Blood Vessels

BLOOD VESSELS

390 the circulation in them is correspondingly very weak. The main mass of blood passes by the lungs through the ductus arteriosus into the dorsal aorta. Since very little blood enters the left atrium from the pulmonary vein, the blood pressure here must be very low. The difference in blood pressure in the right and left halves of the heart is equalized, however, due to the existence of an opening (for. ovale) in the interatrial septum, through which blood flows from the right atrium into the left. Thus, the abdominal aorta with the carotid and subclavian arteries branching off from it also receives mixed blood. By the time of birth, the ductus arteriosus has already narrowed, and a fold develops to the left of the oval opening of the septum, covering it like a valve. This valve is open as long as the pressure on the right side is higher. At birth, however, the inflow of oxidized blood through the umbilical vein ceases immediately, and the pressure in the right atrium falls. At the same time, lung function begins, and the pressure in the left atrium increases. The valve is first passively pressed against the oval opening, and then its edges grow to it. The ductus arteriosus completely closes (lig. Botallii), as well as the umbilical arteries (lig. vesico-umbilicalia) and veins (lig. hepato-umbilicale) and the ductus venosus (lig. venosum Arantii). Lit.—see lit. to art. Blood Vessels and Blood Circulation.