The biggest source of debt demand is our the trade surplus that other nations have with us...they recycle the excess into our debt...and have done so eagerly.
Another source of debt demand has been the net zero interest Yen based rates.
Other countries, corporations and individuals with gobs of cash (Think Arab Sheik) convert to Yen and buy our debt...to take advantage of the interest rate differential.
Our politicians have just kept spending knowing that their very policies are creating both a debt and trade deficit domestically...and the cycle has been spinning wildly since the late 80's..
A whole lot of things are going to change in the coming years with China gobbling up resources, interest rates worldwide on the rise...and according the street (herd), our interest rates are going to flatten out...
If there is flight from our bonds, the fed is going to have one hell of a time trying to keep interest rates flat...
The ten year is sticking its head above a 4.7 yield to have a look around for the first time in about a year...many think this is the critical yield resistence to get through...imho ten-years are headed to 5.0+ easily this year...and higher yet after that...