Females are unable to maintain a high level of cardio respiratory fitness

Males tend to have higher levels of hemoglobin in their blood. Body composition has little to do with cardiorespiratory fitness. Aerobic exercise forces the body to provide extra oxygen to the muscles. Males tend to have higher levels of body fat than females. Males tend to have higher levels but, it is not necessarily true that females are unable to maintain a high level of cardio respiratory fitness.