Final answer:
The foramen ovale and the ductus arteriosus allow blood in the fetus to bypass the lungs, while the ductus venosus allows blood to bypass the liver. These shunts are crucial for fetal circulation but close after birth to adapt to life outside the womb.
Step-by-step explanation:
In the fetus, the foramen ovale and the ductus arteriosus allow blood to bypass the pulmonary (lung) circulation. The foramen ovale is a temporary opening between the right and left atria, diverting blood from going to the lungs. On the other hand, the ductus arteriosus is a temporary vessel connecting the pulmonary trunk to the aorta, shunting oxygenated blood back to the systemic circulation instead of going to the developing fetal lungs.
The ductus venosus is another important fetal structure that allows blood to bypass the liver. Branching from the umbilical vein, it carries oxygenated blood directly to the fetal heart. All these structures are vital for fetal circulation since the fetus does not breathe air and does not require blood to flow to the lungs or liver as it would postnatally. After birth, these shunts close off, redirecting blood flow to accommodate breathing air and the cessation of placental gas and nutrient exchange.