Anatomy and physiology of Fetal circulation

Fetal circulation

1. Fetal Circulation

2. Umbilical vein – oxygenated blood enters the body via the umbilical vein. After mixing with deoxygenated blood in the ductus venosus, it reaches the right atrium (via inferior vena cava that receives blood from trunk and limbs)

3. ● Right Atrium – anatomical relationship of the venae cava ensures that most of the blood in the right atrium (from the inferior vena cava) bypasses the right ventricle and goes directly to the left atrium via the patent foramen ovale.

4. Left atrium – blood from the left atrium mixes with (deoxygenated) blood from the lungs and is expelled via the left ventricle into the aorta, and ultimately around the body Mixing – some blood does not flow directly from the right into the left atrium, but instead it is directed to the right ventricle (mainly blood from the superior vena cava). This deoxygenated blood flows through the lungs and pulmonary trunk, and then via the ductus arteriosus into theaorta

5. Transitional circulation – following the clamping of the umbilical cord at birth, and with the large decrease in pulmonary vessel pressure with inspiration, significant pressure and flow changes occur. The fall in right atrial pressure and increase in left atrial pressure causes the foramen ovale to close, as the septum secundum and septum primum oppose.

6. This is an immediate functional closure only. As a result, all blood from the right atrium is now forced into the right ventricle. The ductus arteriosus also constricts due to the high partial pressure of oxygen (functionally complete by 12 hours). The change to adult circulation is complete by 3 months, by which time the foramen ovale is anatomically fused ( fossaovalis ) and the ductus arteriosus is