Fetal Circulation

Source of oxygen for the fetus is the placenta, so the blood going to the placenta is deoxygenated and the blood coming from the placenta will be oxygenated. The blood goes through the umbilical arteries to the umbilical cord and then to the placenta, where it becomes oxygenated and comes back in through the umbilical vein. The umbilical vein continues as the ductus venosus that eventually drains into the inferior venacava. The venous return from the lower limbs of the fetus is also draining into the IVC and this deoxygenated blood, so the oxygenated blood from the umbilical vein and the deoxygenated blood from the lowerlimbs mix in the inferior venacava. therefore the blood entering the right atrium through the IVC is partially oxygenated.

The blood coming from the superior venacava is deoxygenated blood. Now you must be thinking that the partially oxygenated blood from the IVC mixes with the deoxygenated blood from the SVC. Well thats not the case, in reality these two flow past each other. So lets look at the pathways they take:

• Blood from the IVC: In the right atrium it passes through the foramen ovale and enters the left atrium, from there it goes to the left ventricle and enters the ascending aorta.
• Blood from the SVC: In the right atrium it passes through the tricuspid valve and into the right ventricle. this then goes to the pulmonary trunk and then reaches the pulmonary arteries. At this point instead of going through the pulmonary arteries, this blood shunts and goes through the ductus arteriosus which connects the pulmonary circuit to the aorta. Here it meets the blood coming from the Left ventricle and they mix thus the oxygen level in the blood drops a little. This mixed blood then goes through the rest of the body through the decending aorta except the head and neck.

Now 2 questions rise:

1. What supplies the head and neck?
2. Why did the blood not mix in the heart and mixes in the aorta instead?

Well there is just one explaination to both these questions and that is as follows:

In the aortic arch the brachiocephalic trunk, left common carotid and the left subclaivian are the branches that come off the aortic arch before the ductus arteriosus opens into the arch and these vessels supply the brain. So these vessles carry the partially oxygenated blood from the aorta to the brain befor it mixes with the Blood coming from the SVC and thus the brain recieves blood that has more oxygen as compared to te blood that reaches the entire body.

Changes at birth:

• Umblical arteries close down and form a remnant that is called the medial umbilical ligament.
• Umbilical vein closes down and forms the ligamentum teres or round ligament of liver.
• Ductus venosus closes to form ligamentum venosus.
• The foramen ovale aslo closes to form the fossa ovale.
• The ductus arteriosus closes to form the ligamentum arteriosum.

Now that the arteries and veins to the plalcenta are closed no oxygenated blood from there mixes with the blood in IVC and thus the right atrium  recieves deoxygenated blood from the IVC and SVC. As the foramen ovale is closed thus all the blood goes through the tricuspid valve to the right ventricle. From there blood goes to the pulmonary trunk and as the ductus arteriosus is now closed, blood flows through the pulmonary arteries picks up oxygen from the lungs, comes back through the pulmonary veins to the left artrium. From here the blood goes to the left ventricle and then to the ascending aorta, after this the blood flows to the brain and to the decending aorta from where it is distributes to the entire body.

Patent Ductus Arteriosus:

Causes left to right shunt, blood from the aorta moves into the pulmonary trunk. So it is a noncyanotic defect.

How the Ductus arteriosus closes?

It is important to remember that the smooth muscle in the ductus arteriosus are capable of excessive contraction, so much so that this contraction can close the ductus completely. What inhibits this contraction are prostaglandins and what causes this contraction is oxygen. So at 9 months of age just at the time of birth the prostanglandins lelvel falls and simultaneously the oxygen levels increases as the baby starts to breath.

So now when this oxygen rich blood flows from the aorta to the pulmonary trunk, through the ductus, increased oxygen in the blood stimulates contraction of the ductus and as the inhibitors are small in amount thus ductus closes over aperiod of 24 to 48 hours.

This is why patent ductus arteriosus is more common in preterm infants because there lungs are not properly developed less oxygen in the body and the levels of prostaglandins are quite high before 9 months. All this stops the ductus from closing and thus it persists.

Summary of changes after birth:

Changes after Birth	Remnant in Adult
Closure of right umbilical arteries	Medial Umblical ligament
Closure of umbilical vein	Ligamentum teres of liver
Closure of ductus venosus	Ligamentum venosum
Closure of foramen ovale	Fossa ovalis
Closure of Ductus arteriosus	Ligamentum arteriosum

For further information refer to:

The Human Fetal and Neonatal Circulation. Function and Structure.