth, the systemic and pulmonary circulations become wholly separated and are arranged in series
• Oxygenated blood enters the fetus from the umbilical vein
• The pulmonary system is, therefore, largely bypassed in the fetus with the help of the foramen ovale and the ductus arteriosus.
• The ductus venosus, meanwhile, allows for the oxygenated blood to reach the vital organs (e.g. heart and brain) more effectively
• The cutting of the umbilical cord and changes in the lungs after the newborn’s first couple of breaths lead to four drastic morphological changes in the circulatory system:

  • The ductus venosus degenerates
  • The foramen ovale closes
  • The ductus arteriosus degenerates
  • The umbilical system completely degenerates
• Degeneration of ductus venosus → The ductus venosus closes soon after birth, but rapid constriction of this vessel is not essential to the infant, because blood is no longer flowing through the umbilical vein. It takes a couple of days for the ductus venosus to regress to it remnant, the ligamentum venosum. Blood now passes trough all the liver sinusoids
• Closure of formanen ovale → The opening of the pulmonary circulation and the end of the umbilical blood flow create changes in the pressure and blood in the atria. The initial closing of the foramen ovale is a mechanical effect of the reversal of pressure between the two atria. The higher pressure in the left atrium presses the flexible septum primum towards the rigid septum secundum. The septa fuse by about three months after birth.
• Degeneration ductus arteriosus → The opening of the pulmonary circulation and the end of the umbilical blood flow also leads to restriction of the ductus arteriosus. Constriction of the ductus arteriosus occurs within 24 hours after birth (in infants born at term). The remnant of the ductus arteriosus is called the ligamentum arteriosum.
• degeneration of umbilical system → Cutting the umbilical cord results in an immediate cessation of blood entering the body via the umbilical vein. The intraembryonic parts of the placental circulation (the umbilical artery and veins) regress and form lateral umbilical ligaments. The attachment site of the umbilical cord forms the navel.