Ventilatory response to exercise

Exercise results in an increase in oxygen consumption and in carbon dioxide production by the working muscles. In order to meet the metabolic demands of these tissues, ventilation increases accordingly. Minute ventilation increases linearly in response to oxygen consumption and carbon dioxide production up to a level of approximately 60% of an individual's work capacity. During this period of mild to moderate exercise, mean arterial PO2 and PCO2 remain relatively constant at their normal values. In fact, the partial pressures of these gases may even improve (arterial PO2 is increased; arterial PCO2 is decreased). Therefore, it does not appear that hypoxic or hypercapnic stimulation of the peripheral chemoreceptors plays a role in ventilatory response to mild to moderate exercise.

Beyond this point, during more severe exercise associated with anaerobic metabolism, minute ventilation increases faster than the rate of oxygen consumption, but proportionally to the increase in carbon dioxide production. The mechanism of the ventilatory response to severe exercise involves metabolic acidosis caused by anaerobic metabolism. The lactic acid produced under these conditions liberates an H+ ion that effectively stimulates the peripheral chemoreceptors to increase ventilation.

During exercise, the increase in minute ventilation results from increases in tidal volume and breathing frequency. Initially, the increase in tidal volume is greater than the increase in breathing frequency. As discussed earlier in this chapter, increases in tidal volume increase alveolar ventilation more effectively. Subsequently, however, as metabolic acidosis develops, the increase in breathing frequency predominates.

The mechanisms involved with the ventilatory response to exercise remain quite unclear. No single factor, or combination of factors, can fully account for the increase in ventilation during exercise. Therefore, much of this response remains unexplained. Factors that appear to play a role include:

• Impulses from the cerebral cortex

• Impulses from proprioceptors

• Body temperature

• Epinephrine

At the beginning of exercise, ventilation immediately increases. This increase is thought to be caused by two mechanisms involving the cerebral cortex. Neurons of the primary motor cortex stimulate alpha motor neurons in the spinal cord to cause skeletal muscle contraction. In addition, impulses from the motor cortex, transmitted through collateral interconnections to the medullary respiratory center, stimulate ventilation. The motor cortex is also involved in stimulation of the cardiovascular system during exercise. These adjustments, which occur before any homeostatic factors (e.g., blood gases) have changed, are referred to as anticipatory adjustments. The immediate increase in ventilation may account for as much as 50% of the total ventilatory response to exercise. A conditioned reflex, or a learned response to exercise, may also be involved. Once again, impulses from the cerebral cortex provide input to the medullary respiratory center.

Proprioceptors originating in muscles and joints of the exercising limbs provide substantial input to the medullary respiratory center. In fact, even passive movement of the limbs causes an increase in ventilation. Therefore, the mechanical aspects of exercise also contribute to the ventilatory response. The increased metabolism associated with exercise increases body temperature, which further contributes to the increase in ventilation during exercise. (Not surprisingly, ventilation is also enhanced in response to a fever.) Exercise is associated with a mass sympathetic discharge. As a result, epinephrine release from the adrenal medulla is markedly increased. Epinephrine is believed to stimulate ventilation.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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