Fetal circulation and changes in circulation after birth
Fetal circulation
The fetal lungs are nonfunctional before birth and therefore the heart pumps large quantities of blood through the placenta. Special anatomical arrangements operate in the fetal circulatory system which is different from the adult.
Blood returns from the placenta through the umbilical vein passes through the ductus venosus, bypassing the liver.
Most of the blood that enters the right atrium from the inferior vena cava passes across the posterior region of the right atrium and then through the foramen ovale directly into the left atrium.
Therefore, well oxygenated blood from the placenta enters mainly the left ventricle of the heart rather than the right ventricle and is pumped by the left ventricle into the vessels of the head and forelimbs.
The blood entering the right atrium from the superior vena cava passed downward through the tricuspid valve into the right ventricle. This blood is oxygenated and is pumped into the pulmonary artery and through the ductus arteriosus into the descending aorta and through the two umbilical arteries into the placenta where deoxygenated blood becomes oxygenated.
Changes in circulation after birth
- Primary changes occurs in the pulmonary and systemic vascular resistance at birth
- Loss of tremendous blood flow through the placenta and increases the systemic vascular resistance. This increases the pressure in the aorta, left atrium and left ventricle.
- Decrease in pulmonary vascular resistance due to expansion of lungs. This increases blood flow to lungs with a reduction in the pressures of pulmonary artery, right ventricles and right atrium.
- Closure of foramen ovale
- Closure of ductus arteriosus
- Closure of ductus venosus