endocardium vs serous membrane

endocardium and serous membrane both are nouns.

Word	Noun	Adjective	Verb	Adverb
endocardium	Yes	No	No	No
serous membrane	Yes	No	No	No

As nouns, serous membrane is a hypernym of endocardium; that is, serous membrane is a word with a broader meaning than endocardium:

endocardium: the membrane that lines the cavities of the heart and forms part of the heart valves
serous membrane: a thin membrane lining the closed cavities of the body; has two layers with a space between that is filled with serous fluid

Other hypernyms of endocardium include serosa.

endocardium (noun)	serous membrane (noun)
the membrane that lines the cavities of the heart and forms part of the heart valves	a thin membrane lining the closed cavities of the body; has two layers with a space between that is filled with serous fluid