Dr. Rich Van Konynenburg gave us a good scientific explanation of this. I don't have it on this computer but will try to find it later. Maybe others have it saved. It involves HPA axis dysfunction and vasopressin. Basically, we pee it out. It is possible to measure your blood volume though there are not many medical centers have the equipment. Again, maybe others can give more details.
Aside from drinking fluids, increasing "good" salt and taking more electrolytes, whatever is wrong with the HPA axis needs to heal. There are also drugs that increase vasopressin too, but they don't deal with the cause. Methylation therapy was one method Rich recommended for normalizing the HPA axis.
I'll look for his post....
Sushi