The separation of powers and concomitant system of checks and balances that reside in the U.S. Constitution are clearly spelled out in the first three articles:
Article 1: "All legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and a House of Representatives."
Article 2: "The executive Power shall be vested in a President of the United States of America."
Article 3: "The judicial Power shall extend all Cases, in Law and Equity, arising under this Constitution, the Laws of the United States, and Treaties made, or which shall be made, under this Authority..."
Within each of those three articles, the Constitution spells out in far greater detail the roles and responsibilities of each of the three branches of the federal government. That division of labor and responsibility was designed to prevent the consolidation in any one branch of a preponderance of power. The drafters of the Constitution were so concerned with establishing a system of checks and balances, that the principle of separation of powers was even applied within the legislative branch, Congress. The requirement for the establishment of both a House of Representatives and a Senate was designed to ensure a balance of power within the most representative of three branches. As such, the Constitution mandates that spending bills (appropriations) originate in the House of Representatives, and that only the Senate enjoys the power of consent over Executive Branch nominations of ambassadors and high level officials, and only the Senate is vested with authority to ratify international treaties negotiated by the Executive Branch.