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Stress as a Biological Process

In simple terms, the stress mechanism is an evolutionary adaptation for living in a threatening environment. For most of hominid evolution, life has been very threatening. The ancestors of modern humans periodically needed to increase their level of physical performance in order to defend themselves or to get out of harm’s way. What we now call the ‘Fight or Flight’ mechanism is the outcome of that evolutionary process. The mechanism works over two separate time scales. We have an immediate response to an apparent threat to our well-being, which manifests as fear. But there is a slower-acting response, triggered by an ill-defined or uncertain threat, which we call anxiety, but which triggers the same physiological process.

The stress response or general adaptation syndrome (Selye, 1950) can be divided into three phases: the alarm phase, the resistance phase and the exhaustion phase. In the alarm phase, initiated by the detection of a threat or danger, a sequence of responses is triggered in the endocrine and autonomic nervous systems. Adrenaline (epinephrine) and noradrenaline (norepinephrine) are released into the bloodstream. The responses to this hormonal stimulation are summarised here:

  • • The rate and strength of the heartbeat increase in order to pump oxygenated blood around the body faster.
  • • The spleen contracts to release stored red blood cells to increase the oxygencarrying capacity of the blood.
  • • Sugars stored in the liver are released to provide energy for the muscles.
  • • Blood is redirected from the skin and viscera to the muscles and brain.
  • • The bronchi in the lungs dilate and our breathing deepens.
  • • The pupils dilate, probably in order to improve visual efficiency.
  • • Lymphocytes and coagulating agents are released into the blood to improve the body’s ability to repair wounds.
  • • Sweat production is increased to improve the cooling effect.

If the stress mechanism continues to be stimulated, then the body moves into the resistance phase. A group of chemicals is released that facilitate the conversion of non-sugars into sugars, and the rate of sugar storage in the liver increases. Growth hormones are suppressed, as are functions associated with sexual reproduction. The body continues to adapt itself to the potential demands of increased immediate workload, but now we start to see an impairment of the immune system. In effect, the resistance phase seems to involve an increased adaptation to the specific threat but a reduced ability to defend against other stressors. Finally, if exposure to stress is prolonged, then the body simply runs out of energy while some of the harmful byproducts of the physiological response reach debilitating levels. This is the exhaustion phase, often characterised as ‘burnout’.

One consequence of the disconnect between the evolutionary selection pressure and the modern human condition is how environmental conditions will trigger the stress mechanism. Known as environmental stressors, they include heat, vibration and humidity. Most people feel comfortable at 20°C while actual discomfort can be experienced at temperatures above 30°C and below 15°C. Some helicopter cockpits can get up to temperatures of 50°C while cabin attendants working internal routes in some Gulf States report similar temperatures in the back of older aircraft types. High temperatures lead to an increase in heart rate, blood pressure and sweating. Besides, some information processing tasks can be degraded. Low temperatures lead to a loss of feeling and control of limbs. We can moderate the effects of temperature by controlling such factors as insulation (clothing), ventilation and level of activity (metabolic rate). Shivering is one way of increasing our level of activity, but both sweating and shivering consume energy and can result in fatigue.

Vibration includes the direct vibration of the body and the indirect effects of vibrating air; noise. We ‘hear’ vibrations between 20 and 20,000 Hz. However, the perception of noise is affected by the relationship between the frequency and intensity, measured in decibels. Our ability to detect noise also varies with age. Excessive noise can lead to irritability and, eventually, cardiovascular problems. Low-frequency whole-body vibration can cause a decoupling of sensory inputs, leading to motion sickness. One common source of whole-body vibration in aviation is turbulence. Of course, turbulence can directly lead to injury and death when unrestrained objects and passengers are thrown around the cabin. However, on the flight deck, turbulence can induce a level of stress that impedes information processing while extreme turbulence can make it impossible to read displays accurately. Vibration can interfere with breathing, cause chest pains and lead to speech disorders and headaches.

The extent to which we experience humidity is linked to the ambient temperature. Ordinarily, a comfortable level of humidity, although it varies with temperature, is between 50%-60%. However, the humidity of conditioned cabin air in older generations of aircraft is between 5% and 15%. This can lead to drying of the mucous membranes of the eye, nose and throat and reduced rate of urine production, which, in turn, can lead to additional medical problems. While environmental factors can create a general background level of stress and should not be ignored, it is fear and anxiety that are of more immediate interest as performance inhibitors. I will look at fear in the next section but come back to anxiety when we look at the effects of fatigue on performance.

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