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During short, intense exercise bouts, blood glucose rises above preexercise levels because the autonomic nervous system stimulates hepatic glycogenolysis and in some cases, blood [glucose] actually rises in male athletes during hard exercise. However, the ability of the liver to maintain a high rate of glucose release over time is limited by the amount of glycogen stored and by the activities of the hepatic glycogenolytic and gluconeogenic enzymes. During prolonged exercise, glucose production may be limited to gluconeogenesis because of hepatic glycogen depletion; thus, glucose production may fall below the level required by working muscle and other essential tissues such as the brain. Also, in prolonged exercise leading to dehydration and hyperthermia, shunting of blood flow away from the liver and kidneys occurs. Thus, the levels of gluconeogenic precursors (lactate, pyruvate, alanine) rise, and hepatic glucose production falls. In this case of falling blood glucose, the exercise becomes subjectively more difficult because of CNS starvation and difficulty in oxidizing fats in muscle due to the absence of anaplerotic substrates

“Factors Affecting the Performance of the Biological Machine” continued

Alarm Reaction:

The alarm reaction, the initial response to the stressor, involves the mobilization of systems and processes within the organism. During exercise, for example, the stress of running is supported by the strain of increasing oxygen transport through an augmentation of cardiac output and a redistribution of blood flow to active muscle. The body has a limited capacity to adjust to various stressors; thus, it must adapt its capacity so that the stressor is less of a threat to its homeostasis in the future.

Next stage: Resistance Development

Source: McGraw Hill, Brooks, Fahey, Baldwin – Exercise Physiology, Human Bioenergetics and Its applications – Fourth Ed(book)