I want to show heart's interior i.e., I wanna show the heart's inside portion by cutting heart in two parts, but I am not sure how to start? Anybody guess how to start it or is there any tutorial?
As you asked "how to start", here is a way to begin the modeling:
Keep it simple
A tube is a subdivided open cube:
You can extrude it using CtrlLMB:
You can also extrude or inset in order to have some branching out and AltShiftS allows to round (go to sphere) rapidly:
From a surface, you can also extrude to create vessels in/out puts:
Once the exterior part modeled (be sure to be happy with it). You can create the inner part using a solidify modifier, with no rim:
Then apply the modifier (only the solidify, not the subdivision) and separate the two parts using P then 'by loose parts'. So you can work on the inner model keeping the exterior as it is:
After that you can 'draw' the inner parts, for instance for a valve, using a bevel CtrlB:
Or the knife tool:
Hope these few simple technics can help.