BOX 4-1

Physiologic Changes During the Stress Response

Early Phase of the Stress Response (Duration: Minutes to Hours)

  • Increased heart rate and blood pressure

  • Increased respiration

  • Mobilization of energy from liver and body fat

  • Sharpening of attention and cognition

  • Increased fear conditioning (learning)

  • Blunting of pain

  • Altered intestinal motility

Later Phases of the Stress Response (Duration: Days to Weeks)

  • Enhanced immune system

  • Suppression of appetite and digestion

  • Suppression of growth

  • Suppression of reproduction

  • Persistence of increased heart rate and blood pressure in some cases

  • Persistence of increased cortisol in some cases

  • Release of stress hormones

When a stressor is eliminated, the stress response may shut off quite slowly or not at all (McEwen and Lasley 2002). It is the long-term activation of the stress response—long after the threat has ceased—that poses the greatest risk to human health. Paradoxically, the human stress response is life-saving in the short term (and is adaptive) when immediate stressors are confronted, but it can lead to illness or disease when stressors are severe, recurrent, or persistent in the long term (and is maladaptive).

Although many veterans consider deployment, particularly combat, to be highly stressful and even traumatic, there are many periods during deployment when stressors are not traumatic but are severe and persistent (for example, separation from family and worry about home or work). Prolonged exposure to such stressors, whether in military or civilian life, can overwhelm otherwise healthy people and can lead to health-damaging behaviors, such as smoking and excessive drinking, in an attempt to alleviate the stress.


Once the brain interprets a situation as threatening, it assumes immediate control over the endocrine, cardiovascular, immune, and digestive system. The brain relies on an elaborate communication network that includes hormones, neurotransmitters, chemicals associated with the immune system, and other molecular signals. This section discusses the various regions of the brain involved in the body’s response to stress, the effects of the brain on other organ systems that play roles in the stress response, and the feedback mechanisms that occur in response to acute and chronic stress.

The stress response is spearheaded by the hypothalamus-pituitary-adrenal (HPA) axis in the peripheral stress-response pathway and by the sympathetic nervous system in the central

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement