Which of the following vessels do not carry deoxygenated blood in fetal circulation?
## Core Concept
In fetal circulation, the primary goal is to bypass the lungs, as they are not responsible for gas exchange in the womb. The **foramen ovale**, **ductus arteriosus**, and **ductus venosus** are key shunts that allow blood to bypass the lungs. Fetal circulation ensures that oxygenated blood from the placenta is efficiently distributed to the body.
## Why the Correct Answer is Right
The **pulmonary artery** carries deoxygenated blood in adults, but in fetal circulation, most of the blood in the pulmonary artery is shunted through the **ductus arteriosus** to the aorta, bypassing the lungs. However, the **pulmonary veins** carry oxygenated blood from the lungs to the left atrium in both fetal and adult circulation. Since the lungs are not inflated and are not involved in gas exchange, the blood in the pulmonary veins in the fetus is actually oxygenated blood coming from the **ductus arteriosus** (via the pulmonary arteries) and directly from the **foramen ovale** into the left atrium. Therefore, the pulmonary veins do not carry deoxygenated blood.
## Why Each Wrong Option is Incorrect
- **Option A:** The **superior vena cava** carries deoxygenated blood from the upper body to the right atrium.
- **Option B:** The **inferior vena cava** carries deoxygenated blood from the lower body to the right atrium, but it also receives oxygenated blood from the **ductus venosus** which comes from the placenta.
- **Option C:** The **ductus venosus** primarily carries oxygenated blood from the placenta to the inferior vena cava and then to the heart, but it originates with oxygenated blood.
## Clinical Pearl / High-Yield Fact
A key point to remember is that after birth, with the first breath, the **ductus arteriosus** and **foramen ovale** close, and the **ductus venosus** becomes the **ligamentum venosum**. The changes in circulation after birth are critical for the newborn's adaptation to extrauterine life.
## Correct Answer: D. Pulmonary Veins