Fetal Circulation

The Fetal Kidneys

The fetus grows two sets of kidneys as it develops to facilitate it's rapid growth. As the fetus grows, most of the filtering is handled by the mother's placenta, leading to the strain that pregnancy puts on kidneys towards the end of the pregnancy. While the filtering is mostly handled by the placenta, the fetus' kidneys excrete urine directly into the amniotic fluid, and blockages in any of the urinary tracts can cause not only a back flow of urine that can can damage the kidneys, but also cause lung defects and problems from not having enough amniotic fluid. This is because in latter stages of fetal development, the amniotic fluid is composed mostly of fetal urine.

Fetal Circulation Diagram

Fetal Circulation Diagram
Fetal Circulation Diagram

This is a diagram of the fetal circulation system from the website http://www.embryology.ch/ which is a brilliant website and has a wealth of information on everything one would want to know about the development of a baby from conception to the birth itself. I highly recommend it for future research. This particular diagram shows the flow of blood in a fetus in the later stages of pregnancy. This particular representation is very useful because it actually shows the flow of blood. Oxygenated blood can be seen coming from the placenta, the umbilical cord, then the umbilical vein and flowing into the liver and then the vena cava to begin its circulatory process. Other important information one needs to remember about fetal circulation is that the blood takes three 'shortcuts' during its journey.
First and foremost, half the blood coming from the placenta goes to the liver, and the other half is diverted along the ductus venosus which "skips" the liver. Then there's a connection through which blood can flow directly between the right and left atria called the foramen ovale. The foramen ovale has a small valve called the septum primum which is located on the left side of the atrial septum that prevents the blood from flowing from the left atrium to the right. After birth this valve is sealed by the increased left and decreased right atrial pressure. The last 'shortcut' is the ductus arteriosus, which allows blood to flow from the pulmonary trunk to the descending portion of the aorta to keep blood out of the lungs and from damaging lung capillaries. The ductus arteriosus connects to the lower portion of the aortic arch so the blood traveling to the brain is largely oxygenated.

Differences between the Fetal Circulatory system and the Adult Circulatory system:

The fetal blood has bout a 50% higher concentration of hemoglobin than the maternal blood, and is chemically slightly different. Under the right circumstances, the fetal blood can carry 20-30% more oxygen then the maternal blood.

Changes in a Newborn’s Circulatory system:

The foramen ovale is sealed shut by the change in atrial pressure, and becomes the fossa ovalis. Both the ductus arteriosis and the ductus venosum are closed by sphincter muscles and get turned into ligaments.