The brain structure that controls unconscious but essential functions such as breathing and circulation is the

The correct answer is medulla oblongata.

Medulla oblongata is the part of the central nervous system. It is the structure which continiues to spinal cord and represents the lowest part of the brainstem. Function of medulla oblongata is to control involuntary, autonomic functions. It contains centers for the heart and lungs, thus it regulates functions of breathing, heart rate and blood pressure.