There are many different ways to talk about stress, but by far the most accessible way to understand it is found in the book Why Zebras Don’t Get Ulcers by Robert M. Sapolsky. His way of talking about stress is to consider a zebra on the plains of Africa. If the zebra or a zebra next to them thinks they hear, see, or smell a lion then the body goes into a reaction called a “stress response.” The body goes into fight or flight mode.
The first thing the zebra’s body does is mobilize the body functions that it needs to survive. Hormones are released into the blood stream to increase energy and blood flow. Blood pressure is increased so that if the zebra has to run the body is ready to do so. The energy stores of the body are opened up so that the zebra can sustain running. Memory is increased so that the zebra can remember how to run, where to run, and how to avoid being eaten by the lion. The zebra’s senses are also increased so that it notices even the slightest change in smell, sound, or vision. The focus of the zebra is sharpened so that they are able to stay acutely aware of the potential threat. Perception of pain is also blunted.
At the same time the zebra’s brain puts a halt on any long term expensive building projects. In other words, the body stops doing anything that takes up a lot of energy to do so that all the energy of the body is available for escaping. Digestion is inhibited. In fact, animals in danger often empty their lower track to be lighter for running away. Tissue repair is limited, sex drive decreases, and immunity is inhibited.
The process is essentially the same when a person feels a stressor in their life. It might be a difficult job or a broken family relationship, but the brain does not distinguish between those kind of stressors and lions. This explains why stress is associated with upset stomachs and a loss of energy. People under stress are more susceptible to colds, flues, and infections because their immune system is decreased. They are more jumpy because of heightened senses. They experience a decrease in sex drive. They cannot focus on other things but cannot stop thinking about the stressor. This also explains why people can remember right where they were when they heard about the attacks of 9/11. It should be noted, however, that this heightened memory does not last. Long term memory is inhibited after the initial stress responses. This explains why people do not remember what happens in the hours following a tragedy, but can play the car accident out frame by frame in their head years later.