pulmonary vein vs vena

pulmonary vein and vena both are nouns.

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

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

pulmonary vein: any of four veins that carry arterial blood from the lungs to the left atrium of the heart
vena: a blood vessel that carries blood from the capillaries toward the heart

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

pulmonary vein (noun)	vena (noun)
any of four veins that carry arterial blood from the lungs to the left atrium of the heart	a blood vessel that carries blood from the capillaries toward the heart