Fetal Circulation

• Fetal circulation differs markedly from adult circulation.
• Gas exchange in the fetus occurs in the placenta, not in the lungs.
• The placenta gets deoxygenated blood from fetal organs and sends oxygen-rich blood back into fetal arterial circulation.
• The fetal cardiovascular system delivers highly oxygenated blood to the myocardium and brain.
• In the fetus, preferred streaming and intracardiac plus extracardiac shunts achieve oxygenation.
• Fetal circulation can be defined as a 'shunt-dependent' circulation.
• Deoxygenated blood reaches the placenta via the umbilical arteries and returns to the fetus via the umbilical vein.
• Between 50-60% of placental venous flow goes around the hepatic circulation through the ductus venosus (DV) into the inferior vena cava (IVC).
• In the IVC, better oxygenated blood from the DV flows separately from desaturated systemic venous blood returning from the lower body.
• The Eustachian valve, a tissue flap at the junction of the IVC and right atrium, directs highly oxygenated blood.
• The Eustachian valve directs highly oxygenated blood along the dorsal aspect of the IVC, across foramen ovale (FO) into the left atrium (LA).
• The oxygen saturation of fetal blood in the LA is 65%.
• Better oxygenated blood goes into the left ventricle (LV) and is ejected into the ascending aorta.
• A large portion of LV blood gets delivered to the brain and coronary circulation, ensuring that blood with the highest oxygen concentration reaches these vital structures.

Transition to Newborn Circulation

• At birth, the umbilical cord is clamped, so the baby stops receiving oxygen and nutrients from the mother.
• With the first breaths, the lungs expand, and the alveoli clear of fluid.
• Increased blood pressure of the baby, and reduced pulmonary pressures reduce the need for shunting blood through the ductus arteriosus.
• Pressure changes encourage the shunt's closure.
• Pressure increase occurs in the left atrium of the heart along with decrease in the right atrium.
• The shift in pressure stimulates the foramen ovale to close.
• Closure of both the ductus arteriosus and foramen ovale finishes the transition from fetal to newborn circulation.

Fate of Fetal Circulation Closure

• Functional closure of the ductus arteriosus takes 10-96 hours.
• Anatomical closure of the ductus arteriosus takes 2-3 weeks.
• The remnant of the ducturs arteriosum becomes the ligamentum arteriosum.
• Functional closure of the foramen ovale occurs within minutes.
• Anatomical closure of the foramen ovale takes one year.
• The remnant of the foramen ovale becomes the fossa ovalis.
• Functional closure of the ductus venosus occurs within minutes.
• Anatomical closure of the ductus venosus takes 3-7 days.
• The remnant of the ductus venosum is the ligamentum venosum.
• Umbilical arteries become medial umbilical ligaments.
• Umbilical vein becomes the ligamentum teres.

Fetal vs Infant Circulation

• The fetal system is a low pressure systems compared with the infant's high pressure system
• The fetal circulatory system allows for right to left shunting; the infant allows left ot right blood flow
• The fetal lungs are non-functional, the infant lungs are functional.
• In the fetus there is increased pulmonary resistance; in the infant there is decreased pulmonary resistance
• The fetus has decreased systemic resistance whilst the infant has increased systemic resistance

Cardiac problems related to defects of foramen and ducts

• Cardiac problems that can occur are Atrial Septal Defect
• Cardiac problems that can occur is Tetralogy of Fallot (TOF) which includes
  • Ventricular septal defect (VSD)
  • RV outflow tract obstruction (Subpulmonary stenosis)
  • Right ventricular hypertrophy
  • Overriding aorta
• TOF can be surgically repaired with both valve sparing repair and RV to PA Conduit