"Total debt of the federal government can increase in two ways. First, debt increases when the government sells debt to the public to finance budget deficits and acquire the financial resources needed to meet its obligations. This increases debt held by the public. Second, debt increases when the federal government issues debt to certain government accounts, such as the Social Security, Medicare, and Transportation trust funds, in exchange for their reported surpluses. This increases debt held by government accounts. The sum of debt held by the public and debt held by government accounts is the total federal debt. Surpluses reduce debt held by the public, while deficits raise it. Total federal debt outstanding was $14,288 billion on April 29, 2011, of which $14,236 billion was subject to the debt limit. The U.S. Treasury projects the federal debt will reach its statutory limit before May 16, 2011. The Treasury Secretary wrote that he would declare a debt issuance suspension period on that date, unless Congress acted beforehand, which would allow certain extraordinary measures to extend Treasury's borrowing capacity until early August 2011. Funding federal operations could soon become complicated without a debt limit increase."
CRS Report for Congress, RL31967