it depends on task U have to do.

It can be  as complex as you wish to make it.

In general it is a quite simple apparatus. Like others mentioned it depends on what measurements you want to make.

A traditional Langendorff Isolated Heart Model you can buy almost as a unit and build it together mostly from glas parts. The old Dr. Langendorff modell is basicly not really simulate nature anymore.

If you want to simulate nature and the real daily human cardio-thoraci c surgery or cardiology praxis /real life/ than you have to educate yourself  to the direction of cardioprotrction, even blood cardioprotection and to consultate with the cardio-techniques collegues. (They are controlling the heart-lung mashines during open heart surgery).  A modern Up -to -Date Langendorff type heart model even the working heart model are today blood perfused and memebrane oxygenated and  frequently lab. (Astrup) controlled. The fluid is calibrated by osmolality and measured on-line for temperature. /heart and blood simultanously/