creating a looping formula between different spreadsheets
 

I think half of my problem is that I don't know what to call what I'm trying
to do. I have two different spreadsheets, for simplicity, we can call them A
and B.

Spreadsheet A has 3 columns, one is the name of the row, the second is a set
of numbers, and the third is the cumulative sum of the set of numbers.

Spreadsheet B has 2 columns, one is the title of the rows, and the other
will be a cumulative sum by the time I'm done.

What I would like to do is start spreadsheet B with a 0 in the second column
of the first row. That zero would start the cumulative sum of spreadsheet A.
Spreadsheet A would run through the cumulative sum and put that value in the
second row of spreadsheet B. What I would then like to happen is have this
value be placed at the beginning of the cumulative sum on spreadsheet A, but
I can't figure out how to do this without creating a circular reference. So
basically the cumulative sum in spreadsheet A is nested inside of the
cumulative sum of spreadsheet B.

I realize that I could just expand spreadsheet A, but the reason I am doing
this is to create a model for a hypothetical child's exposure to pollutants
from birth to 18 years, and since I'm working with minute by minute data,
this would result in a spreadsheet that is 9.5 million rows long, which is
not exactly something I want.


If there is anything not clear about this I will gladly clarify.
Any help on this problem would be vastly appreciated!