Trace path of blood in diagram of fetal circulation (see diagram)
2. Ductus venosus
fetal blood vessel connecting the umbilical vein to the IVC
blood flow regulated via sphincter
carries mostly hi oxygenated blood
3. Foramen ovale
shunts highly oxygenated blood from right atrium to left atrium
As soon as the baby is born, the foramen ovale, ductus arteriosus ductus venosus and umbilical vessels are no longer needed.
The sphincter in the ductus venosus constricts, so that all blood entering the liver passes through the hepatic sinusoids.
Occlusion of the placental circulation causes an immediate fall of blood pressure in the IVC and right atrium.
Aeration of the lungs at birth is associated with
1. a dramatic fall in pulmonary vascular resistance due to lung expansion.
2. a marked increase in pulmonary blood flow (thus raising the left atrial pressure above that of IVC)
3. a progressive thinning of the walls of the pulmonary arteries (due to stretching as lungs increase in size with first few breaths)
The first breath:
the pulmonary alveoli open up:
pressure in the pulmonary tissues decreases
Blood from the right heart rushes to fill the alveolar capillaries
Pressure in the right side of the heart decreases
Pressure in the left side of the heart increases as more blood is returned from the well-vascularized pulmonary tissue via the pulmonary veins to the left atrium
Resulting circulatory changes include:
blood pressure is now high in the aorta and systemic circulation is well established
Control of circulation is a reflex function regulated:
Peripherally by the baroreceptors in the aortic artch and carotid sinus
Centrally by baroreceptors in the cardiovascular center of the medulla (in close proximity of the chemoreceptors that regulate respiration)
Respiratory and circulatory reflexes are usually strong in the healthy full-term newborn, but their efficiency in controlling cardiovascular function is susceptible to environmental factors.
What happens to these shunts at birth?
Foramen ovale (see drawing)
Before birth the foramen ovale allows most of the oxygenated blood entering the right atrium from the IVC to pass into the left atrium
Prevents passage of blood in the opposite direction because the septum primum closes against the relatively rigid septum secundum.
Closes at birth due to decreased flow from placenta and IVC to hold open foramen, and
More importantly because of increased pulmonary blood flow and pulmonary venous return to left heart causing the pressure in the left atrium to be higher than in the right atrium.
The increased left atrial pressure then closes the foramen ovale against the septum segundum.
The output from the right ventricle now flows entirely into the pulmonary circulation.
Other changes in the heart
The right ventricular wall is thicker than the left ventricular wall in fetuses and newborn infants because the right ventricle has been working harder. By the end of the first month the left ventricular wall is thicker than the right because it is now working harder than the right one. The right ventricular wall becomes thinner because of atrophy associated with its lighter workload.
Umbilical Arteries constrict at birth
To prevent loss of infantνs blood.
Umbilical cord is not tied for 30-60 seconds so that blood flow thru umbilical vein continues, transferring fetal blood from placenta to the infant.
Blood change from fetal to adult pattern of circulation is not a sudden occurrence in some changes occur during the first breath, others over hours and days.
During the transitional stage right to left flow may occur through the foramen ovale. The closure of the fetal vessels and the foramen ovale is initially a functional change; later anatomic closure results from proliferation of endothelial and fibrous tissues.
|Fetal Structure||Adult Structure|
|Foramen Ovale||Fossa Ovalis|
|Ductus Venosus||Ligamentum venosum|
|Umbilical Arteries and abdominal ligaments||medial umbilical
superior vesicular artery (supplies bladder)
|Ductus Arteriosum||Ligamentum arteriosum|