Why the lungs are largely bypassed by circulating blood in fetus?

In the fetal circulatory system, the lungs are largely bypassed by circulating blood because the fetus receives oxygen and nutrients from the mother's circulation. The fetal lungs are not yet fully developed and are not responsible for gas exchange. Instead, the placenta, which is an organ that connects the mother's and fetal circulatory systems, serves as the primary respiratory and nutrient exchange site.

Here's a detailed explanation of why the lungs are largely bypassed in the fetus:

1. Placental Circulation: The mother's oxygenated blood enters the placenta through the uterine arteries. Within the placenta, this oxygen-rich maternal blood comes into close proximity with the fetal blood vessels, but the two circulations do not mix. The fetal blood absorbs oxygen and nutrients from the maternal circulation through a process called diffusion.

2. Ductus Venosus: Oxygenated blood from the placenta is carried to the fetus through the umbilical vein. Instead of flowing directly to the liver like it would after birth, the majority of this oxygenated blood bypasses the liver in the fetus. It does so through a specialized blood vessel called the ductus venosus.

3. Foramen Ovale: The ductus venosus empties into the inferior vena cava, which normally carries deoxygenated blood back to the heart. However, in the fetus, the foramen ovale, an opening between the right and left atria of the heart, is open. This allows most of the oxygenated blood from the inferior vena cava to cross directly into the left atrium, bypassing the lungs.

4. Ductus Arteriosus: The oxygenated blood from the left atrium is pumped into the systemic circulation through the left ventricle and aorta. However, some of this blood is diverted away from the lungs and into the descending aorta through a vessel called the ductus arteriosus. The ductus arteriosus connects the pulmonary artery, which carries blood to the lungs, with the descending aorta. This ensures that most of the oxygenated blood reaches the body's tissues and organs, rather than being sent to the non-functional fetal lungs.

5. Pulmonary Circulation: Only a small amount of blood is directed to the fetal lungs through the pulmonary circulation. This blood provides some oxygen and nutrients to the developing lungs, but it is not sufficient for gas exchange. The lungs are primarily engaged in fluid production and maturation during fetal development.

After birth, with the first breath of air, the baby's lungs expand, and placental circulation ceases. The foramen ovale and the ductus arteriosus close, and the pulmonary circulation becomes fully functional. This allows the lungs to take over the primary role of gas exchange, and the fetus transitions to independent respiration.