A 23-year-old man undergoes exercise physiology testing at sea level. He jogs on a treadmill to achieve a moderate-intensity physical activity level based on a target heart rate of 50%-70% of his estimated maximum heart rate. An increase in which of the following is expected at the peak of his exertion?

Question

A 23-year-old man undergoes exercise physiology testing at sea level.  He jogs on a treadmill to achieve a moderate-intensity physical activity level based on a target heart rate of 50%-70% of his estimated maximum heart rate.  An increase in which of the following is expected at the peak of his exertion?

Quizplus · Accepted Answer

Long-distance running and other forms of physical exercise cause increased oxidative metabolism of glucose and fatty acids in the skeletal muscle. This markedly increases the rates of O₂ consumption and CO₂ production, which must be balanced by increases in skeletal muscle perfusion and alveolar ventilation. The cardiovascular response to exercise involves vasoconstriction in the splanchnic circulation and vasodilation in skeletal muscle, shunting blood toward exercising muscle. The vasodilation is predominant, resulting in overall decreased systemic vascular resistance. There is also an increase in heart rate and stroke volume that markedly increases cardiac output and O₂ delivery to the tissues. Increased O₂ extraction by the tissues leads to a concomitant increase in CO₂ production, which is absorbed by the systemic capillaries and transported in high quantities via venous blood to the lungs, where it is expired. (Choices A and B) Mean values for arterial O₂ and CO₂ content remain essentially constant during exercise, even during periods of intense exertion. This is likely accomplished via tight regulation of increases in alveolar ventilation and gas exchange efficiency. (Choices C and D) Tissue pH typically decreases during exercise due to production of carbonic and lactic acid in active skeletal muscle. The acidic pH causes a decrease in hemoglobin affinity for O₂ (ie, right shift in the O₂-hemoglobin dissociation curve) to facilitate unloading of O₂ in the tissues. Although venous pH may decrease significantly during moderate exercise, there is typically little change in arterial pH due to respiratory compensation. (Choice E) Physiologic dead space (ie, the air in the respiratory system that does not participate in gas exchange) is decreased during exercise due to a decrease in pulmonary vascular resistance that allows perfusion of additional pulmonary capillary beds. (Choice G) The mean O₂ content of venous blood remains the same or decreases with exercise because the rate of O₂ extraction by the tissues outpaces the rate of oxygen delivery (cardiac output). In most healthy individuals, cardiac output is the major limiting factor to O₂ consumption during exercise. Educational objective: During physical exercise, there is increased skeletal muscle CO₂ production that increases the CO₂ content of venous blood. Arterial O₂ and CO₂ content remains constant via increases in alveolar ventilation and gas exchange efficiency. Venous O₂ content remains constant or is decreased due to increased O₂ extraction by the tissues that matches or exceeds the rate of oxygen delivery (ie, O₂ consumption during exercise is limited by cardiac output).

A 23-Year-Old Man Undergoes Exercise Physiology Testing at Sea Level