That would depend on you. Do you have an affinity for the sciences? Do you suck at math? If so, then chemistry may be challenging. Does it take you a long time to memorize stuff? If so, then anatomy will be challenging. If you have an interest in it all as well as an ability to do that kind of work then you'll be fine.
I took human anatomy and physiology I and II with labs last summer. I work full-time, and they were more for a hobby than anything else. I actually used to teach that subject as an AP H.S. class. However, I found the courses littered with busy work. The book was awesome. The material was awesome. The work was stupid.