Intramuscular pressure reduces blood flow and muscle becomes oxygen depleted. Responses to both types are different. At start of exercise muscle oxygen use immediately rises. Lung oxygen uptake lags but reaches steady state within 2 min. Fig A Oxygen consumption is proportional to work rate initially but then levels off.
Further work relies on anaerobic mechanism.
Fig A For light or moderate exercise, increase in ventilation is proportional to O 2 consumption and CO 2 production. Mean arterial PCO 2 DOES NOT RISE in exercise. In severe exercise ventilation can become inappropriately high and alveolar and arterial PCO 2 will then fall. This is when lactic acid appears in significant amounts. Ventilation is unlikely to be a limiting factor in exercise in a healthy subject.