Good day. Atriums have thing walls , ventricles wall is thick due to thicker miocardium. Left ventricle miocardium thicker than right ventricle. But sorry, it looks like tangetial slice of ventricles. May be try, to use for example indian ink for marking different parts of heart. Like it used for marking in histopathology dissection of cancers.

Here is example https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.turkjpath.org/pdf.php3%3Fid%3D1788&ved=2ahUKEwjL7935irDtAhWLGewKHZkFCU4QFjAAegQIAhAB&usg=AOvVaw32Dk2KeUOm2RqX0vNugTxN

Thanks Dmitry !

So you think it's left ventricle ? but i see valves ... maybe between left ventricle and left atrium ?

I can used marking because the heart is fixed in resin and after we do micrometer section