1. Review of Fetal Circulation
2. Changes at Birth
3. Postnatal circulation
4. Defects

I. Review of Fetal Circulation

 The development of the cardiovascular system -
… Begins to develop toward the end of the third week
… Heart starts to beat at the beginning of the fourth week
… The critical period of heart development is from day 20 to day 50 after fertilization.
… Many critical events occur during cardiac development, and any deviation from this normal pattern can cause congenital heart defects, if development of heart doesnνt occur properly.
… However, we will concern ourselves with the events surrounding the circulatory changes at birth.

Trace path of blood in diagram of fetal circulation (see diagram)

Three shunts in the fetal circulation

 1. Ductus arteriosus
… protects lungs against circulatory overload
… allows the right ventricle to strengthen
… hi pulmonary vascular resistance, low pulmonary blood flow
… carries mostly med oxygen saturated blood

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

II. Review of respiratory changes and other changes at birth

… 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.

Circulatory Adjustments at birth:

Increasing uptake of oxygen by lungs (first and subsequent breaths) induces a vasoconstriction of ductus venosus and ductus arteriosis

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.

II. Ductus Arteriosus

… The DA constricts at birth, but there is often a small shunt of blood from the aorta to the left pulmonary artery for a few days in a healthy, full-term infant.
… In premature infants and in those with persistent hypoxia the DA may remain open for much longer.
… Oxygen is the most important factor in controlling closure of the DA in full-term infants. Closure of the DA appears to be mediated by bradykinin, a substance released by the lungs upon initial inflation.
… Bradykinin has potent contractile effects on smooth muscle. Action depends upon the high oxygen content of the aortic blood resulting from aeration of the lungs at birth.
… When the PO2 of blood passing through the DA reaches about 50 mm Hg, the wall of the DA constricts. (May be mediated direct or may be mediated by Oxygenνs effect on decreasing PG E2 and prostacylcin secretion. (unlike in a coarctation of aorta which requires PGE2 infusion to reopen the DA for blood flow.
… As a result of reduced pulmonary vascular resistance, the pulmonary arterial pressure falls below the systemic level and the blood flow thru the ductus arteriosis is diminished.

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.

Adult Derivatives of Fetal Vascular Structures

… Because of certain changes in the cardiovascular system at birth, certain vessels and structures are no longer required.
… Over a period of months these fetal vessels form nonfunctional ligaments, and fetal structures such as the foramen ovale persist as anatomic vestiges of the prenatal circulatory system.

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

Two common defects:

1. patent ductus arteriosus
… common in females 2-3 times more than males, unknown reason why
… If instead of functional closure after birth there is patent structure then aortic blood is shunted into the pulmonary artery.
… Most common congenital anomaly associated with maternal rubella infection during early pregnancy (mode of action by virus unclear)
… Premature infants usually have a PDA due to hypoxia and immaturity.
… Surgical closure of PDA is achieved by ligation and division of the DA.
2. Patent foramen ovale
… most common form of an Atrial Septal Defects (ASDs)
… a small isolated patent foramen ovale is of no hemodynamic significance; but if other defects present (e.g. pulmonary stenosis or atresia), blood is shunted through the foramen ovale into the left ventricle, producing cyanosis, a dark bluish coloration of the skin and mucous membranes resulting from deficient oxygenation of the blood.
… A probe patent foramen ovale is present in up to 25% of people. A probe can be passed from one atrium to the other through the superior part of the floor of the fossa ovalis. Though not clinically significant (usually small) but may be forced open because of other cardiac defects and contribute to functional pathology of the heart. Results from incomplete adhesion between the original flap of the valve of the foramen ovale and the septum secundum after birth.