関連ワード:  how superior vena cava is formed    superior vena cava is formed by the union of    superior vena cava is formed at the level of    superior vena cava is formed by the union of the two    superior vena cava is formed by which vein    superior vena cava is formed by the merging of    superior vena cava is formed by fusion of    superior vena cava is formed by the junction of    superior vena cava is formed by uniting veins of    the superior vena cava is formed by the merging of the blank veins