Credit goes to your brain stem, the autopilot for your most important automatic functions: breathing, blood pressure, and heart rate.
The brain controls some of our body activities without us having to think about them. Such examples include breathing, heartbeat, and body temperature. When we sleep or even if we faint, these activities keep working. They are all automatic.
The brain sends out signals along nerves to the muscles of the heart, intestine, lungs, stomach, liver, kidneys, bladder, and other internal organs telling them when and how to move. Usually you are not aware of this unless something happens, like when you are frightened and your heartbeat and breathing become faster.
The part of the nervous system called the autonomic nervous system is a network of nerves that carries these automatic signals back and forth between the brain and the different organs. This system makes sure that your normal body functions continue to run smoothly.
Picture Credit : Google