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The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republiccomposed of 50 states, the federal district of Washington, D.C., five major territories, and various possessions.[fn 1] The 48 contiguous states and Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is in the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The territories are scattered about the Pacific Ocean and the Caribbean Sea. At 3.8 million square miles (9.9 million km2)[18] and with over 320 million people, the country is the world's third largest by total area (fourth largest by land area)[fn 2] and the third most populous. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.[24]

The geography and climate of the United States are also extremely diverse, and the country is home to a wide variety of wildlife. The term "United States", when used in the geographical sense, is the contiguous United States, the state of Alaska, the island state of Hawaii, the five insular territories of Puerto Rico, Northern Mariana Islands, U.S. Virgin Islands, Guam, and American Samoa, andminor outlying possessions.[1] The United States shares land borders with Canada and Mexico and maritime borders with Russia,Cuba, and the Bahamas in addition to Canada and Mexico. From 1989 through 1996, the total area of the US was listed as 9,372,610 km2 (3,618,780 sq mi) (land + inland water only). The listed total area changed to 9,629,091 km2 (3,717,813 sq mi) in 1997 (Great Lakes area and coastal waters added), to 9,631,418 km2 (3,718,711 sq mi) in 2004, to 9,631,420 km2 (3,718,710 sq mi) in 2006, and to 9,826,630 km2 (3,794,080 sq mi) in 2007 (territorial waters added). Currently, the CIA World Factbook gives 9,826,675 km2 (3,794,100 sq mi),[2] the United Nations Statistics Division gives 9,629,091 km2 (3,717,813 sq mi),[3] and the Encyclopædia Britannica gives 9,522,055 km2(3,676,486 sq mi)(Great Lakes area included but not coastal waters).[4] These source consider only the 50 states and the Federal District, and exclude overseas territories

The United States is the world's largest national economy in nominal terms and second largest according to purchasing power parity (PPP), representing 22% of nominal global GDP and 17% of gross world product (GWP).[2] The United States' GDP was estimated to be $17.914 trillion as of Q2 2015.[2][30] The U.S. dollar is the currency most used in international transactions and is the world's foremost reserve currency Over the same period, the three heavyweights, the European Union, USA and Japan, have registered a decrease. In 2002, almost 83% of research and development was carried out in developed countries; by 2007 this share had dropped to 76%. This trend is even clearer when industry’s contribution to GERD is considered. Between 2000 and 2007, the private sector share of R&D spending, as a proportion of GDP, saw a sharp increase in Japan, China, Singapore and especially the Republic of Korea, while it remained stable in Germany, France, and the United Kingdom and even saw a slight decrease in the Russian Federation and the USA.

Nephrology continues to be in transition. While rates of kidney diseases and injury continue to rise in the US, changes in the general health care system and the delivery of kidney care make it unclear how increases in need will be translated into demand for nephrologists. The changes in the delivery system also raise questions as to the future roles and career paths for nephrologists. ESRD patients, and nephrology fellowship programs is not evenly distributed across the US. They also show that there are few fellowship programs in HRRs that have the highest number of ESRD patients per nephrologist. These regions are likely to be among the areas with greatest need. Any effort to align the number and location of training of nephrologists should focus on these areas. Not surprisingly, many of the HRRs near clusters of fellowship programs do not have high numbers of ESRD patients per nephrologist. This likely reflects the fact that many physicians locate in areas near where they have trained. A question that needs to be explored is whether nephrologists completing training in relatively rich nephrology supply areas are more likely to move to areas with greater need or stay in the region where the supply is already relatively high. The 68% fill rate for nephrology is well below most other IM subspecialties which generally matched in the >90% range. Only infectious disease (ID) was close (70% match). Geriatrics, which is not in the SMS, was the only specialty with an even lower match rate (44%). » Of all IM subspecialties, nephrology had the lowest success in matching USMDs measured as a percentage of all matched. Only 31% of nephrology matches were USMDs compared to 50% for all IM subspecialties. » Nephrology did slightly better than average with osteopathic graduates, with DOs accounting for 9% of the matches. » Nephrology had the highest percent of matches both for US citizen IMGs (17%) and non-US IMGs (42%)—a total of 59%.