The structure indicated is the left gonadal vein.

The gonadal veins are the veins which drain the testes in men (testicular vein) and the ovaries (ovarian vein) in women.  The left gonadal vein drains into the left renal vein, which then drains into the inferior vena cava. The right gonadal vein however, drains directly into the inferior vena cava.

The inferior vena cava is responsible for carrying deoxygenated blood into the right atrium of the heart from the lower part of the body. The inferior vena cava is formed at the point of union of the right and left common iliac veins at the L5 vertebral level.

Several veins drain into the inferior vena cava:

• Lumbar veins
• Right gonadal vein (the left gonadal vein drains into the left renal vein)
• Renal veins
• Right suprarenal vein
• Inferior phrenic vein
• Hepatic veins

Learn more about the veins of the body in this tutorial.