<s>[INST] context: Fetal circulation

Fetal circulation differs from adult circulation in several ways. Almost all differences are attributable to the fundamental difference in the site of gas exchange.
In the adult, gas exchange occurs in the lungs, while in the fetus placenta provides the exchange of gases and nutrients.

The course of fetal circulation
A) Blood returning to the heart

Blood oxygenated in the placenta returns by way of the umbilical vein, which enter the fetus at the umbilicus and course through to join the portal vein.
The ductus venous is a bypass shunt between the portal vein and IVC.
Most of the umbilical venous blood shunt through the ductus venosus to the IVC.
Blood from IVC (which carries the blood drained from lower extremities, umbilical veins, hepatic veins and renal veins) enters the right atrium.
On reaching the right atrium, about 1/3rd of total inferior vena caval return of blood enters the left atrium through formen oval.
The rest two third mixes with the venous return from SVC (which carries the blood drained from head, neck and upper limbs) to enter the right ventricles.

B) Blood coming out of the heart
1. Right side

The right ventricle pumps out the blood into pulmonary trunk very small amount of this blood enters the pulmonary circulation because the lungs are collapsed and pulmonary vascular resistance is high.
Most of this blood passes to descending aorta through ductus arteriosus, a shunt between pulmonary trunk and descending Aorta.
The descending aorta carries blood to umbilicus through the umbilical artery, for oxygenation.
Descending aorta also supplies the blood to lower limbs, kidney and abdominal viscera.

2. Left side

The left ventricle pumps blood into the ascending aorta for distribution in the coronary circulation, head and upper limbs. [/INST] question: Oxygenated blood to the fetus is carried by – | options: A: Umblical artery B: Umblical vein C: SVC D: Pulmonary artery | answer: B </s>