Name one difference between the composition of the blood in the veins and the vena cava

There is no difference in blood composition of vein and vena cava. Both vein and vena cava carries deoxygenated blood from cells to the heart. From heart, the blood goes to the lungs for cleaning. In lungs, the carbondioxode in the blood is removed and oxygen is loaded in the blood. From lungs the blood goes to the heart and then pumps to all parts of the body.