Understanding any particular organ system at the level expected of a medical student is not difficult, especially relative to technical knowledge expected to function in other fields like engineering for example. The answer to your question is completely dependent on how difficult a course director at a particular institution chooses to make their course. Most subjects can be made more difficult by incorporating more detail and course directors sometimes do it if they feel if providing mechanisms adds value to the medical student's education or if something will be translatable to their practice later (ex pulmonary physiology-mechanical ventilation). Cardiovascular physiology can get more challenging if you get into the details of the various sodium channels and fluid mechanics beyond CO=SVxHR. Pulmonology can be made more difficult given the practical aspects with ventilation. Nephrology can be made more difficult by delving deeper into acid/base chemistry. Again, the NBME emphasizes knowledge of a few concepts/facts which are easily graspable. Another way things can be made difficult is if a basic science lecturer chooses to make you memorize a ton of information. In heme, memorizing various heme malignancy subtypes can be challenging. Also, depending on what extent the course director wants to grill histology, the subjects of heme/onc, GI, and nephrology can be made more challenging.
