The first breath makes the begining of a very short dramatic transition within the circulatory system of a newborn. In the fetal circulation purely oxygenated blood enters the body through the left umbilical vein from placenta to the ductus venosus, this blood mixes with a small volume of deoxygenated portal blood coming from the GIT and then enters the IVC, where it mixes with deoxygenated blood returning from the trunk and legs. This stream of blood, still highly oxygenated enters the right atrium and largely shunted through the foramen ovale to the left atrium. The oxygenated blood entering the fetal right atrium from the IVC and the deoxygenated blood entering from the SVC form hemodynamically distinct streams and undergo very little mixing in the atrium. This separation of streams is accomplished partly by the shape and placement of the valve of the IVC.
Oxygenated blood from the right atrium mixes with the very small amount of blood returning from the lungs via the pulmonary veins within the left atrium. The vascular resistance of the collapsed fetal lungs is very thus little amount of blood flows through the pulmonary circulation . The oxygenated blood in the left ventricle is then pushed into the aorta for distribution first to the head, neck, and arms and then, via the descending aorta, to the trunk and limbs. As blood enters the descending aorta, it mixes with the deoxygenated blood shunted through the ductus arteriosus. This blood consists mainly of the blood entering the right atrium from the superior vena cava and expelled via the right ventricle and pulmonary trunk. Thus, the blood delivered to the head, neck, and arms by the fetal circulation is more highly oxygenated than the blood delivered to the trunk and lower limbs. After the descending aorta has distributed blood to the trunk and lower limbs, the remaining blood enters the umbilical arteries and returns to the placenta for oxygenation.
Changes at first breath
the foramen oval becomes obliterated as the pressure in the left ventricles increases septum primium to push onto septum secundum. this becomes a bit depressed from the left ventricular side due to the pressure forming a fossa: fossaovalis
the ductus arteriosus obliterated and regenerate to form ligamentum arteriosus as the ductus venous, the proximal part of the left umbilical vein do obliterate and degenrate to form the ligamentum venous or teres. Lack of obliteration is a conjenital abnormaliy
with this changes, the IVC contains purely venous blood as well as the descending aorta contains purely oxygenated blood.