Fetal Circulation PDF Notes

Fetal circulation 1 • The fetal circulation is the circulatory system of a human fetus, often encompassing the entire feto-placental circulation which includes the umbilical cord and the blood vessels within the placenta that carry fetal blood. 2 • The fetal circulation works differently from...

Fetal circulation 1 • The fetal circulation is the circulatory system of a human fetus, often encompassing the entire feto-placental circulation which includes the umbilical cord and the blood vessels within the placenta that carry fetal blood. 2 • The fetal circulation works differently from that of born humans, mainly because the lungs are not in use. • Instead, the fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord. 3 4 5 6 7 Placental role • The core concept behind fetal circulation is that fetal hemoglobin (HbF) has a higher affinity for oxygen than does adult hemoglobin, which allows a diffusion of oxygen from the mother's circulatory system to the fetus. 8 • The circulatory system of the mother is not directly connected to that of the fetus, so the placenta functions as the respiratory center for the fetus as well as a site of filtration for plasma nutrients and wastes. • Water, glucose, amino acids, vitamins, and inorganic salts freely diffuse across the placenta along with oxygen. 9 • The uterine arteries carry blood to the placenta, and the blood permeates the sponge-like material there. • Oxygen then diffuses from the placenta to the chorionic villus, an alveolus-like structure, where it is then carried to the umbilical vein. 10 Circuit • Blood from the placenta is carried to the fetus by the umbilical vein. • Less than a third of this enters the fetal ductus venosus and is carried to the inferior vena cava, while the rest enters the liver proper from the inferior border of the liver. 11 • The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein. • The blood then moves to the right atrium of the heart. • In the fetus, there is an opening between the right and left atrium (the foramen ovale), and most of the blood flows through this hole directly into the left atrium from the right atrium, thus bypassing pulmonary circulation. 12 • The continuation of this blood flow is into the left ventricle, and from there it is pumped through the aorta into the body. • Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the maternal circulation. 13 • Some of the blood entering the right atrium does not pass directly to the left atrium through the foramen ovale, but enters the right ventricle and is pumped into the pulmonary artery. • In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus, which directs most of this blood away from the lungs (which are not being used for respiration at this point as the fetus is suspended in amniotic fluid). 14 At birth • At birth, when the infant breathes for the first time, there is a decrease in the resistance in the pulmonary vasculature, which causes the pressure in the left atrium to increase relative to the pressure in the right atrium. 15 • This leads to the closure of the foramen ovale, which is then referred to as the fossa ovalis. • Additionally, the increase in the concentration of oxygen in the blood leads to a decrease in prostaglandins, causing closure of the ductus arteriosus. • These closures prevent blood from bypassing pulmonary circulation, and therefore allow the neonate's blood to become oxygenated in the newly operational lungs. 16 17 18 • Due to the higher pressure of the blood in the inferior vena cava, more blood flows from it directly into the left atrium via the foramen ovale. • The foramen ovale opens like a valve and can direct the blood stream that comes from below directly into the left atrium. Taken together, the diameters of the inferior and superior vena cava are larger than that of the foramen ovale and therefore a small portion of the blood seeps into the right ventricle via the tricuspid valve. The heart is filled only with a mixed blood (O2 saturation). 19 • Shunts for the blood in the developing heart serve a very practical need. Initially all of the blood returns to the right atrium. • However, because the lungs develop very late, the pulmonary vessels are limited in their capacity and the resulting resistance is very high. 20 • Thus, the pulmonary circulation system cannot deal with all the blood. • So the circulation system remains balanced nonetheless, there are two shunts that provide shortcuts for most of the pulmonary circulation system. • First there is a direct connection between the right and left atria in that the blood from the right atrium flows directly into the left via the foramen ovale and thus goes around the pulmonary circulation system. 21 • This shunt allows a normal development of the left atrium and the left ventricle in that the cardiac musculature on this side is trained. • On the other hand, the right ventricle could also not develop correctly when no blood would flow via the right side. • Thus in fetal hearts blood also flows in small amounts from the right atrium via the tricuspid valve into the right ventricle but, going through the truncus pulmonalis it takes a shortcut into the aorta via the ductus arteriosus. 22 Fetal shunts The two cardiac shunts: • Connection between the right and left atria via the foramen ovale • Connection between the truncus pulmonalis and the aorta via the ductus arteriosus The Liver shunt: • ductus venosus 23 24