THE RETAL CIRCULATION (FIG. 97) The circulation of the blood in the haus differs from normal ex tra-uteline circulation in that the systemic and the pulmonary circu lations are not separated in the fcetus and that, excepting the umbilical vein which contains pure arterial blood, most of the fcctal vessels carry mixed blood.
The fcetal circulatory communications, all of which atrophy after birth, are the following: I, Ductus venomm Arantii.—This is a continuation of the umbilical vein, i.e., a communication from the latter to the liver. The umbilical vein, rising from the placenta, divides below the liver into two branches, the longer of which reaches the superior vena cava duetus Arantii, the shorter one going to the left branch of the portal vein. Umbilical vein and ductus venosus Arantii atrophy to form the ligamentuni teres.
2. Umbilical Arterice.—They wise from the hypogastrie arteries of the fcctus, pass out of the child's body through the umbilicus, sur round the umbilical vein in their twisted course and break up into branches in the placenta. Before birth they carry blood from the fmtus to the placenta; after birth they atrophy' to form the ligamenta vesicalia lateralia.
3. Duchm artericsas Baia the fcetus this furnishes the direct blood vessel connection between pulmonary artery and aorta. The blood is carried through it from the right ventricle directly into the systemic circulation. The ductus arteriosus Botalli arises just where the pulmonary artery divides, and runs obliquely upward to the con cavity of the aortic areh. This vessel atrophies soon after birth. The atrophy is the result of expansion of the lungs with inspiration, which causes the blood to be sucked from the pulmonary artery into the lungs, thoroughly filling the branches of the pulmonary artery. Under normal conditions a sound can no longer lie passed through the duetus artetiosus Rota even a few days after birth; in extra-uterine life, from thrombosis and the growth of tissue arising from the blood vessel walls, it forms a solid cord, the ligamentum arteriosum.
4. Foramen Orale.—This is an egg-shaped opening in the inter auricular septum. The blood from the inferior vena cavil, which flows into the right auricle, chiefly arterial blood, is directed by a projecting semilunar membrane, the Eustachian valve, toward the foramen ovale, through which most of it reaches the left auricle. The final closure of this
foramen occurs toward the end of the first year of life, hy the semilunar valve gradually growing forward over the upper edge of the oval opening.
The blood stream in the frettis takes the following path: first, arterial blood from the placenta is carried by the umbilical vein to the liver of the fcetus. Before reaching the liver, the blood stream divides into two branches, one going through the ductus Arantii into the in ferior vena cava, the other joining the portal vein directly, also finally reaching the inferior vena cava through the hepatic veins, mixed with the blood of the portal vein. Before the inferior vena cave enters the right auricle, it carries mixed blood from the following sources: arterial blood from the umbilical vein, venous blood from the portal vein, and venous blood from the lower half of the body.
The great part of the blood from the inferior vena cave is carried from the right auricle, as a tesult of the peculiar position of the Eus tachian valve already described, into the left auricle; thence it enters the systemic circulation, and through the hypogastric and umbilical arteries of the hfftus, it again reaches the placenta.
The blood from the superior vena cava is sent from the right side of the heart through the pulmonary artery into the lungs on the one side, and on the other, through the ductus Botalli into the aorta.
There is, nevertheless, a decided difference in the distribution of the blood within both halves of the heart, although both receive mixed blood. The great mass of the blood in the left half of .the heart arises from the inferior vena cava, only a small part coming from the lungs. The right side of the heart contains blood poor in oxygen, as it receives an overwhelming quantity of almost pure venous blood from the supe rior vena cava. The blood carried back into the left side of the heart by the pulmonary veins is also pure venous blood, since it receives no oxygen in the lungs of the fcetus.