In Physio the 3 big killers that will stay with you until your residency and beyond (if your're going into IM, FP, Peds, or Anesthesia) are pulmonary, cardiovascular, and renal physiology...OK?
As you get farther along in your clinical training, for example in cardio., you'll be more concerned with extra heart sounds (S3, S4) or murmurs rather than things like the relation of cardiac muscle sarcomere length to amount of force generated.
Renal physiology gets really hairy too...You basically need to retain everything you've learned in your 1st renal, because you'll need it for understanding of how diuretics work, acid-base imbalances (a ruthless killer!), or electrolyte imbalances, all of which are of clinical relevance.
For Pulmonary I suggest you retain knowledge of V/Q mismatch, alveolar gas exchange, and arterial blood gases.
WARNING: Everything and anything you learning in your first two years is fair game for USMLE Step 1, so even if you don't think you might not need it for your second year (after all, you have to make some room for all the Micro., Path, and Pharm), you should learn it well so that when you study for your boards, you don't need to start learning from scratch.
I hope this helped...Good Luck!!