renal vein vs vena

renal vein and vena both are nouns.

Word	Noun	Adjective	Verb	Adverb
renal vein	Yes	No	No	No
vena	Yes	No	No	No

As nouns, vena is a hypernym of renal vein; that is, vena is a word with a broader meaning than renal vein:

renal vein: veins that accompany renal arteries; open into the vena cava at the level of the 2nd lumbar vertebra
vena: a blood vessel that carries blood from the capillaries toward the heart

Other hypernyms of renal vein include vein, venous blood vessel.

renal vein (noun)	vena (noun)
veins that accompany renal arteries; open into the vena cava at the level of the 2nd lumbar vertebra	a blood vessel that carries blood from the capillaries toward the heart