Effect of Exercise on the Respiratory System
Many cardiovascular and respiratory mechanisms come into play to meet the oxygen needs of active tis-sue and to remove the extra carbon dioxide and heat produced during exercise.
Even before the start of exercise, respiratory rate and depth are altered as a result of the psychic stim-uli on the respiratory center. Soon after the onset of exercise, there is an abrupt increase in ventilation, probably a result of afferent stimuli from propriocep-tors. When exercise is continued, there is a gradual increase in ventilation, presumably a result of changes in arterial pH, Pco2 and Po2.
The amount of oxygen entering the pulmonary capillaries is increased by many mechanisms. During exercise, the volume of blood in the pulmonary cir-culation is increased from 5 L/minute (1.3 gal/min) at rest to as high as 20–35 liters/minute (5.3–9.2 gal). The gas exchange surface area is increased as more pulmonary capillaries open up and are better per-fused. In addition, the pressure gradient between alveoli air and blood is increased, speeding the rate of diffusion. This is because more oxygen is used by ac-tive tissue and the deoxygenated blood carried to the lungs has a lower partial pressure of oxygen. At the tissue level, the increase in carbon dioxide produc-tion, temperature increase, and lactic acid produc-tion all contribute to rapid unloading of oxygen from hemoglobin to the tissues.
If exercise is prolonged and strenuous, the respira-tory and cardiovascular changes are not sufficient to keep up with oxygen use. As a result, energy is de-rived by anaerobic means, with accumulation of lac-tic acid (oxygen debt). After cessation of exercise, respiration reduces abruptly as a result of reduced neural stimulus. Respiration slows down further as arterial pH and Pco2 return to normal. However, the respiration does not reach basal levels until the O2 debt is repaid, which may take as long as 90 minutes.