Allow me to butt in. And I'm not after credit; that goes to Deborah. I'm wearing my professor's hat now.
The "cm-5" is "centimeter to the minus fifth power" and should be superscript. That is the negative fifth power, or the number of centimeters, say 10, multiplied by itself five time, but instead of being a large positive number, it is a small decimal fraction 10 x 10 x10 x 10 x 10 x 10 = 1,000,000. With the neative sign, it's 0.0000001.
The minus fifth power derives from the fact that area is in cm squared and flow is volume/sec and is cm cubed/sec.,Next force (dynes) is convrted to mm Hg or force /( cm x cm) pressure. Thus, the pressure exerted by the moving column of blood laterally on the wall of the vessel is a function of force, the area on which the force is bearing, and the volume/sec bearing on it.
Dynes is the unit of force and can be converted to mm Hg.
The formula you ask about is derived from flow (ml/sec) or (cubic cm per second), area (cm squared), and pressure (mm Hg/ cm x cm). That gives you cm to the fifth, but as we resolve the equation by multiplcation, the cm moves into the denomination and the positive fifth power becomes the negative fifth power.
Wood units come from the late English cardiologist Paul Wood, M.D. who said, "That's a hell-of-an -awakward unit to be putting into print." Someone then said, "Right-O, Paul, we'll call it a Wood unit." And so it came to pass.
A nice explanation should be found in Arthur C. Guyton's "Human Physiology and Mechanisms of Disease"
Wow, isn't this fun?
If it's not clear enough, let me know. I'l have one of my students translate it.
Teaching Doc
What I tell you is what I teach. I'm here to help improve your quality of life, and I'll work with you as long as necessary to help you.