Outstanding mortgages cause a huge part of the cash flow out of the real economy. Here's a link to total mortgages outstanding:
FRB: Mortgages Outstanding
The balance as of the end of 2009 was $14.3 trillion. This includes both residential and commercial mortgages. If we guess at 6% P&I on that amount, we have an outflow of over $200 billion per quarter.
The main money flows in the economy.
Click the image to get to a full-size diagram.