Question

which is the largest vein and artery in human body

Answer 1

The largest vein in the body isVena Cava. 
The largest artery in your body is theAorta.

Answer 2

The largest artery is the aorta, the main high-pressure pipeline connected to the heart's left ventricle. The aorta branches into a network of smaller arteries that extend throughout the body. The arteries' smaller branches are called arterioles and capillaries.

The largest vein in the body is Vena Cava. The aorta is the largest blood vessel in the body. It is an artery that directly arises from the heart itself and descends through the thorax and into the abdomen. All the arteries of the body, save the pulmonary arteries, stem from the aorta or one of its main branches.