The central nervous system is divided into two main parts; the voluntary and involuntary nervous system. The voluntary nervous system controls things we consciously tell our bodies to do. Tasks such as walking talking, eating or writing are all controlled by the voluntary nervous system. The involuntary nervous system controls all of the things we don't have to remind ourselves to do; breathing, pulse, blood pressure, digestion. Imagine how busy you would be if you had to remind your heart to beat 60 to 80 times each minute, or your lugs to take a breath every five seconds. You would have virtually no time left to do anything else.

The involuntary nervous system is further divided into two parts. They are identified as the sympathetic and parasympathetic nervous system. The sympathetic nervous system speeds things up while the parasympathetic slows things down.

The sympathetic nervous system is responsible for speeding up your heart rate, you breathing rate and  increasing your blood pressure. In addition to this, it also shunts blood through your body. Shunting is the body's way of efficiently utilizing the finite amount of blood our bodies contain. When you've eaten a big meal and your body needs to digest, your sympathetic nervous system will shunt blood to your stomach to begin that process. this is also true  in a medical emergency. When someone is having a heart attack, their body will shunt blood to their heart to try and repair the damage being done. Subsequently, their skin will appear pale and waxy due to the lack of blood in the extremities and skin.

The parasympathetic nervous system works to counteract the sympathetic. When your blood pressure increases, the parasympathetic works to lower it. A great example of these two systems at work is your reaction to a near-miss car accident. Your initial reaction is rapid breathing, rapid pulse, increased blood pressure and possibly nausea (due to shunting). After a few minutes, the parasympathetic will take over to lower your pulse and respiratory rate and decrease your blood pressure. The parasympathetic nervous system has no effect on shunting.