The Autonomic Nervous System (ANS)
The Autonomic Nervous System is a complex neural network that controls and regulates most visceral functions in our body (eg blood pressure, circulation, digestion, body temperature, etc.). We are not able to control the ANS directly with our will, but on the other hand we do not need to worry about these functions because they are regulated automatically in the background.
NB : The counterpart to this is the somatic nervous system which enables us to think, feel and move, to receive impulses from our sensory organs and react with coordinated muscle action.
The autonomic nervous system is divided into two subsystems, the Sympathetic and the Parasympathetic nervous system.
Depending on the situation, one or the other of these two subsystems will be predominant in its activity.
Further reading: Wikipedia: Autonomic Nervous System
The Parasympathetic nervous system increases its level of activity when our body is at rest or in a phase of recovery: some body functions slow down, such as heart rate and respiration, other activities increase, such as the digestion.
The Sympathetic Nervous System, on the other hand, prepares us for activity. This already starts in the morning when we open the eyes while awaking from sleep. The sympathetic nervous system enables us to keep our eyelids open. Contraction of circular smooth muscles surrounding the peripheral vessels will increase the blood pressure and adapted it to different situations and needs, in order to keep a sufficient blood flow to the brain. The sympathetic nervous system is also responsible for the activity of the sweat glands, whose main task is to cool the body at elevated body temperature. When the interaction of the two subsystems of the autonomic nervous system is well balanced, the body functions are continuously and accurately adjusted to the changing requirements.
E.g. in conditions of psychological anxiety and tension, the sympathetic nervous system will cause increasing heart rate, rising blood pressure, slowering of the bowel activity, increased perspiration, release of adrenaline (epinephrine) from the adrenal gland into the bloodstream, etc.