Which fetal shunt connects the umbilical vein to the inferior vena cava?

A. They form alternate channels in the lungs: Venous anastomoses are not specific to the lungs, and their primary role is not limited to pulmonary circulation. While collateral pathways exist in many organs, venous anastomoses occur throughout the body and serve a broader function in maintaining venous return.

B. They restrict the flow of blood back to the heart: Venous anastomoses do not impede venous return. Instead, they provide alternate routes for blood to reach the heart if a primary vein is obstructed, maintaining tissue perfusion and reducing the risk of congestion.

C. They reconnect previously branched blood vessels: Venous anastomoses are direct connections between veins that previously branched from a common vessel or ran in parallel. These connections allow blood to bypass blockages or take alternative pathways, providing redundancy in venous circulation and stabilizing hemodynamics.

D. They only occur in arteries: Anastomoses are not exclusive to arteries; veins commonly form anastomotic networks. Venous anastomoses are particularly important in regions with variable flow or potential obstruction, ensuring continuous venous drainage from tissues.

Correct answer: True

In veins, the tunica externa (also called the tunica adventitia) is typically the thickest layer of the vessel wall, providing structural support and elasticity to accommodate large volumes of blood at low pressure. In some larger arteries, while the tunica media is generally the thickest layer to withstand high pressure, the tunica externa can still be relatively thick to anchor the vessel to surrounding tissues and provide additional tensile strength. This structural arrangement allows veins to be highly compliant and arteries to maintain their shape under pulsatile blood flow.