Osmosis makes some of the best medical tutorials I've seen

Osmosis is producing medical tutorials of consistently excellent quality. They start out explaining concepts in understandable layman's terms, but go on to delve into topics at a level more appropriate for college or graduate study. This makes them particularly useful if you're somewhat knowledgeable about a topic, but want to review and build on what you already know. 

This video on opioid addiction is a concise introduction to the subject that doesn't get terribly technical. Videos on more specialized topics, such as cardiac dysrhythmias, are far more detailed and complex.