Fetal Circulation by Lisa McCabe. Hello, my name is Lisa McCabe. I'm a clinical nurse specialist at Children's Hospital Boston in the cardiovascular program.
We are now going to talk about fetal circulation and the transition to postnatal circulation that occurs after birth. The specific structures associated with fetal circulation include the placenta, the umbilical vein, the ductus venosus, the foramen ovale, the ductus arteriosus, and the umbilical arteries. Blood rich in nutrients and oxygen supplied via the placenta flows through the umbilical vein to the ductus venosus.
Blood flows from the ductus venosus into the inferior vena cava up to the right atrium. This blood mixes with blood returning to the heart from the upper body via the superior vena cava and from the lower body via the inferior vena cava. Once in the right atrium, some of the blood flows to the right ventricle and some of the blood flows through the foramen ovale to the left atrium and into the left ventricle, where it is then pumped out into the aorta to the body.
Point of clarification. The flow of blood from the right atrium to the left atrium via the foramen ovale enables oxygenated blood from the placenta to reach the coronary arteries and cerebral circulation. the two most metabolic active organs in the developing fetus.
Blood that flows into the right ventricle is then pumped into the pulmonary artery. Because the lungs are fluid-filled instead of air-filled, the vessels in the lungs are narrow, creating higher resistance to blood flow into the lungs. Due to this high resistance to blood flow in the pulmonary circulation and the low resistance to blood flow, In the systemic circulation, blood pumped into the pulmonary artery by the right ventricle is more likely to flow into the ductus arteriosus and then into the aorta.
Due to the high blood flow from the superior vena cava and the inferior vena cava up to the right atrium, pressures in the right atrium are higher than in the left atrium. This promotes blood flow through the foramen ovale at the atrial level. Only 8% of right ventricular output flows into the lungs, providing nutrients for the developing lung tissues.
Most of the blood passes through the ductus arteriosus into the aorta and out to the body. The umbilical arteries allow blood to flow from the body back to the placenta to be enriched with oxygen and nutrients. Once the baby is born and takes a breath, the umbilical cord is clamped and the placenta is removed from the systemic circulation. Immediately the transition from fetal to postnatal circulation begins. With the elimination of the placenta from the circulation, systemic vascular resistance begins to rise.
With each breath, more alveoli in the lungs expand and the vessels surrounding them dilate in response to the presence of oxygen. Pulmonary pressures begin to decrease. Although pulmonary pressures are lower than systemic pressures within minutes after birth, it is six to eight weeks before pulmonary vascular resistance decreases to normal.
In the postnatal circulation, blood no longer flows through the foramen ovale or the ductus arteriosus. The increase in left atrial pressure after birth forces the septum primum against the septum secundum. functionally closing the foramen ovale. Within three months, the foramen ovale permanently closes as fibrin deposits fuse the layers of septal wall together. Point of clarification.
While the patent foramen ovale or PFO may close by this time, it also may not. Autopsy studies in adults indicate that 15 to 25% of adults have a patent foramen ovale. which never close. The shunt via the PFO after birth is usually small. The ductus arteriosus begins to close shortly after birth, once the infant begins to breathe.
Normally, the ductus closes completely in 4 to 10 days. The ductus venosus is open at the time of birth, making central venous access possible through the umbilical vein. As fibrin infiltrates the ductus venosus, it usually closes within 3 to 7 days.
After it closes, the remnant is known as ligamentum venosum. Within a week after birth, the umbilical vein and umbilical arteries are infiltrated with fibrin and also become ligaments. Thank you for your time. This concludes our lecture on fetal circulation.
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