What structure is formed when the external jugular vein drains into the subclavian vein?

The correct choice indicates that when the external jugular vein drains into the subclavian vein, it contributes to the formation of the right and left brachiocephalic veins.

The subclavian vein itself receives blood from both the external and internal jugular veins in the region of the neck. When the subclavian vein on either side (right or left) combines with the internal jugular vein, they form the corresponding brachiocephalic vein. This is a vital part of the venous system, as the right and left brachiocephalic veins subsequently converge to form the superior vena cava, which then drains deoxygenated blood back to the heart.

Understanding this anatomical relationship is crucial for grasping how systemic venous return occurs in the upper body. Knowledge of vein drainage patterns, particularly those involving the brachiocephalic veins, is essential in clinical settings as it has implications for various medical procedures and understanding cardiovascular anatomy.