Physics of diving question...

[DetectorGuy]

I am studying for a class and I have some questions about the physics of diving. Some of the questions I had have become crystal clear and some of the things I thought I knew have become confusing. I am struggling with the Haldanean model. I realize that in 1906 Haldane came up with the theory of compartments, which is just a mathematical model reflecting that parts of the body absorb and off gas at different rates. The tissue compartments are a theoretical model of the body. Each part of the body loads N2 at different rates, and also off gases at different rates. For instance blood absorbs N2 rather quickly, and bone much more slowly. Instead of doing a model for every tissue in the body,  His original model was based on 5 compartments, the Navy tables on 6 compartments, and the PADI RDP on 14. This much I get...

Where I start to get lost is when Haldane assigned a halftime to each compartment. A compartment half time is the time in minutes for a particular compartment to go "half way" from its beginning tissue pressure to equilibrium at a new depth. Haldane's original model had halftimes ranging from 5 minutes to 75 minutes. Modern models range from 3 minutes to over 600 minutes. My first set of questions would be this: What body parts relate to each of the compartments and is it important to know which body parts are fast compartments and which are slow? I am assuming that blood is a fast compartment and bone is slow, but other than that I can not find reference to answer this in any books. I might be over thinking this...
The second set of questions would be: How do I find out which compartment has a 5 minute halftime and which has a 20 minute halftime? Are these minutes just theoretical per compartment or are they written somewhere?
The third question is an example out of the book asking: A 20 minute halftime compartment will have how much tissue pressure after 40 minutes at 80fsw?
Their answer was: 60fsw of pressure (40 minutes = 2 halftimes for a 20 minute halftime. After 1st halftime, pressure goes halfway from 0 to 80fsw, which is 40fsw. After the second halftime, pressure goes halfway from 40fsw to 80fsw, which is 60FSW).  I get that part of the theory, but I am still confused as to which part of the body the 20 minute halftime compartment is assigned to.
Then I was wondering about M-values. I read that M-value is the maximum tissue pressure allowed in the compartment when the diver surfaces, so as to prevent exceeding the maximum acceptable gradient. If the diver exceeds the M-value in any compartment, there's an unacceptable risk of decompression sickness. Also that when calculating decompression dives, there are M-values for each compartment at each decompression stop depth. For Recreational diving we only need to be concerned with the surface M-value. The faster compartments have a higher M-value and slower compartments have a lower M-value.
My fourth and final set of questions for the night are relating to M-values: Is it important for a recreational diver to know what the M-value is for a specific compartment, or is this just a theoretical value given to a theoretical compartment by the people that created the model that our computers and tables are based on?

Let me know if you can explain these questions to me in laymans terms, John