Combing meshes smoothly

Hi all

Am trying to combine 2 and more different meshes in Blender and then create a smooth transition between them (you’ll get an idea when you see the heart & vessels that I’m working on).

Does this have to be done via topology so combining the meshes by manually going through and joining vertices?

Or can I combine them & then use, for instance clay and the sculpt tool to ‘pad’ the angles, - I feel not going on the funky results I’ve got so far…

I’ve been combining the different meshes using ctrl-J…

Ctrl-J only makes the separate meshes part of one object, but does not resolve any edge issues.
If you want the ‘join’ to be ‘clean’ you must make a hole in the heart of the same shape as the blood vessel and then stitch them together using a ‘bridge edge loops’ function, either the new native blender one or the looptools one. I am assuming there is no gemotry inside that you want to keep, but if you want the inside to be anatomically accurate - as in you want to follow a blood cell through its journey there’s a lot more modeling involved.
in some ways it might be easier to start with the heart shape and choose some quad on it s surface and extrude it to follow the path of the blood vessel. This involves some rotating of the model to get the extrusion to follow the right path but is quite fast and you can scale during the process to taper the tube.
A similar approach would be to cut a hole in the heart and position duplicates of that edge properly and then bridge them all in one operation. This requires a bit more planning but is quite effective.
If you have trouble following what I have mentioned you might try posting a Blend file to get more meaningful help. Your screenshot does not show a mesh so I don’t know exactly what you’re starting with, in terms of quads, triangles and mesh density.
Good luck!