How does the nervous system control the blood flow to the various organs?
Think for a bit about your muscles. You know if you are resting, not doing any exercise, your muscles don't need as much blood. On the other hand, if you are exercising (such as running) your muscles need more blood because they are in a greater need of oxygen and glucose to keep functioning while you are running.

The arteries pumping to any organ receive commands from the central nervous system about how to behave. For example, if you are running substances which are secreted via the nerve endings which encircle the small arteries supplying the muscle tissue and make them wide (dialate) so more blood will be getting to them.

On the other hand, if you are resting or sleeping, your body does not produce those same hormones and your arteries supplying the muscles tend to become smaller (narrow). The blood supply to the arms and muscles when you don't need them is not excessive.

If you are eating and relaxing the tiny arteries going to your guts will become as wide as possible so they can get as much blood to your guts as possible to absorb the nutrients from your guts and return it to your body. Now imagine someone yelling "Fire!" while you are eating. The arteries going to your guts will have to shut off and try to divert the blood to your muscles, brain, and heart because now those are the organs which need to work harder and need the blood coming to them.

This very rapid response is started with the fear that you were confronted with when someone yelled "Fire!" Your brain sent the command that there is danger. This danger got communicated to the subserving nervous system which controls the function of your glands and the intricate inter-actions of your organs and the very meticulous arrangement of distribution of blood to go where it is needed the most. What happened in fact, is the arteries going to your brain, heart, and muscles widened (dialated) and now they are receiving more blood.

So, when you are running fast your muscles are getting enough blood and oxygen. Your brain is trying to organize the whole body and put your body in a very ready state it also needs more blood and that is why these three organs are getting more blood. At this time of danger, eating is not a priority for your body. This is why it shuts down the blood supply to your guts. It also tries to shut down the blood supply to your skin because you don't need much blood flowing to your skin when you are running. This activates your glands, especially the sweating gland so you can use more water to get rid of the heat forming while you are running.

Hormones: Adrenaline
If you are interested in adrenaline and want to know more about it, let me tell you your interest is well worth it. Adrenaline (Epinephrine) is a hormone secreted by a little gland above the kidney. It is a first cousin (almost a brother) to another hormone called Noradreanaline (Norepinephrine).

Noradreanaline (Norepinephrine) is secreted from certain sympathetic nerve endings. Noradreanaline is always a constricting hormone. For example, any time the nerve ending secrete Noradreanaline it will cause the arteries to constrict (become narrow or vasal). It also makes your heart pump harder and faster (improves the rate of heart beating and contraction).

On the other hand, Adrenaline (Epinephrine) is usually constricting when it acts on the arteries, but in certain arteries it dialates them. For example, when it is secreted to the nerve endings going to the muscles instead of constricting them it dialates them.

When you are confronted with fear, like when there is a fire alarm, your body secretes both epinephrine and norepinephrine (adrenaline and noradrenaline). Epinephrine makes your heart beat faster and stronger, makes your eye pupils dialate, and improves your visual acuity a little bit.

Norepinephrine goes to the skin circulation and constricts it because your body doesn't need it during exercise. At the same time, epinephrine goes to the sweating glands in your skin and makes them secrete sweat to cool your body. Here you can see how important it is for these hormone to be related, yet to have unique differences.

Another hormone which is secreted by the parasympathetic nervous system is called Acetylcholine. This hormone acts in an opposite way by trying to slow the heart and decrease the heart's force of contraction. This is the hormone that dominates when we are sleeping. It is mainly produced by the Vagas Nerve.