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
Overview
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 |
Umbilical Vein (intra-abdominal part) |
Ligamentum teres |
Ductus Venosus | Ligamentum venosum |
Umbilical Arteries and abdominal ligaments | medial umbilical
ligaments, superior vesicular artery (supplies bladder) |
Ductus Arteriosum | Ligamentum arteriosum |