A reference hand-book for nurses

by Amanda K. Beck

From Kimber’s Anatomy.

The peculiarities of the fetal circulation, leaving details aside, are: the direct communication between the two auricles of the heart, through an opening called the foramen ovale; the communication between the pulmonary artery and descending portion of the arch of the aorta, by means of a tube called the ductus arteriosus; and the communication between the placenta and the fetus, by means of the umbilical cord.

The arterial blood for the nutrition of the fetus is carried from the placenta along the umbilical cord by the umbilical vein. Entering the fetus at the umbilicus, the blood passes upward to the liver and is conveyed into the inferior vena cava in two different ways. The larger quantity first enters the liver, and, alone or in conjunction with the blood from the portal vein, ramifies through the liver before entering the inferior vena cava, by means of the hepatic veins. The smaller quantity of blood passes directly from the umbilical vein into the inferior vena cava, by a tube called the ductus venosus.

In the inferior vena cava the blood from the placenta becomes mixed with the blood returning from the lower extremities of the fetus. It enters the right auricle and, guided by a valve—the Eustachian valve, passes through the foramen ovale into the left auricle. In the left auricle it unites with a small quantity of blood returned from the lungs by the pulmonary veins. From the left auricle the blood passes into the left ventricle and is distributed by the aorta almost entirely to the upper extremities by the superior vena cava, the blood enters the right auricle, and, passing over the Eustachian valve, descends into the right ventricle, and from the right ventricle into the pulmonary artery. As the lungs in the fetus are solid, they require very little blood, and the greater part of the blood passes through the ductus arteriosus into the descending aorta, where, mixing with the blood delivered to the aorta by the left ventricle, it descends to supply the lower extremities of the fetus, the chief portions of this blood, however, being carried back to the placenta by the two umbilical arteries.

Diagrammatic view of the fetal circulation
(Dorland).

From this description of the fetal circulation it will be seen:

1. That the placenta serves the double purpose of a respiratory and nutritive organ, receiving the venous blood from the fetus, and returning it again charged with oxygen and additional nutritive material.

2. That the greater part of the blood traverses the liver before entering the inferior vena cava; hence the large size of this organ at birth.

3. That the blood from the placenta passes almost directly into the arch of the aorta and is distributed by its branches to the head and upper extremities; hence the large size and perfect development of those parts at birth.