Educators juggle the four Fair Use factors to determine if the balance leans toward Fair Use. There is no precise description or formula to use. All four Fair Use factors have to go into your consideration of whether your use of a copyrighted work is Fair Use.
Purpose and character of the use
Face-to-Face teaching in a non-profit educational institution is considered Fair Use. Good for us! Most academic health sciences centers are non-profit educational institutions teaching students formally enrolled in their courses and programs.
Nature of the copyrighted work
Use of fact-based works is much more likely to be considered Fair Use than highly creative works. The author's original creativity is judged to be a strong factor. More good news! Medical textbooks, most journal articles, documents, reports, and other materials that you are likely to be sharing with your students are heavily fact-based. The information in a table may just list facts and may not really be a matter of copyright. It can be used that way, or the facts can be entered into a new table or other layout you create. However, the representation or expression of this information in an animation, graph, diagram or such may be highly creative. In the case of highly creative works or expressions of ideas, Fair Use will allow you to distribute the item one time but require that you get permission for continued use.
Amount and substantiality of the portion used in relation to the whole
The proportion of the material that you use and whether that portion that you are using is the most substantial or significant portion of the work is considered in the determination of Fair Use. This one is in your hands as you control what portion of any copyrighted material you use. The most common interpretation of an appropriate amount is a chapter of a book, an article in a journal, a small portion of the tables or images from a book.
Effect on the potential market or value of the copyrighted work
This is perhaps the most important factor in determining Fair Use. If you are sharing so much of a copyrighted work that it takes the place of a student textbook that your students would ordinarily purchase, you may be infringing on the copyright holder's rights. Consider whether your use of a copyrighted material realistically, not just hypothetically, affects the potential market for that material.