Alteration in Na+, Cl-, and H2O

I am having a hard time trying to understand:

• Isotonic,
• Isotonic Fluid lost, Isotonic Fluid Excess
• (Why did my book state that “it is caused by Hypovolemia and dehydration” & “Isotonic Fluid Excess is caused by hypervolemia” This make no sense. How is it normal but low and vice versa!)
• Hypervolemia
• Hypernatremia, Hyponatremia, and all those other electrolytes
• Hypovolemia
• Hypernatremia ,Hyponatremia, and all those other electrolytes
• (Same question. Why will it be Hypovolemia Hypernaremia, if the both the Na+ and H2O is low, why is it Hypernatermia? Hypernatermia= >145 mEq/L. Since it went down shouldn’t the Na+ be lower than 145?)

If there is any lab values, much will be appreciated. Thank you.