The U.S. government does not issue U.S. currency - the Federal Reserve does. The Federal Reserve is a private bank owned and operated for profit by a very powerful group of elite international bankers.
The U.S. government cannot simply go out and create new money whenever it wants under our current system. Instead, it must get it from the Federal Reserve. So, when the U.S. government needs to borrow more money it goes over to the Federal Reserve and asks them for some more green pieces of paper called Federal Reserve Notes.
The Federal Reserve swaps these green pieces of paper for pink pieces of paper called U.S. Treasury bonds. The Federal Reserve either sells these U.S. Treasury bonds or they keep the bonds for themselves.
The U.S. national debt is approximately 12 trillion dollars. So how much money actually exists in the United States today?
The "M0" money supply is the total of all physical bills and currency, plus the money on hand in bank vaults and all of the deposits those banks have at reserve banks. As of mid-2009, the Federal Reserve said that this amount was about 908 billion dollars.
The "M1" money supply includes all of the currency in the "M0" money supply, along with all of the money held in checking accounts and other checkable accounts at banks, as well as all money contained in travelers' checks. According to the Federal Reserve, this totaled approximately 1.7 trillion dollars in December 2009, but not all of this money actually "exists" as we will see in a moment.
The "M2" money supply includes everything in the "M1" money supply plus most other savings accounts, money market accounts, retail money market mutual funds, and small denomination time deposits (certificates of deposit of under $100,000). According to the Federal Reserve, this totaled approximately 8.5 trillion dollars in December 2009, but once again, not all of this money actually "exists" as we will see in a moment.
The "M3" money supply includes everything in the "M2" money supply plus all other CDs (large time deposits and institutional money market mutual fund balances), deposits of eurodollars and repurchase agreements. The Federal Reserve does not keep track of M3 anymore, but according to ShadowStats.com it is currently somewhere in the neighborhood of 14 trillion dollars. But again, not all of this "money" actually "exists" either.
So why doesn't it exist? It is because our financial system is based on something called fractional reserve banking.