Stress can be defined as anything that throws your body out of allostatic balance.
Stress comes in many ways:
INJURY - STARVING - TOO HOT - TOO COLD - PSYCHOLOGICAL - WEIGHT TRAINING - LONG RUN - COLD SHOWER.
And many other things, regardless of the type of stress you turn on the same stress response.
One of the hallmarks of the stress-response is the rapid mobilization of energy from storage sites and the inhibition of further storage.
Glucose and the simplest forms of proteins and fats come pouring out of your fat cells, liver, and muscles, all to stoke whichever muscles are struggling to save your neck.
If your body has mobilized all that glucose, it also needs to deliver it to the critical muscles as rapidly as possible. Heart rate, blood pressure, and breathing rate increase, all to transport nutrients and oxygen at greater rates.
During an emergency, it makes sense that your body halts long-term, expensive building projects. If there is a tornado bearing down on the house, this isn’t the day to repaint the garage.
Hold off on the long-term projects until you know there is a long term. Thus, during stress, digestion is inhibited, there isn’t enough time to derive the energetic benefits of the slow process of digestion, so why waste energy on it?
The same thing goes for growth and reproduction, both expensive, optimistic things to be doing with your body (especially if you are female).
During stress, growth and tissue repair is curtailed, sexual drive decreases in both sexes; females are less likely to ovulate or to carry pregnancies to term, while males begin to have trouble with erections and secrete less testosterone.
Along with these changes, immunity is also inhibited. The immune system, which defends against infections and illness, is ideal for spotting the tumor cell that will kill you in a year, or making enough antibodies to protect you in a few weeks, but is it really needed this instant?
The logic here appears to be the same, look for tumors some other time; expend the energy more wisely now.
Another feature of the stress-response becomes apparent during times of extreme physical pain. With sufficiently sustained stress, our perception of pain can become blunted.
It’s the middle of a battle; soldiers are storming a stronghold with wild abandon. A soldier is shot, grievously injured, and the man doesn’t even notice it. He’ll see blood on his clothes and worry that one of his buddies near him has been wounded, or he’ll wonder why his innards feel numb. As the battle fades, someone will point with amazement at his injury, didn’t it hurt like hell? It didn’t.
Such stress-induced analgesia is highly adaptive and well documented.
Finally, during stress, shifts occur in cognitive and sensory skills. Suddenly certain aspects of memory improve, which is always helpful if you’re trying to figure out how to get out of an emergency. Moreover, your senses become sharper.
Think about watching a terrifying movie on television, on the edge of your seat at the tensest part. The slightest noise, a creaking door, and you nearly jump out of your skin. Better memory, sharper detection of sensations, all quiet adaptive and helpful.
The above information is taken from the The book Why Zebras Don't Get Ulcers