Fetal circulation

Question 1

Describe fetal circulation.

Answer 1

1. Blood from the placenta is carried to the fetus by the umbilical vein.
2. Less than a third of this enters the fetal ductus venosus and is carried to the inferior vena cava, while the rest enters the liver proper from the inferior border of the liver.
3. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein.
4. The blood then moves to the right atrium of the heart.
5. In the fetus, there is an opening between the right and left atrium (the foramen ovale), and most of the blood flows through this hole directly into the left atrium from the right atrium, thus bypassing pulmonary circulation.
6. The continuation of this blood flow is into the left ventricle, and from there it is pumped through the aorta into the body.
7. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the maternal circulation.[1]
8. Some of the blood entering the right atrium does not pass directly to the left atrium through the foramen ovale, but enters the right ventricle and is pumped into the pulmonary artery.
9. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus, which directs most of this blood away from the lungs (which aren’t being used for respiration at this point as the fetus is suspended in amniotic fluid).

Question 2

Describe postnatal circulation

Answer 2

1. With the first breath, increased alveolar O2 pressure causes vasodilation in the pulmonary vessels
2. Obstetrical clamping induces spontaneous constriction and change of the umbilical vein to ligamentum teres and umbilical arteries to medical umbilical ligaments
3. W/in 10-15 hours after birth, the ductus arteriosus constricts and will become the ligamentum arteriosum
4. Increased left atrial pressure and decreased right atrial pressure causes the foramen ovale to close and become the fossa ovalis
5. The ductus venosus also constricts and will become the ligamentum venosum

Question 3

Describe venous system development

Answer 3

The general pattern develops from three pairs of veins:

1. Vitelline veins
   
   • Develop into portion of the IVC, hepatic veins and sinusoids, ductus arteriosus, portal vein, inferior mesenteric vein, sup. mesenteric vein, and splenic vein
2. Umbilical veinsRt. umbilical vein develops into hepatic sinusoids, and degenerates early in fetal lifeLeft umbilical vein develops into hepatic sinusoids, and ligamentum teres
3. Cardinal veinsAnt. cardinal vein develops into SVC and IJVPost. cardinal vein develops into portion of IVC and common iliac veinsSubcardinal vein develops into portion of IVC, renal veins, and gonadal veinsSupracardinal vein develops into portion of IVC, intercostal veins, hemiazygos vein, and azygos vein