Final answer:
Increased hemoglobin in the blood is not a physiological adaptation to endurance exercise training. Endurance training instead increases mitochondria volume, muscle blood flow, and capillaries in the muscles to enhance the oxygen extraction during aerobic activities.
Step-by-step explanation:
The factor out of the options provided that does NOT contribute to the increase in the maximal arterial-venous oxygen difference (a-vO2 difference) with endurance exercise training is d) increased hemoglobin in the blood. While increased hemoglobin levels can improve the oxygen-carrying capacity of the blood, it is not a direct physiological adaptation elicited by endurance training. Instead, endurance training enhances the oxygen extraction by the active muscles primarily through the following mechanisms: a) increased mitochondria volume in the muscle, b) increased muscle blood flow, and c) increased capillaries in the muscle. These adaptations improve the ability of muscles to utilize oxygen more efficiently during sustained aerobic exercise.
It is worth noting that substances like erythropoietin (EPO) can be used to artificially increase hemoglobin levels, and hence the oxygen-carrying capacity of the blood, which could benefit aerobic respiration in muscles. However, this is not a natural adaptation to endurance training, but rather a pharmacological intervention.