Oxygenated blood from the placenta reaches the fetal heart in utero via:
**Core Concept**
The fetal circulation is a unique adaptation that ensures the exchange of oxygen and nutrients between the mother and the fetus through the placenta. The umbilical cord contains two arteries and one vein, which play a crucial role in this process.
**Why the Correct Answer is Right**
Oxygenated blood from the placenta reaches the fetal heart through the **umbilical vein**, specifically the **left umbilical vein**, which merges with the **left portal vein**. This blood then flows through the **ductus venosus**, a shunt that directs the oxygenated blood from the placenta to the fetal heart, bypassing the liver. The ductus venosus is a highly specialized structure that allows the oxygenated blood to reach the fetal heart efficiently.
**Why Each Wrong Option is Incorrect**
**Option A:** The **umbilical arteries** carry deoxygenated blood from the fetus to the placenta, not oxygenated blood from the placenta to the fetus.
**Option B:** The **ductus arteriosus** is a shunt that connects the pulmonary artery to the aortic arch, allowing blood to bypass the lungs in the fetus. It is not involved in the transport of oxygenated blood from the placenta to the fetal heart.
**Option D:** The **inferior vena cava** carries deoxygenated blood from the lower body to the fetal heart, not oxygenated blood from the placenta.
**Clinical Pearl / High-Yield Fact**
Remember the "4Ds" of the fetal circulation: **ductus venosus**, **ductus arteriosus**, **descending aorta**, and **distal aorta**. These shunts and vessels play a crucial role in the unique circulation of the fetus.
**Correct Answer:** C.