Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Cardiovascular System
Ontogeny of the Human Circulatory System
Fetal Circulation

The fetus receives the nutrients and oxygen necessary for life from the mother through the Vessels of the Placenta (Atl. Figs. 77, 80).

The placenta is connected to the fetus by the umbilical cord, which contains two umbilical Arteries (Branches of the fetal internal iliac arteries) and the umbilical vein. These vessels pass from the cord into the fetus through an opening in its anterior abdominal wall (the umbilical ring). Venous Blood is carried by the arteries from the fetus to the placenta, where it is enriched with nutrients and oxygen, becoming arterial blood. The blood then returns to the fetus via the umbilical vein, which approaches The Liver and divides into two branches. One of these branches empties directly into the INFERIOR VENA CAVA (as the ductus venosus). The other branch enters the porta hepatis and divides into capillaries within the liver tissue. From there, blood flows through the hepatic Veins into the inferior vena cava, where it mixes with venous blood from the lower body and enters the right atrium. The opening of the inferior vena cava is located opposite the foramen ovale in the interatrial septum (Fig. 2.17). Consequently, most of the blood from the inferior vena cava enters the left atrium, and from there, the left ventricle. In addition, the pulsating blood flow from the placenta via the umbilical vein can temporarily block Blood flow through the portal vein. Under these conditions, primarily oxygen-enriched blood will enter The Heart. In the intervals, venous blood enters the heart via the superior and inferior venae cavae.

As previously described, most of the venous blood from the right atrium enters the right ventricle and then the pulmonary artery. A small volume of blood goes to the Lungs, while most of it passes through the ductus arteriosus into the descending aorta, after the branching of the arteries to the HEAD and upper limbs, and is distributed throughout the systemic Circulation, which is connected to the placenta via the umbilical arteries.

The posterior cardinal veins are mainly associated with the mesonephros (mesonephric Kidney). The renal veins merge to form the subcardinal sinus, which receives veins from the posterior part of the trunk. When the mesonephros degenerates, the posterior cardinal veins also undergo atrophy. The inferior vena cava is formed through the growth and fusion of small veins. Through it, blood from the sinus enters the heart. Veins from the right mesonephros connect with the vessels of the liver. The vitelline veins, which collect blood from the gut, also pass through the liver. Following the regression of the yolk sac, their mesenteric branches transform into the corresponding veins. Through The formation of transverse anastomoses, they develop into the portal vein.

Thus, both ventricles pump blood into the systemic circulation, which is why their walls are of almost equal thickness. Purely arterial blood flows in the fetus only within the umbilical vein and the ductus venosus. In all other fetal vessels, mixed blood circulates; however, the head and upper body, especially During the first half of intrauterine development, receive blood from the inferior vena cava that is less mixed than that supplied to the rest of the body. This promotes better and more intensive Brain development.