The Fœtal Circulation.—The placenta is an organ of nutrition as well as respiration, and through the umbilical vessels the food materials are brought to the fœtus and the waste products removed.
Surrounded by the jelly of Wharton that fills out the cord, and running in and out between the two arteries, the umbilical vein passes into the fœtal abdomen and divides into two branches, one, the larger, short-circuits directly into the inferior vena cava. This branch is called the ductus venosus. The other joins the portal vein and passes through the liver, after which it also enters the vena cava.
Thus the heart is fed with a mixed blood, part coming fresh from the placenta and part coming up from the lower half of the fœtus. This blood is poured into the right auricle, where it becomes mixed again with the blood coming down from the upper pole of the fœtus through the superior vena cava.
Fig. 23.—The fœtal circulation. (Edgar.)
Now a small part goes down into the right ventricle and is forced into the pulmonary arteries to supply the lungs. But the lungs are not functionating, hence the greater part is again short-circuited through the ductus arteriosus into the arch of the aorta, where it meets with the great volume of blood which passed over into the left auricle through the hole in the septum between the right and left auricles, called the foramen ovale, thence down into the left ventricle and out through the aorta to supply the rest of the fœtal body.
With the exception of the ductus venosus and the ductus arteriosus and the foramen ovale, the circulation is the same as in the adult.
The blood in the descending aorta again divides and part goes on to supply the lower extremities while the greater part leaves the internal iliac arteries by means of the hypogastric vessels and returns through the umbilical arteries to the placenta for oxygenation.
As soon as the child is born, the fœtal structures are altered. The child breathes, the pulmonary circulation is established and the ductus arteriosus is closed. The placental circulation is abolished, and the ductus venosus and the hypogastric arteries are converted into solid fibrous cords. Owing to the immediate change of pressure in the auricles, the foramen ovale closes and the circulation assumes the adult type.