Federal trust funds are created in the U.S. Treasury to account for all program income, such as Social Security and Medicare taxes, and disbursements, such as benefit payments and program administrative costs. Revenues not needed in a particular year are invested in special non-marketable government securities; therefore, the trust funds represent the total value, including interest, of all prior program annual surpluses and deficits. There are two Medicare trust funds: the Hospital Insurance Trust Fund (HI), which pays for inpatient hospital and related care, and the Supplementary Medical Insurance Trust Fund (SMI), which pays for physician and outpatient services. Medicare Part D prescription drug expenditures are paid out of the SMI Trust Fund.