I don't think the adjectives: nice, easy to use, and inexpensive can all be applied to any of the major 3D applications.
Blender would be inexpensive (free/open source) but definitely has a learning curve and you might not get the best results without a lot of time spent tweaking.
Maya is the industry standard, and runs "okay" on Mac, but is thousands of dollars, and also has a pretty steep learning curve.
3D Studio Max is windows only, expensive, and difficult to use.
Sketchup, as far as I know, would be too basic and doesn't do animations.
Maxon Cinema 4D runs great on the Mac, is supposed to be more intuitive than the rest, but is also going to be thousands of dollars. This is the 3D package I use, and have seen many examples of creating blood cells, using dynamics and fancy rendering to make good body simulations.
Any other packages that come to mind will all have the same flaws as stated above. I would recommend downloading the demos of some of these programs and looking for some tutorials online if you are interested. If you don't have any prior experience, or you don't want to invest a lot of time/money into this, I would recommend trying to freelance out this work. Sorry, that is probably not what you were hoping to hear.