sigmoid vein vs vena

sigmoid vein and vena both are nouns.

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

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

sigmoid vein: tributaries of the inferior mesenteric vein; drains the sigmoid colon
vena: a blood vessel that carries blood from the capillaries toward the heart

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

sigmoid vein (noun)	vena (noun)
tributaries of the inferior mesenteric vein; drains the sigmoid colon	a blood vessel that carries blood from the capillaries toward the heart