During a normal session in the U.S. Senate, the Vice President presides. In the absence of the Vice President, the Constitution dictates that a President pro tempore takes over the position.
If both the Vice President and the President pro tempore are absent during a Senate meeting, a junior senator takes up the seat. In the role of the presiding officer, the Vice President has no official vote in the Senate unless as a tiebreaker. The presiding officer is also unable to address the Senate formally unless permission is given. This requires the Senate's unanimous consent.
The President pro tempore is elected to the position by the Senate. This position is typically filled by the majority party's most senior member.