| Placenta: | The organ responsible for delivery of nutrients, removal of
waste products and delivery of oxygenated
blood to the fetus via diffusion and active transport mechanisms. Primarily, placental circulation is low resistance. |
| Fetal lungs: | Are filled with fetal lung liquid, not used to oxygenate blood.
Because the alveoli are filled with liquid
most of the arteries and arterioles are surrounded by liquid which increases resistance to blood flow through the vessel. This results in most of the blood flow bypassing the lungs and therefore directed to the systemic periphery. |
| Umbilical vein &
Ductus Venosus: |
A vessel which delivers oxygenated blood from the placenta to the fetus.
The umbilical vein enters portal venous system where it empties most of the blood flow into the ductus venosus which connects to the inferior vena cava. Average oxygen saturation of blood is 80% in the umbilical vein before it mixes with unoxygenated blood in the ductus venosus. After mixing, the oxygen saturation is approximately 67%. |
| Foramen ovale: | The majority of inferior vena cava blood flow crosses the foramen
ovale and into the left atrium
bypassing the lungs, some blood flow enters the right atrium. The foramen ovale is anatomical opening between the right atrium and left atrium which closes shortly after birth. |
| Ductus arteriosus: | A vessel that connects the main pulmonary artery to the aorta.
The blood flow that does enter the right atrium (mainly from the superior
vena cava, 52% oxygen saturated) enters the right ventricle and then the
main pulmonary artery where the blood flow then enters the ductus arteriosus
which connects to the aorta. Once again, most blood flow
bypasses the lungs and is directed to the systemic circulation. Blood flow is flowing in a right to left direction. The ductus arteriosus should functionally close within 15 hours and structurally within a few weeks (in mature infants). |
| Umbilical arteries: |
2 vessels that allows unoxygenated blood to flow from the descending aorta back to the placenta. |
| The First Breath: | The lungs are filled with air instead of fluid. Higher
oxygen levels in the blood and alveoli
filled with air instead of fluid allows for vascular resistance to decrease. This results in a greater increase in pulmonary blood flow. |
| Anatomical Changes: | Placenta is removed from circulation. Foramen ovale and
Ductus Venosus and Arteriosus
close. Higher pressure in the left atrium due to increased pulmonary blood flow cause the foraman ovale to close. Higher concentrations of oxygen in the blood, decreased prostaglandin levels and decreased pulmonary vascular resistance closes the ductus arteriosus. When the umbilical cord is clamped, the umbilical vein closes, systemic vascular resistance is increased and this causes the ductus venosus to close.
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