It is merely a theory that explains how free enterprise works. Banks for example, a very important part of the economy, are left out. Some socities do not accept the circular flow model theory. These socities, therefore, choose another economic system other than capitalism. Socialists and communists see the role of the government in a different light than capitalists do.
The United States Constitution states that the Vice President of the United States is the President of the Senate, despite not being a senator, and that the Senate must choose a president pro tempore.
So the Senate elects a President pro tempore, currently Daniel Inouye (Democrat of Hawaii) who is basically a much less powerful version of the Speaker of The House (Since the Senate doesn't have formal rules for debate the Senators themselves more or less determine their own speaking time).