"The complex has its roots in the simple."  Lau-Tzu 

It is one thing to be alerted to a threat but altogether another to over react with a tendency towards fear, to fight, to flee, to feint, freeze, fake it, fumble, mumble or stumble.

There are many causes of anxiety.  Food additives, sugar, adrenal dysfunction and other internally consumed substances.  Situations in life that are threatening or seem so will cause anxiety. The idea you do not have enough time is a huge cause of anxiety. Taking on too many projects or responsibilities invites anxiety and we too often forget we chose and often can un-choose.   Spending too much money, getting into debt and worrying about that will cause anxiety.  Eventually a certain level anxiety becomes an every day occurrence that we sort of get used to. The breathing pattern sets in and never leaves. How sad.  

Modern stressful living as well as past traumas have resulted in vast numbers of people over reacting and even imagining threats that do not exist. When the breathing is not allowed to stay in balance every reaction to life becomes distorted or negatively magnified.

Generally the first line of defense if to remove a negative stimulus. The caffeine you ingested is driving you up the wall? Stop drinking caffeinated beverages.  If there is snake in the area, remove the snake or leave the area.  If this is impractical, (your boss is the "snake" or he/she is just so exciting you just cannot sit still), you take life saving prescription drugs that make you nervous, you can try to reduce your anxious reaction(s) by the way you are breathing.  You can consciously slow your breathing down and reduce the anxiety level; at will.

This can be quite helpful but is often a temporary approach and your body may need more oxygen in the first place so slowing the breathing down may or  may not be a good idea albeit a momentary quick fix and even life saving.  The long term best approach is to manage the immediate anxiety and then change/develop your breathing so that it automatically adjusts and you stay calm and do not get anxious in the first place. "Courage under fire" might be one way of looking at it but it is really more about grounding, relaxing, and feeling safe inside.  

Basically, one's mind interprets something as negative and the breathing over-reacts in an unbalanced way and that makes nervousness get worse even up to full blown panic, high blood pressure, migraine, hot flash, ulcer, constipation, heart attack or stroke. It also occurs in the reverse. The breathing is now semi-permanently out of balance and invites overreaction to perfectly harmless events and what was an imagined threat now causes a reaction that can be just as damaging as a real threat. 

The way you breathe can make you sick.
When your breathing overreacts in an unbalanced way your body often tightens in several areas.  Or you hold your breath or breathe shallowly to try to stay in control which only worsens the situation by creating oxygen deprivation and more tension which heightens the stress response and then the tension sets in to stay. This further restricts respiration and produces shallow, rapid distorted breathing.  Shallow breathing numbs our feelings and traps the anxiety inside, blocking smooth energy flow, which in turn triggers more physiological or psychological arousal, sending us up the anxiety and confusion escalator and then possibly down into depression, malaise and confusion.  I see what is often called psychosomatic illness greatly stemming from this.