During pregnancy, the fetal circulatory system works differently than after birth: The fetus is connected by the umbilical cord to the placenta, the organ that develops and implants in the mother's uterus during pregnancy. Through the blood vessels in the umbilical cord, the fetus receives all the necessary nutrition, oxygen, and life support from the mother through the placenta. Waste products and carbon dioxide from
the fetus are sent back through the umbilical cord and placenta to the mother's circulation to be eliminated. The fetal circulatory system uses two right to left shunts, which are small passages that direct blood that needs to be oxygenated. The purpose of these shunts is to bypass certain body parts? in particular, the lungs and liver ? that are not fully developed while the fetus is still in the womb. The shunts that bypass the lungs are called the foramen ovale, which moves blood from the right atrium of the heart to the left atrium, and the ductus arteriosus, which moves blood from the pulmonary artery to the aorta. Oxygen and nutrients from the mother's blood are transferred across the placenta to the fetus. The enriched blood flows through the umbilical cord to the liver and splits into three branches. The blood then reaches the inferior vena cava, a major vein connected to the heart. Most of this blood is sent through the ductus venosus, also a shunt that passes highly oxygenated blood through the liver to the inferior vena cava and then to the right atrium of the heart. A small amount of this blood goes directly to the liver to give it the oxygen and nutrients it needs. Waste products from the fetal blood are transferred back across the placenta to the mother's blood. Inside the fetal heart:
At birth, the umbilical cord is clamped and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of life, the lungs begin to expand. As the lungs expand, the alveoli in the lungs are cleared of fluid. An increase in the baby's blood pressure and a significant reduction in the pulmonary pressures reduces the need for the ductus arteriosus to shunt blood. These changes promote the closure of the shunt. These changes increase the pressure in the left atrium of the heart, which decrease the pressure in the right atrium. The shift in pressure stimulates the foramen ovale to close. The closure of the ductus arteriosus and foramen ovale completes the transition of fetal circulation to newborn circulation.
1.4. The transition from intra-uterine to extra-uterine lifeTo appreciate the reasons why a newly born infant may require more than routine care following birth it is imperative to have at least a basic understanding of the physiological changes that need to occur in order for an infant to undergo a successful transition from intra-uterine to extra-uterine life. The boxes below compare fetal life in utero, where the fetus is entirely dependent upon its mother and the function of the placenta for survival, to extra-uterine life, where the newborn must take over the role of oxygenation in order to survive. Features of intra-uterine (fetal) life
Features of transition to extra-uterine (post-natal) life
Table 1: Comparison of fetal and postnatal circulation
Oxygen saturations after birthWith the rise in blood oxygen content over the first few minutes of life, arterial oxygen saturation (SaO2) will rise. Recent studies have demonstrated that in healthy, uncompromised newborns, oxygen saturation rises from intrapartum levels of 40- 50%, increasing to a mean of 60% by one minute of age and reaching a mean of 90% by seven to ten minutes of post natal age when measured via pulse oximetry (SpO2). (Dawson, et al., 2007).
What changes occur in the baby's cardiovascular and respiratory systems at birth?Once the baby takes the first breath, a number of changes occur in the infant's lungs and circulatory system: Increased oxygen in the lungs causes a decrease in blood flow resistance to the lungs. Blood flow resistance of the baby's blood vessels also increases. Fluid drains or is absorbed from the respiratory system.
What cardiovascular changes occur after birth?The transition to newborn life at birth involves major cardiovascular changes that are triggered by lung aeration. These include a large increase in pulmonary blood flow (PBF), which is required for pulmonary gas exchange and to replace umbilical venous return as the source of preload for the left heart.
What are the 3 changes that happen to the cardio pulmonary system at birth?The newborns transition from fetal to neonatal life includes aeration of the lungs, establishment of pulmonary gas exchange and a changing the fetal circulation into the adult phenotype.
What adaptations help newborn infants to survive?The most essential adaptation to birth is the initiation of breathing, but the airspaces of the fetal lung are filled with fetal lung fluid.
|