Chapter 11. The Respiratory System under Stress

The reader uses the knowledge he or she gained from the preceding chapters to predict the response of the respiratory system to 4 physiologic stresses: exercise, ascent to altitude, diving, and sleep.

• Identifies the physiologic stresses involved in exercise.
• Predicts the responses of the respiratory system to acute exercise.
• Describes the effects of long-term exercise programs (training) on the respiratory system.
• Identifies the physiologic stresses involved in the ascent to altitude.
• Predicts the initial responses of the respiratory system to the ascent to altitude.
• Describes the acclimatization of the cardiovascular and respiratory systems to residence at high altitudes.
• Identifies the physiologic stresses involved in diving.
• Predicts the responses of the respiratory system to various types of diving.
• Identifies the physiologic stresses involved in sleep.
• Relates the alterations that occur in the respiratory system during sleep to the pathophysiology of obstructive sleep apnea.

This chapter is mainly intended to be a review of the preceding chapters of the book. The responses of the respiratory system to 4 physiologic stresses are examined as they relate to the material already covered; the discussions of the responses to each stress will therefore be brief and rather superficial. For a more complete discussion of each stress, consult the Suggested Readings at the end of this chapter.

Exercise increases the metabolism of the working muscles. It stresses the respiratory system by increasing the demand for oxygen and increasing the production of carbon dioxide. Moderate to strenuous levels of exercise also cause increased lactic acid production. The respiratory and cardiovascular systems must increase the volume of oxygen supplied to the exercising tissues and increase the removal of carbon dioxide and hydrogen ions from the body.

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Acute Effects

The effects of exercise in an untrained person are mainly a function of an increase in the cardiac output coupled with an increase in alveolar ventilation.

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Control of Breathing

As discussed at the end of Chapter 9, both the tidal volume and the breathing frequency are increased during exercise. The causes of the increased alveolar ventilation during exercise were discussed in that section.

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Mechanics of Breathing

The work of breathing is increased during exercise. Larger tidal volumes result in increased work necessary to overcome the elastic recoil of the lungs and chest wall during inspiration because the lungs are less compliant at higher lung volumes and because the elastic recoil of the chest wall is inward at high thoracic volumes. Of course, the greater elastic recoil tends to make expiration easier, but this is offset by other factors. The high airflow rates generated during exercise result in a much greater airways resistance component of the work of breathing. Greater turbulence and dynamic compression of airways secondary to active expiration combine to greatly increase the work of breathing. (Recall that during turbulent airflow

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