Obstetrical Nursing / A Text-Book on the Nursing Care of the Expectant Mother, the Woman in Labor, the Young Mother and Her Baby

by Carolyn Conant Van Blarcom

The ascending vena cava, then, pours into the right auricle a mixture of arterial blood, which has come directly from the placenta, and venous blood returned from the liver, intestines and lower extremities. There is a difference of opinion concerning the course of the blood stream after reaching the right auricle. The general teaching, however, is that the eustachian valve, guarding the foramen ovale, deflects the current through this opening from the right into the left auricle. It then pours into the left ventricle, is pumped into the arch of the aorta, from which most of the blood is sent to the head and upper extremities, though a small part carries nourishment to other parts of the body.

The descending, or superior, vena cava, carrying blood returning from the head and arms also empties into the right auricle; this stream presumably crosses the stream which is directed toward the foramen ovale, flows into the right ventricle by which it is pumped into the pulmonary artery. The circulation of blood through the lungs, however, is for their own nourishment, and not for aëration as with the adult. For this reason most of the contents of the fetal pulmonary artery empties into the aorta through the ductus venosus, one of the temporary fetal structures already referred to. From the aorta the stream is directed in part to the lower extremities and the pelvic and abdominal viscera, but most of it flows into the hypogastric arteries. These are also temporary arteries. They lead to the umbilical cord and, as the umbilical arteries, carry the venous or vitiated blood through the cord to the placenta where it is oxygenated, freed of its waste in the chorionic villi and returned to the fetus through the umbilical vein.

As soon as the child is born and it is obliged to obtain its oxygen from the surrounding air, its pulmonary circulation of necessity becomes immediately more important and is greatly increased in volume. In fact, the entire fetal circulation is readjusted to meet the needs of the new and independent functions which the little body now assumes. The temporary structures are obliterated, since they are no longer needed, and the lungs and intestines become more active in compensation.

Fig. 29.—Diagram showing circulation of the blood after birth, with hypogastric arteries, ductus venosus, ductus arteriosus and foramen ovale in process of obliteration and pulmonary circulation greatly increased. (From The American Textbook on Obstetrics.)

As the ductus venosus and hypogastric arteries terminate in blind ends and become useless as soon as the umbilical cord is cut, they soon begin to atrophy and are obliterated within a few days after birth. This means that less blood is poured into the right auricle, which naturally results in relatively less tension in the right heart and an increased pressure in the left, which tends to close the foramen ovale. The foramen ovale does not entirely disappear at once, however, but closes gradually, sometimes remaining open for months. Occasionally it remains open permanently, and though some people have gone through life comfortably with a patent foramen ovale, its ultimate failure to close usually results in serious circulatory trouble. This is also true of the ductus arteriosus, which sometimes, but not often, fails to close.

The rule is that as the lungs expand and an increased amount of blood is carried to them for aëration, the ductus arteriosus deflects a steadily diminishing stream from the right ventricle to the arch of the aorta. Thus it gradually ceases functioning in most cases and disappears in the course of a few weeks. The abandoned vessels may degenerate and disappear in time or they may persist in the form of small fibrous cords. (Fig. [29].)

Although the circulatory system shows the most elaborate adjustments to the protection afforded by intra-uterine life, there are also other adaptations made by the fetal organism.

The baby acquires about 90 per cent of its weight during the latter half of pregnancy, as well as a steadily increasing proportion of solids and a decrease of fluids in its tissues, for in its early days the embryo consists largely of water. But for all of that, its existence and growth in utero, and the functioning of its heat producing centre require surprisingly little oxygen and nourishment. The amniotic fluid keeps the fetus at an equable temperature, about 1° above that of the mother, and as space within the uterine cavity permits of only limited movement, there is very little combustion for the liberation of heat and energy.

The kidneys assume functional form at a very early fetal age, probably about the seventh week, and the presence of albumen and urea in the amniotic fluid suggest that small amounts of urine may be voided, particularly during the latter part of pregnancy.