fetal
• FETAL BLOOD AND CIRCULATION
• Introduction
• Throughout the fetal stage of development, the maternal blood supplies the fetus with O2 and nutrients and carries away its wastes.
o These substances diffuse between the maternal and fetal blood through the placental membrane.
o They are carried to and from the fetal body by the umbilical blood vessels.
• Adaptations of fetal blood and vascular system.
• The concentration of hemoglobin in fetal blood is about 50 % greater than in maternal blood.
• Fetal hemoglobin is slightly different chemically and has a greater affinity for O2 than maternal hemoglobin.
o At a particular oxygen partial pressure, fetal hemoglobin can carry 20-30% more O2 than maternal hemoglobin.
• Fetal Circulation OH-98
• In the fetal circulatory system, the umbilical vein transports blood rich in O2 and nutrients from the placenta to the fetal body.
o The umbilical vein enters the body through the umbilical ring and travels along the anterior abdominal wall to the liver.
About 1/2 the blood it carries passes into the liver.
The other 1/2 of the blood enters a vessel called the ductus venosus which bypasses the liver.
o The ductus venosus travels a short distance and joins the inferior vena cava.
There, the oxygenated blood from the placenta is mixed with the deoxygenated blood from the lower parts of the body.
This mixture continues through the vena cava to the right atrium.
o In the adult heart, blood flows from the right atrium to the right ventricle then through the pulmonary arteries to the lungs.
In the fetus however, the lungs are nonfunctional and the blood largely bypasses them.
o As the blood from the inferior vena cava enters the right atrium, a large proportion of it is shunted directly into the left atrium through an opening called the foramen ovale.
A small valve, septum primum is located on the left side of the atrial septum overlies the foramen ovale and helps prevent blood from moving in the reverse direction.
o The rest of the fetal blood entering the right atrium, including a large proportion of the deoxygenated blood entering from the superior vena cava passes into the right ventricle and out through the pulmonary trunk.
Only a small volume of blood enters the pulmonary circuit, because the lungs are collapsed, and their blood vessels have a high resistance to flow.
Enough blood reaches the lung tissue to sustain them.
o Most of the blood in the pulmonary trunk bypasses the lungs by entering a fetal vessel called the ductus arteriosus which connects the pulmonary trunk to the descending portion of the aortic arch.
As a result of this connection, the blood with a relatively low O2 concentration which is returning to the heart through the superior vena cava, bypasses the lungs.
At the same time, the blood is prevented from entering the portion of the aorta that provides branches leading to the brain.
o The more highly oxygenated blood that enters the left atrium through the foramen ovale is mixed with a small amount of deoxygenated blood returning from the pulmonary veins.
This mixture moves into the left ventricle and is pumped into the aorta.
Some of it reaches the myocardium through the coronary arteries and some reaches the brain through the carotid arteries.
o The blood carried by the descending aorta is partially oxygenated and partially deoxygenated.
Some of it is carries into the branches of the aorta that lead to various parts of the lower regions of the body.
The rest passes into the umbilical arteries, which branch from the internal iliac arteries and lead to the placenta.
There the blood is reoxygenated.
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.
Blood from the mother enters the fetus through the vein in the umbilical cord. It goes to the liver and splits into three branches. The blood then reaches the inferior vena cava, a major vein connected to the heart.
Inside the fetal heart:
• Blood enters the right atrium, the chamber on the upper right side of the heart. Most of the blood flows to the left side through a special fetal opening between the left and right atria, called the foramen ovale.
• Blood then passes into the left ventricle (lower chamber of the heart) and then to the aorta, (the large artery coming from the heart).
• From the aorta, blood is sent to the head and upper extremities. After circulating there, the blood returns to the right atrium of the heart through the superior vena cava.
• About one-third of the blood entering the right atrium does not flow through the foramen ovale, but, instead, stays in the right side of the heart, eventually flowing into the pulmonary artery.
Because the placenta does the work of exchanging oxygen (O2) and carbon dioxide (CO2) through the mother's circulation, the fetal lungs are not used for breathing. Instead of blood flowing to the lungs to pick up oxygen and then flowing to the rest of the body, the fetal circulation shunts (bypasses) most of the blood away from the lungs. In the fetus, blood is shunted from the pulmonary artery to the aorta through a connecting blood vessel called the ductus arteriosus.
Blood circulation after birth:
With the first breaths of air the baby takes at birth, the fetal circulation changes. A larger amount of blood is sent to the lungs to pick up oxygen.
• Because the ductus arteriosus (the normal connection between the aorta and the pulmonary valve) is no longer needed, it begins to wither and close off.
• The circulation in the lungs increases and more blood flows into the left atrium of the heart. This increased pressure causes the foramen ovale to close and blood circulates normally.
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