Human fetal circulation differs from adult human circulation or even those of children in so many ways. This article outlines most of these differences.
What are the reasons for the differences in fetal circulation and adult circulation?
Fetal circulation differs from humans who are already born because of the difference in the circulatory system. Fetuses do not use their lungs, but rely instead on obtaining oxygen and nutrients from the mother through the placenta and umbilical cord.
How does the mother supply oxygen and nutrients to the fetus?
The maternal blood supplies the fetus with oxygen and nutrients (water, glucose, amino acids, vitamins, and inorganic salts) and carries away its wastes. The mother does this all throughout the fetal stage of development. Oxygen, nutrients and waste products diffuse back and forth between the maternal and fetal blood through the placental membrane. The umbilical blood vessels carry these substances to and from the fetal body.
Fetal hemoglobin exists in higher percentages in the fetal circulation. Fetal hemoglobin differs chemically from adult hemoglobin. It possesses greater affinity for oxygen and can carry twenty to thirty percent more oxygen at a particular oxygen partial pressure.
Fetal circulation relies on the difference between fetal and maternal hemoglobin. The difference allows a diffusion of oxygen from the mother's circulatory system to the fetus. The placenta acts as the respiratory center for the fetus. It also filters for plasma nutrients and wastes. The uterine arteries carry oxygenated blood to the placenta and permeate the sponge-like material there. Oxygen then diffuses from the placenta to the chorionic villus and then to the umbilical vein.
What is the mechanism of fetal circulation?
In fetal circulation, the placenta, through the umbilical vein, carries blood to the fetus. The umbilical vein delivers half of this blood volume to the fetal ductus venosus. The ductus venosus carries the blood into the inferior vena cava. The umbilical vein delivers the other half of the blood volume to the liver through the inferior border of the liver in the right lobe by joining with the portal vein. From the liver, the blood then moves to the right atrium of the heart.
The foramen ovale exists in the fetus. This is an opening between the right and left atrium. Most of the blood flows through this foramen directly into the left atrium from the right atrium; thus, bypassing pulmonary circulation. From here, the blood flows into the left ventricle. The left ventricle pumps blood 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.
Another mechanism different in fetal circulation is that some of the blood enters the right ventricle. The right ventricle pumps the blood into the pulmonary artery. The fetus does not use its lungs yet for respiration. Amniotic fluid suspends the fetus and, through the ductus arteriosus, the pulmonary artery and the aorta are connected. This connection directs most of the fetal blood away from the lungs.
Fetal circulation changes at birth. When the fetus becomes an infant, the infant breathes for the first time and the foramen ovale and ductus arteriosus close. The blood no longer bypasses the pulmonary circulation; thus, the neonate’s blood becomes oxygenated in the newly-operational lungs.