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.
The President of the Senate is the head of the Senate and this function is always served by the Vice-President of the United States. However, the Vice-President is usually committed to other work and can't in practice we present at the Senate. The person who leads the Senate when the President of the Senate can't is called the President Pro Tempore of the Senate: this is usually the person with the biggest seniority, that is experience in the Senate.