Why does blood bypass the lungs

The enriched blood flows through the umbilical cord to the liver and splits into 3 branches. The blood then reaches the inferior vena cava. This is a major vein connected to the heart. Most of this blood is sent through the ductus venosus. This is also a shunt that lets highly oxygenated blood bypass the liver to the inferior vena cava and then to the right atrium of the heart. A small amount of this blood goes straight to the liver to give it the oxygen and nutrients it needs.

Waste products from the fetal blood are transferred back across the placenta to the mother's blood. Blood enters the right atrium. This is the chamber on the upper right side of the heart. When the blood enters the right atrium, most of it flows through the foramen ovale into the left atrium.

Blood then passes into the left ventricle. This is the lower chamber of the heart. Blood then passes to the aorta. This is the large artery coming from the heart. From the aorta, blood is sent to the heart muscle itself and to the brain and arms. The blood is bluish in colour because it is low in oxygen. When the heart relaxes, the blood flows through the tricuspid valve into the right ventricle.

The right ventricle contracts to send the blood through the pulmonary valve into the pulmonary arteries. The blood travels to the lungs, where it gets fresh oxygen and turns bright red again. It returns to the left atrium through the pulmonary veins. Then it moves through the mitral valve to the left ventricle.

The left ventricle pumps blood high in oxygen out through the aortic valve into the aorta. The aorta delivers the blood to the body and the process begins again.

This happens in two ways:. These passages usually close after birth, typically sometime between a few days to a few weeks after the baby is born. Skip to main content. It looks like your browser does not have JavaScript enabled. Vascular structures formed early in gestation provide an initial platform for gas exchange and nutrient delivery.

Specialized circulatory structures required for systemic circulation then form later in gestation to support the metabolic needs of the fetus before birth. Once there is adequate fetal-placental circulation established, blood transports between fetus and placenta through the umbilical cord containing two umbilical arteries and one umbilical vein.

The umbilical arteries carry deoxygenated fetal blood toward the placenta for replenishment, and the umbilical vein carries newly oxygenated and nutrient-rich blood back to the fetus.

When delivering oxygenated blood throughout the developing fetus, there are unique physiologic needs, supported by specific structures unique to the fetus which facilitate these needs. The first important structure is a vessel termed the ductus venosus.

When the umbilical vein enters the fetal umbilicus, it branches into two veins: the larger ductus venosus, and a smaller portal sinus. The ductus venosus is a vein which largely bypasses the liver and drains most oxygenated blood directly into the inferior vena cava. However, the liver tissue itself still needs an oxygenated blood supply; to provide this, a portion of blood from the umbilical vein enters the smaller second branch, the portal sinus, which carries oxygenated blood to the liver tissue.

These branches of the portal sinus and the ductus venosus allow delivery of some oxygenated blood to be distributed throughout the liver while most oxygenated blood is shunted around the liver to continue on through the ductus venosus and into the inferior vena cava. Once blood has circulated through the liver, the resulting deoxygenated blood from the liver needs to be drained.

Deoxygenated blood leaves the liver through the hepatic vein which drains into the inferior vena cava. Thus, deoxygenated blood from the liver drains to the same vessel which the ductus venosus drains to. Therefore, the inferior vena cava in the fetus carries oxygenated blood from the ductus venosus mixed with a small amount of deoxygenated blood from the hepatic vein.

This somewhat decreases the oxygen content of the blood in the inferior vena cava; oxygen content is therefore lower in the inferior vena cava than it was in the umbilical vein when it first entered the fetus. After being carried through the inferior vena cava, the next stop for blood is the right atrium of the fetal heart. In an adult, the heart and lungs work together to both replenish the oxygen content of blood, and then pump that oxygenated blood to be distributed throughout the body.

In the fetus though, developing lungs are bathed in amniotic fluid and have no access to oxygen. Thus, the placenta supplies all oxygenated blood, and there is no reason for blood to enter the lungs for oxygenation. The fetal heart therefore serves the sole purpose of pumping blood to be distributed throughout the rest of the developing fetus while allowing the vast majority of blood to bypass the lungs.

There are two structures that allow for this bypass of the lungs. The first structure is encountered in the right atrium. An opening within the atrial septum termed the foramen ovale carries blood from the right atrium directly into the left atrium, thereby bypassing the right ventricle and lungs. Blood that traverses through the foramen ovale then travels from left atrium to the left ventricle to the ascending aorta and is then pumped into systemic circulation.

This is the method by which most blood bypasses the lungs. A small portion of blood which entered the right atrium will not pass through the foramen ovale though. The direction in which blood in the right atrium is subsequently directed is based mainly on its pattern of flow. Blood may enter the right atrium through the inferior vena cava, which in the fetus carries mostly well-oxygenated blood from the umbilical vein and some deoxygenated blood from the hepatic vein; or blood may enter through the superior vena cava, which carries oxygen-deprived blood from the superior structures of the fetus, including the head and upper limbs.

When following inferior vena cava blood, the well-oxygenated blood from the ductus venosus courses mainly on the medial side of the vessel. When this blood then enters the right atrium, it stays medial, therefore being directed toward the medially-placed foramen ovale.

The less-oxygenated blood that came from the hepatic vein courses on the lateral side of the vessel; this less-oxygenated blood then stays lateral when it enters the right atrium, therefore missing the foramen ovale and instead entering the right ventricle, as it would in adult circulation.

Similarly, deoxygenated blood entering the right atrium via the superior vena cava misses the foramen ovale in its downward pattern of flow, and it enters the right ventricle as well. This mixture of blood is then pumped from the right ventricle to the pulmonary trunk.

However, the lungs are filled with amniotic fluid, creating a pressure which is too high for blood to enter the pulmonary arteries at this point. This is another protective measure, ensuring minimal fetal blood enters the lungs during development. Instead, a second fetal structure for bypassing the lungs exists, connecting the pulmonary artery directly to the descending aorta. This vessel is termed the ductus arteriosus.