Describe fetal circulation. BE IN GREAT DETAIL.
The pressure found inside the fetal vasculature are the reverse of those found in teh adult. Pressures int he venous system are higher than those in the arterial system. There are two reasons for this.
- 1) During the development of the fetus, the growing lungs provide a very high resistance to blood flow. This high resistance is secondary to pulmonary vasculature constriction caused byt he low Pa02 in the fetal blood and also by teh fact that the lungs are for the most part collapsed.
- (In short low Pa02, lungs collapsed, pulmonary artery constriction.)
- Thus this increases the prsesures in teh right ventricle, right atrium, and vena cava.
2) Placenta offers very little resistance to blood flow. Low resistance causes low pressures in the aorta, left ventricle, left atrium, and the entire arterial system.
- Now onto the actual circulation...
- 1) Umbilical vein carries into first shunt, the ductus venosus. 50% is in the liver, the rest goes through the ductus venosus towards the inferior vena cava.
2)After arriving at the right atrium from the inferior vena cava, most of the blood shunts through the foramen ovale to the left atrium. There is a flap that acts as a one way valve for the foramen ovale that closes shut when the pressures int he left atrium are greater than right. This allows no shunted blood to move backwards. The rest of the blood moves on towards the right ventricle.
3)Upon the right ventricle meeting the pulmonary artery, there is another shunt, the ductus arteriosus. It joins the pulmonary artery to the aorta. Most of the blood enters the shunt, leaving only 10% to perfuse the lungs.
4) After the blood reaches the aorta, blood perfuses the mothers extremeities while 60% goes through the umbilical arteries where the cycle starts again.