Fetal Circulation

Fetal Circulation

Salient Features of Fetal Growth Placenta is very active. Liver and lungs are passive. Head is growing faster.

Characteristics of Fetal Circulation Venous and arterial blood mix up in atria The venacava carries mixed blood to the right atrium bypassing liver and lungs. Aorta carries mixed blood back to placenta Oxygen saturation is less in fetus than in adults. Difference in oxygen tension between arterial and venous blood is less.

Fetal Circulation

Course of Fetal Circulation Pre-delivery phase The arterial blood flows to the fetus through the umbilical vein. Umbilical vein divides into two branches. One is joined by the portal vein and enters the right lobe of the liver. The other continues upwards as ductus venosus and joins the inferior venacava. Hepatic veins and blood returning from lower extremities is also carried by the inferior venacava hence mixture of oxygenated and deoxygenated blood.

Continued It enters the right atrium and is directed through the foramen ovale into the left atrium. In the left atrium, it mixes with a small quantity of blood returning from the lungs through the pulmonary veins. From the left atrium, blood passes into the left ventricle, and from the left ventricle into the aorta. From the aorta it is distributed almost entirely to the head and upper extremities – a small quantity of blood continuing into the descending aorta.

Continued After circulation in the head and upper extremities, blood is returned by the superior vena cava to the right atrium, where it mixes with a small portion of that blood entering from the inferior vena cava. From the right atrium it descends into the right ventricle and passes into the main pulmonary artery. From the pulmonary artery, most of the blood passes trough the ductus arterioss into the aorta, where it mixes with blood from the left ventricle.

Continued Some of the blood is distributed to the lungs by the right and left pulmonary arteries. The pulmonary veins return this blood to the left atrium. The blood from the main pulmonary artery through the ductus arteriosus passes through descending aorta. Descending aorta supplies viscera of the abdomen and pelvis and lower extremities . The umbilical arteries return the blood to the placenta.

Intermediate Phase Immediately after delivery , the baby begins to breathe, Placental circulation ceases to function as soon as the umbilical cord is tied. Pulmonary circulation changes, An increased quantity of blood is pumped by the right ventricle into the pulmonary arteries than through the ductus arterosis More blood is returned from the lungs to the left atrium. and Pressure rises in the left atrium, causing the foramen ovale to close. The ends of the hypogastric arteries atrophy.

Adult Phase Closure of ductus arterosus in early postnatal period due to reflex action secondary to increased oxygen tension and the interaction of prosta glandin – Ligamentum arteriosum PDA machinery murmur Obliteration of foramen ovale – 6 to 8 weeks Patent foramen ovale with few or no symptoms Occluded umblical vein – ligamentum teres Obliterated ductus venosus – ligamentum venosum Umblical arteries – hypogastric ligaments

Characteristics of Normal Adult Venous and arterial blood no longer mix in the atria The venacava carries only deoxygenated blood into the right atrium and right ventricle which is pumped to the pulmonary arteries and finally to the pulmonary capillaries Aorta carries only oxygenated blood from the left heart via the pulmonary veins for distribution to the rest of the body.

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