Stress and homeostasis are at odds with one another within the body of a living organism. An organism is designed to be in a state of homeostasis, where all the systems within its body are functioning optimally. Stress pushes the organism out of homeostasis, forcing it to compensate for changes in the environment or within its internal systems. In biology, stress may refer to physical and environmental challenges as well as emotional and psychological ones that can have an effect on an organism's body.
Organisms are never fully free from stress and are thus never in a true state of homeostasis, though they are usually in a state that is relatively stable. Stresses in the environment, such as those caused by temperature or the presence of danger, as well as those within the organism, such as those caused by hunger, thirst, or emotional discomfort will all cause the organism to move out of homeostasis as the stressor is dealt with. Most of the time, stress and homeostasis are in a balance that allows the organism to survive though not always to thrive.
One of the most easily recognizable examples of the connection between stress and homeostasis is the adrenal response, which is seen in humans and many other animals. In this process, a stressor, such as the fear brought on by the presence of a predator, causes the body to release adrenaline so that it can escape from danger and survive. The stressor causes the body to leave a state of homeostasis and to enter a state of "fight-or-flight," in which the heart beats faster, the respiration quickens, and unneeded systems, such as digestion and color vision are temporarily switched off. Once the threat is no longer present, the body will calm down and return to a state of homeostasis.
Although stress and homeostasis are often discussed because of their relationship in the adrenal response, they are also linked in other aspects of an organism's life. If there are not enough nutrients in the organism's system, this will create stress within the organism, prompting the organism to find more food. If it is too cold, stress will cause the organism to try to warm itself, such as through shivering. Emotional stress will also cause the body to leave homeostasis, often through the adrenal response. With emotional stress, it is more difficult to balance stress and homeostasis because the threat is not as well-defined as other threats, such as predation.