Understanding the Bypass of Blood Flow to a Fetus's Liver

Introduction

Blood flow to a fetus bypasses its liver primarily due to the presence of two key blood shunts: the ductus venosus and the foramen ovale. This unique vascular arrangement ensures that oxygen-rich blood can reach vital organs, especially the heart and brain, without passing through the liver. In fetal circulation, the liver's functions are primarily fulfilled by the mother's placenta. This article delves into these processes and their implications.

Key Blood Shunts

Ductus Venosus

The ductus venosus is a blood vessel that connects the umbilical vein, which carries oxygenated blood from the placenta, directly to the inferior vena cava. This shunt allows a significant portion of the blood to bypass the liver and enter the fetal circulation, ensuring that vital organs like the heart and brain receive adequate oxygen. The following diagram illustrates the pathway of the blood through these structures:

Foramen Ovale

The foramen ovale is an opening between the right and left atria of the fetal heart. Since the fetus does not have fully functional lungs, the majority of oxygen-rich blood is redirected from the right atrium to the left atrium via this foramen, bypassing the lungs and ensuring that blood is oxygenated through the placenta.

Liver Function in Fetal Circulation

In fetal circulation, the primary function of the liver is not to process nutrients and detoxify blood as it does in adults. Instead, these tasks are performed by the mother's placenta. The placenta supplies the fetus with necessary nutrients and removes waste products, reducing the liver's workload. After birth, the shunts close, allowing blood to flow through the liver and the lungs as in an adult.

Postnatal Adaptation and Closure of Shunts

After birth, the shunts close to adapt the newborn's circulatory system to breathing air. The ductus venosus closes approximately 2-3 days after birth, and the foramen ovale closes shortly after birth. This process ensures that the newborn's body can now breathe air, and blood can flow through the liver and lungs. The following diagram illustrates the postnatal closure of these shunts:

Functional Heterogeneity of the Fetal Liver

The fetal liver shows functional heterogeneity of the parenchymal cells dependent on the microcirculation. The right side of the liver is perfused with less well-oxygenated blood mainly from the portal blood and has a greater role in hematopoiesis. In contrast, the left side of the liver receives blood mainly from the umbilical vein, has a higher content of oxygen-dependent enzymes, and is more active in drug binding and metabolism. This functional heterogeneity is an area of active research, as shown in the microsphere distribution protocols by Joe Zink and Clive Greenway.

References:

Survival Linx :: The book [Fetal and Neonatal Hepatic Circulation] Hepatic Circulation Colloquium Series on Integrated Systems Physiology by W.Wayne Lautt, D. Neil Granger, Joey Granger, Morgan Claypool Publishers, 2009 Hepatic Circulation