Oxygen and nutrients are delivered to the fetus from the mother’s blood with the help of the placenta – placental circulation. It occurs as follows. The arterial blood enriched with oxygen and nutrients flows from the mother’s placenta into the umbilical vein, which enters the fetal body in the navel and goes up to the liver, lying down in its left longitudinal sulcus. At the level of the gate of the liver v. The umbilicalis is divided into two branches, one of which immediately flows into the portal vein, and the other, called ductus venosus, rambles along the lower surface of the liver to its posterior margin, where it flows into the trunk of the inferior vena cava.
The fact that one of the branches of the umbilical vein delivers pure arterial blood through the portal vein of the liver gives rise to a relatively large liver; The latter circumstance is associated with the necessary for the developing organism the function of the blood formation of the liver, which prevails in the fetus and decreases after birth. After passing through the liver, blood through the hepatic veins flows into the inferior vena cava.
Thus, all the blood from v. Umbilicalis, either directly (through ductus venosus), or indirectly (through the liver) enters the inferior vena cava, where it is mixed with venous blood flowing through the inferior vena cava inferior from the lower half of the fetus.
Mixed (arterial and venous) blood through the inferior vena cava flows into the right atrium. From the right atrium, it is guided by a valve of the inferior vena cava, valvula venae cavae inferioris, through the foramen ovale (located in the atrial septum) into the left atrium. From the left atrium, the mixed blood enters the left ventricle, then into the aorta, bypassing the pulmonary circulation that is not yet functioning.
In addition to the inferior vena cava, the superior vena cava and the venous (coronary) sinus of the heart flow into the right atrium. Venous blood entering the superior vena cava from the upper half of the body, then enters the right ventricle, and from the latter into the pulmonary trunk. However, due to the fact that the lungs do not function as a respiratory organ, only a small part of the blood enters the lung parenchyma and from there through the pulmonary veins into the left atrium. Most of the blood from the pulmonary trunk along the ductus arteriosus passes into the descending aorta and from there to the viscera and lower extremities. Thus, despite the fact that in general the mixed blood flows through the vessels of the fetus (with the exception of v. Umbilicalis and ductus venosus before its inflow into the inferior vena cava), its quality below the confluence of the ductus arteriosus deteriorates significantly. Consequently, the upper body (head) receives blood richer in oxygen and nutrients. The lower half of the body eats worse than the upper, and lags behind in its development. This explains the relatively small size of the pelvis and lower limbs of the newborn.
The act of birth represents a leap in the development of an organism, during which fundamental qualitative changes of vital processes take place. The developing fetus moves from one environment (uterine cavity with its relatively constant conditions: temperature, humidity, etc.) to another (outside world with its changing conditions), as a result of which the metabolism, as well as the ways of nutrition and respiration, change radically. Instead of nutrients previously obtained through blood, food enters the digestive tract, where it undergoes digestion and absorption, and oxygen begins to flow not from the mother’s blood, but from the outside air due to the inclusion of respiratory organs. All this is reflected in the blood circulation.
At birth, there is a sharp transition from placental circulation to the pulmonary. At the first inhalation and stretching of the lungs with air, the pulmonary vessels greatly expand and fill with blood. Then ductus arteriosus collapses and obliterates during the first 8–10 days, turning into ligamentum arteriosum.
The umbilical artery overgrown during the first 2 – 3 days of life, the umbilical vein – a little later (6 – 7 days). The flow of blood from the right atrium to the left through the oval hole stops immediately after birth, as the left atrium is filled with blood coming from the lungs, and the difference in blood pressure between the right and left atria is equalized. The closure of the oval hole occurs much later than the obliteration of ductus arteriosus, and often the hole persists during the first year of life, and in 1/3 of cases it lasts a lifetime. The described changes are confirmed by X-ray live research.