Final answer:
The transition from fetal to neonatal circulation involves the closure of fetal shunts and initiation of lung function. The foramen ovale becomes the fossa ovalis, the ductus arteriosus becomes the ligamentum arteriosum, and severed umbilical vessels close or become fibrotic. Newborns undergo thermoregulatory adjustments to maintain body temperature in a cooler environment.
Step-by-step explanation:
Neonatal Circulatory Adjustments
The transition from fetal to neonatal circulation is a significant event in a newborn's life. This change involves the closure of the three primary fetal shunts: the ductus arteriosus, the foramen ovale, and the ductus venosus. At birth, a newborn's first breath is critical, as the inflation of the lungs leads to a decrease in blood pressure within the pulmonary system and the right side of the heart. This change in pressure reverses the flow of blood through the foramen ovale from the left to the right atrium, leading to the closure of this shunt. Subsequently, the tissue flaps over the foramen ovale usually fuse, creating the fossa ovalis within the first year of life.
The ductus arteriosus also constricts in response to the increased oxygen levels following the initiation of breathing, becoming the ligamentum arteriosum. The umbilical cord, once severed, leads to the collapse of the umbilical blood vessels, with the proximal umbilical arteries remaining part of the circulatory system, while the distal portions and the umbilical vein become fibrotic.
Thermoregulatory Adjustments
In the womb, the fetus is protected by the amniotic fluid's stable temperature. The birth process exposes the newborn to a colder environment, prompting the need for thermoregulatory adjustments. Newborns have a large surface area to volume ratio, immature musculature, and an underdeveloped nervous system, which all contribute to difficulty in maintaining body temperature. They rely on nonshivering thermogenesis, breaking down brown adipose tissue, to produce heat.