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The most startling fact about these results is the sev-eralfold increase in diffusing capacity between the resting state and the state of maximal exercise. This results mainly from the fact that blood flow through many of the pulmonary capillaries is sluggish or even dormant in the resting state, whereas in maximal exercise, increased blood flow through the lungs causes all the pulmonary capillaries to be perfused at their maximal rates, thus providing a far greater surface area through which oxygen can diffuse into the pulmonary capillary blood.

It is also clear from these values that those athletes who require greater amounts of oxygen per minute have higher diffusing capacities. Is this because people with naturally greater diffusing capacities choose these types of sports, or is it because something about the training procedures increases the diffusing capacity? The answer is not known, but it is very likely that training, particularly endurance training, does play an important role.

Blood Gases During Exercise. Because of the great usage of oxygen by the muscles in exercise, one would expect the oxygen pressure of the arterial blood to decrease markedly during strenuous athletics and the carbon dioxide pressure of the venous blood to increase far above normal. However, this normally is not the case. Both of these values remain nearly normal, demonstrating the extreme ability of the respiratory system to provide adequate aeration of the blood even during heavy exercise.

This demonstrates another important point: The blood gases do not always have to become abnormal for respiration to be stimulated in exercise. Instead, respiration is stimulated mainly by neurogenic mechanisms during exercise, as discussed in Chapter 41. Part of this stimulation results from direct stimulation of the respiratory center by the same nervous signals that are transmitted from the brain to the muscles to cause the exercise. An additional part is believed to result from sensory signals transmitted into the respiratory center from the contracting muscles and moving joints. All this extra nervous stimulation of respiration is normally sufficient to provide almost exactly the necessary increase in pulmonary ventilation required to keep the blood respiratory gases—the oxygen and the carbon dioxide— very near to normal.

Effect of Smoking on Pulmonary Ventilation in Exercise. It is widely known that smoking can decrease an athlete's "wind." This is true for many reasons. First, one effect of nicotine is constriction of the terminal bronchioles of the lungs, which increases the resistance of airflow into and out of the lungs. Second, the irritating effects of the smoke itself cause increased fluid secretion into the bronchial tree, as well as some swelling of the epithelial linings.Third, nicotine paralyzes the cilia on the surfaces of the respiratory epithelial cells that normally beat continuously to remove excess fluids and foreign particles from the respiratory passageways. As a result, much debris accumulates in the passageways and adds further to the difficulty of breathing. Putting all these factors together, even a light smoker often feels respiratory strain during maximal exercise, and the level of performance may be reduced.

Much more severe are the effects of chronic smoking. There are few chronic smokers in whom some degree of emphysema does not develop. In this disease, the following occur: (1) chronic bronchitis, (2) obstruction of many of the terminal bronchioles, and (3) destruction of many alveolar walls. In severe emphysema, as much as four fifths of the respiratory membrane can be destroyed; then even the slightest exercise can cause respiratory distress. In fact, many such patients cannot even perform the simple feat of walking across the floor of a single room without gasping for breath.

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