Blood Flow:

Blood carrying the highest oxygen saturation and nutrients is returned from the placenta to the fetus via the umbilical vein.  This blood travels upward to the underside of the liver where it separates into 2 portions:

One portion entering the portal and hepatic circulations

One portion entering the ductus venosus (bypassing the hepatic circulation) and ultimately emptying into the inferior vena cava (IVC).  

Once in the IVC, the blood mixes with unoxygenated blood returning from the lower extremities and the wall of the abdomen.  It continues to travel upward eventually entering the right atrium where it mixes with blood returning from the head and upper extremities via the superior vena cava (SVC).

Thirty to thirty-five percent of blood returned to the right atrium passes through the foramen ovale to the left atrium.  Here it mixes with blood returning from the lungs via the 4 pulmonary veins; enters the left ventricle;  and exits the left side of the heart via the aorta.  This preferential shunting of blood with the highest oxygen saturation to the left side of the heart assures adequate oxygenated blood flow to the coronary and cerebral circulations—tissues with the greatest need.  

The remaining blood in the right atrium mixes with blood returning from the SVC.  This blood is directed downward toward the tricuspid valve; enters the right ventricle; and exits the right side of the heart via the main pulmonary artery on its way to the pulmonary arterial bed.  However, during fetal days, the pulmonary arterial bed is vasoconstricted and hypertrophied and relatively resistant to blood flow.  Taking into consideration the increased (elevated) pulmonary vascular resistance in the pulmonary arterial bed most of this blood will never arrive in the pulmonary arterial bed, instead, it will be diverted away thorough the ductus arteriosus and into the aorta.  Only about 8% of fetal cardiac output actually flows to/from the non-functioning fetal lungs and pulmonary arterial bed.  Remember the vast majority of blood bypasses pulmonary circulation, instead passing through the ductus arteriosus into the descending aorta to perfuse the trunk and lower extremities. 

Blood going to the lungs is transported to the pulmonary arterial bed by the right and left pulmonary arteries and is returned from the lungs to the left atrium via the 4 pulmonary veins.  Once in the left atrium it mixes with blood coming through the foramen ovale, is directed downward toward the mitral valve and into the left ventricle.  Blood leaves the fetal cardiac system when it enters the ascending aorta, flows up to the aortic arch, mixes with blood coming across the ductus arteriosus, and flows down the descending aorta on its way to the placenta via the two umbilical arteries.  

Following gas exchange and uptake of nutrients in the placenta, blood again enters the umbilical vein and the process starts over once again.  As a result of this process, in fetal days, approximately ²/₃ of the blood volume passes through the right side of the heart while only ¹/₃ passes through the left side of the heart. 

Differences:  

    Fetal or intra-uterine circulation is anatomically and physiologically different from extra-uterine circulation. 

First, during intra-uterine days the vast majority of the fetus’ cardiac output is delivered to the placenta due to its low vascular resistance—the organ of oxygenation for the fetus.  Since placental oxygenation is not as efficient as pulmonary oxygenation, the fetus’ pulmonary arterial oxygen tension is only about 20-30 Torr or mmHg, which corresponds to an adult extra-uterine value of 60-70%.  Nonetheless, the fetus relies on the placenta: 

Secondly, fetal circulation must meet the needs of selected organs and tissues for optimum growth.  Fetal lungs are essentially non-functional, the fetal liver is only partially functional, and, as such, these organs need less blood during this time frame than will be required after birth.  Conversely, the fetal brain and developing central nervous system (CNS) both have high oxygen requirements and will receive a larger portion of fetal blood. Therefore, fetal circulation’s blood flow allows most of the right ventricular output to bypass the lungs and go, instead, to the placenta to pick up oxygen.  On the other hand, flow exiting the left ventricle, blood with the highest oxygen saturation, is preferentially directed to the coronary and cerebral circulations thereby ensuring adequate oxygenation of these systems. 

Thirdly, three major anatomic differences exist between intra-uterine and extra-uterine circulation:  the ductus venosus, the foramen ovale, and the ductus arteriosus.  The ductus venosus is a portion of the umbilical vein located on the under side of the liver, giving rise to the portal vein.  It carries oxygenated blood from the placenta and umbilical vein to the inferior vena cava.  The foramen ovale, an opening in the septum between the right & left atria, shunts blood from the right atrium to the left atrium during fetal days.  This allows the majority of blood returned to the heart to bypass pulmonary circulation; roughly 30-35% of right atrial contents pass through the foramen ovale into the left atrium mixing with blood returning from the lungs.  About mid-gestation a flap forms in the atrial septum, a sort of one-way valve, permitting blood flow in one direction only—right to left; flow in the opposite direction is not permitted.  Again, this contributes to the fetus’ ability to bypass pulmonary circulation.  The ductus arteriosus diverts blood from the main pulmonary artery and pulmonary circulation into the descending aorta just below the origin of the left sub-clavian artery.  About 90% of right ventricular output crosses from the pulmonary artery, through the ductus arteriosus, and into the descending aorta where it combines with left ventricular output.  This is due to increased or elevated pulmonary vascular resistance. 

Finally, a unique feature of fetal circulation is the low oxygen saturation level of fetal blood.  The fetal oxygen saturation level is considerably lower than that of the newborn infant because of the lower efficiency of the placenta as compared to the lungs as an organ of gas exchange.  

Fetal hemoglobin has a greater affinity for oxygen than normal hemoglobin.  This leftward shift of the oxygen dissociation curve enhances oxygen uptake despite low placental oxygen levels.