Well this might be a bit barebones and quite simplistic, (after all, much of the MCAT is based on basic science) but I'd recommend BoozemanBiology (YouTube channel). And Khan, without question. Khan is much longer than your typical YouTube vid, but that's because he so exhaustively covers the topic (usually), and you'll understand it damn well.
For the renal system, Khan is a must. Digestive system is very well covered by Bozeman.
YouTube is seriously a goldmine of information - search and you'll find what you need!
Good luck!!