Article 1 of the U.S. Constitution is the largest and most important section of the document. It prescribes the make up of the Congress.
The Upper House, or Senate, was designed to give representation to the the States themselves and the Lower House, or House of Representatives, was designed to represent the people. Originally the Senate was elected by the legislature of each State. This was amended to become a popular vote and weakened the FEDERALIST system the Founder’s created. The Senate is the strongest “check” against the President of the U.S. Though recently the Obama administration has weakened the Senate further by trying to limit their ability to confirm appointments that the President makes.
The House of Representatives have also been heavily weakened by abrogating their duties to bodies like the Federal Reserve and the Bureaucracy of the Executive Branch.