fetal circulation
Rules for Circulation
Introduction to Circulation Rules
The discussion begins with the recognition that there are specific rules for understanding circulation, particularly fetal and neonatal circulation.
Rule number one clarifies that when asked about changes in circulation, the focus should be on what happens at birth, rather than any later point in time.
Key Differences in Circulation
Rule number two highlights that there are six specific differences between fetal circulation and postnatal circulation. Each difference must be addressed in both comparisons.
Rule number three emphasizes not to obsess over the fetus; a light-hearted tone is suggested to ease the serious demeanor in the classroom.
Major Differences Between Fetal and Neonatal Circulation
Presence of Placenta:
Fetal circulation incorporates a placenta, which performs multiple organ functions for the fetus.
The placenta facilitates gas exchange, functioning as the lungs; it also performs kidney and liver functions due to the underdeveloped state of these organs in the fetus.
Umbilical Arteries:
Fetal circulation features two umbilical arteries that carry deoxygenated blood from the fetus to the placenta. This contrasts with a baby that has no functional umbilical arteries post-delivery.
Umbilical Vein:
Fetal circulation includes one umbilical vein that carries oxygenated blood back from the placenta to the fetus.
Confusion with color coding can arise since umbilical arteries (carrying deoxygenated blood) resemble venous blood, while the umbilical vein (carrying oxygenated blood) is represented in red.
Ductus Venosus:
The umbilical vein branches into one small branch that perfuses the liver and a larger vessel known as the ductus venosus, which bypasses the liver.
This structure allows blood to bypass the liver since the placenta performs liver functions, allowing the fetal liver to grow without engaging in these processes immediately.
Foramen Ovale:
The foramen ovale is a hole in the interatrial septum allowing blood to flow from the right atrium to the left atrium, bypassing the non-functioning fetal lungs due to increased pressure in the right atrium compared to the left.
Ductus Arteriosus:
This connection allows some blood to flow from the pulmonary trunk to the aorta, bypassing the pulmonary circulation. Just a small amount of blood actually flows into the pulmonary arteries for lung development.
Changes at Birth
After birth, essential changes occur in circulation that transition the fetus to a functioning neonate.
The first change is the clamping and cutting of the umbilical cord, severing the direct connection to the placenta.
This eliminates the functional capacity of the placenta for the fetus, marking the first transformative step.
The umbilical arteries, responsible for sending blood to the placenta, are cut, depriving the system of this pathway.
The umbilical vein is also severed, stopping blood flow back from the placenta to the fetus, impacting structures reliant on this blood supply.
Impact of Birth on Structures
The lack of blood supply to the ductus venosus leads to changes in how blood circulates through the liver, as post-birth blood flow will utilize the smaller branch for liver perfusion, integrating with normal circulation.
Initial Breathing and Circulatory Adjustments
Upon the baby's first breaths, the lungs expand, which decreases the resistance in pulmonary circulation.
The decrease in resistance leads to lower pressure on the right side of the heart, allowing the left atrium to gain higher pressure than its right counterpart.
The pressure difference causes the flap of tissue associated with the foramen ovale to close, leading to a gradual permanent closure over time (forming the fossa ovalis).
Closure of Ductus Arteriosus
The ductus arteriosus responds to increased oxygen in the blood after birth, leading to a contraction due to spiral smooth muscle instead of circular smooth muscle, effectively closing off the detour between pulmonary trunk and aorta.
The circulatory pathway returns to standard anatomy that includes proper oxygenation through the lungs.
Consequences of Circulatory Changes
If the pressure dynamics do not shift post-birth—where the right heart pressure stays above the left pressure—the foramen ovale may remain open, potentially leading to complications.
Hypoxia can also prevent contractions in the ductus arteriosus, allowing blood to skip the lungs improperly, resembling persistent fetal circulation.
Common Congenital Issues
The most common congenital heart defect is an atrial septal defect (ASD), which involves the failure of closure of the foramen ovale.
Monitoring is often sufficient for small ASDs, while larger defects may necessitate surgical intervention.
Ventricular septal defects (VSDs) are less common in newborns since they do not originate with an opening in the septum.
fetal circulation
Clamping and Cutting of the Umbilical Cord
The umbilical cord is severed, eliminating the direct connection to the placenta, marking the first transformative step.
Cutting of Umbilical Arteries and Vein
The umbilical arteries are cut, depriving blood circulation to the placenta, and the
umbilical vein is also severed, stopping blood flow back from the placenta to the fetus.
Changes in Blood Circulation through the Liver
Lack of blood supply to the ductus venosus leads to changes in liver circulation, using the smaller branch for liver perfusion as it integrates with normal circulation. The ductus venous shrivels up due to lack of blood flow.
Initial Breathing and Expansion of Lungs
Upon the baby's first breaths, lungs expand, decreasing pulmonary circulation resistance, shifting pressure dynamics in the heart.
Closure of the Foramen Ovale
As pressure in the left atrium exceeds that in the right atrium, the flap associated with foramen ovale closes, leading to a gradual permanent closure.
Closure of Ductus Arteriosus
Increased oxygen in the blood causes the ductus arteriosus to contract, closing off the detour between the pulmonary trunk and aorta, returning to standard circulatory anatomy.